Energy Glossary

A unit of energy equal to 1,000 watts used for one hour. This is how utilities measure your electricity usage.

A billing mechanism that credits solar energy system owners for the electricity they add to the grid. The excess power generated during the day offsets power consumed at night.

A photovoltaic (PV) module that converts sunlight into electricity. Multiple panels are combined to form a solar array.

A device that converts direct current (DC) electricity from solar panels into alternating current (AC) electricity used by your home.

A system that stores excess solar energy for use when the sun isn't shining, providing backup power during outages.

An electricity pricing structure where rates vary by time of day. Peak hours (usually 4-9 PM) have higher rates.

Investment Tax Credit allowing homeowners to deduct a percentage of solar installation costs from federal taxes. Currently 30% through 2032.

A tradable certificate representing the environmental benefits of generating 1 MWh of solar electricity. Can be sold for additional income in some states.

A solar energy system connected to the utility grid, allowing you to draw power when needed and export excess generation.

A solar system with battery storage that operates independently from the utility grid, requiring full self-sufficiency.

The ratio of actual energy produced by a system to its maximum potential output. Solar typically has a 15-25% capacity factor.

The annual decrease in solar panel efficiency. Modern panels typically degrade 0.3-0.5% per year.

The process of connecting your solar system to the utility grid, requiring utility approval and inspection.

A financing arrangement where a third party owns the solar system and you pay for the electricity it generates.

A financing option where you pay a fixed monthly fee to rent a solar system, while a third party owns and maintains it.

A small inverter attached to each solar panel, allowing independent operation and optimization of each panel.

A central inverter that connects to multiple solar panels in series (strings). Less expensive than microinverters but less flexible.

The total cost of generating electricity from a system divided by its lifetime energy production. Used to compare energy sources.

The equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter. Varies by location.

The compass direction of the sun. In the Northern Hemisphere, south-facing panels (180° azimuth) typically produce the most energy.

The angle of solar panels relative to horizontal. Optimal tilt often equals your latitude for year-round production.

An assessment of how trees, buildings, or other objects cast shadows on your roof throughout the year, affecting solar production.

A pattern of your electricity usage throughout the day and year. Important for sizing solar and battery systems.

The amount of CO2 emissions per unit of electricity generated. Varies by grid mix (coal, gas, renewables).

A utility charge based on your peak power usage (measured in kW) during a billing period, common for commercial customers.

A policy mechanism that guarantees a fixed payment rate for renewable energy fed into the grid, typically for a set period.

A shared solar installation where multiple customers receive credits on their utility bills for their share of production.

A program allowing customers to receive bill credits from a remote solar facility they don't own, common in community solar.

A contract between you and your utility that governs how your solar system connects to and interacts with the grid.

The percentage of sunlight that a solar panel converts into electricity. Modern panels range from 15-22% efficient.

A unit of power equal to one joule per second. Solar panels are rated in watts (W) or kilowatts (kW = 1,000W).

Electricity that flows in one direction. Solar panels produce DC power, which must be converted to AC for home use.

Electricity that periodically reverses direction. Standard household electricity in the US is 120/240V AC.

The time it takes for your cumulative savings to equal your initial investment in solar. Typically 6-10 years in the US.

The financial gain from solar expressed as a percentage of the initial cost. Most residential solar has 10-20% ROI.

A tradeable certificate representing the environmental attributes of 1 MWh of renewable electricity generated. Utilities buy RECs to meet clean energy mandates.

A state regulation requiring utilities to source a minimum percentage of electricity from renewable resources by a target date. Also called a Renewable Energy Standard (RES).

A solar incentive paid per kWh of actual electricity produced over a set period, rather than as an upfront rebate based on system size.

A federal tax depreciation schedule that allows businesses to recover solar investment costs over 5 years, significantly improving commercial solar ROI.

A financing mechanism where solar and energy efficiency improvements are funded through a voluntary assessment on the property tax bill, repaid over 10-25 years.

A federally funded program that helps low-income households pay energy bills and weatherize homes. Administered by states with varying benefit levels.

A federal program that provides free energy efficiency improvements to low-income homes, including insulation, air sealing, and heating system upgrades.

Landmark 2022 federal legislation that extended and expanded clean energy tax credits, including the 30% solar ITC through 2032, and added new incentives for batteries, heat pumps, and EVs.

California's successor to traditional net metering, which compensates solar exports at a lower "avoided cost" rate rather than the full retail rate. Took effect April 2023.

The cost a utility would have incurred to generate or purchase electricity that was instead provided by a distributed resource like rooftop solar.

The rate at which a utility purchases excess solar electricity from a customer. May equal the retail rate (full net metering) or a lower wholesale/avoided cost rate.

A public or quasi-public financing institution dedicated to accelerating clean energy deployment by offering low-cost loans, credit enhancements, and other financial products.

A state or local incentive that excludes the added home value from a solar installation from property tax assessments, so your taxes don't increase despite higher home value.

A state incentive that waives sales tax on the purchase of solar equipment, reducing the upfront cost of installation.

A direct cash rebate offered by a state, utility, or municipality for installing solar, typically based on system size ($/W) or a flat amount.

A DC-to-DC converter installed behind each solar panel that maximizes the output of individual panels before sending power to a central string inverter.

A solar panel that generates electricity from both sides, capturing reflected light (albedo) on the rear face for 5-30% additional production depending on installation.

A category of devices (microinverters and power optimizers) that manage power conversion or optimization at the individual panel level rather than at a central inverter.

An electrical enclosure that combines the output of multiple solar panel strings into a single feed before it enters the inverter.

An NEC (National Electrical Code) safety requirement that solar systems must be able to reduce rooftop DC voltage to safe levels within 30 seconds of initiating shutdown, protecting firefighters and first responders.

An intelligent breaker panel that provides circuit-level monitoring and control, enabling load management, backup prioritization, and integration with solar and battery systems.

Electric Vehicle Supply Equipment — a device that safely delivers electricity to charge an EV battery. Level 2 (240V) chargers are most common for home use, adding 25-30 miles of range per hour.

An efficient HVAC system that transfers heat rather than generating it, providing both heating and cooling. Air-source heat pumps are most common; ground-source (geothermal) are most efficient.

A mounting system that tilts solar panels to follow the sun's path across the sky, increasing energy production by 20-35% compared to fixed-tilt systems.

A device that regulates the voltage and current from solar panels to a battery bank, preventing overcharging and deep discharge. Common types are PWM and MPPT.

An electricity pricing structure where the per-kWh rate increases as you use more power within a billing period, divided into usage tiers (e.g., Tier 1: 0-500 kWh, Tier 2: 501-1000 kWh).

The amount of electricity a utility allocates at the lowest rate tier, based on your climate zone, season, and heating source. Usage above baseline is billed at higher tiers.

The highest rate of electricity consumption (measured in kW) recorded during a billing period. Commercial customers often pay demand charges based on their peak demand.

The ratio of real power (kW) used to do work versus apparent power (kVA) drawn from the grid. A power factor below 1.0 means some energy is wasted as reactive power.

The portion of your electric bill covering the cost of the local power lines, transformers, and infrastructure that deliver electricity from substations to your home.

Fees for transporting electricity over high-voltage power lines from generation plants to local distribution substations. Usually a smaller portion of residential bills.

A program where utilities pay customers to reduce electricity usage during peak demand events, often by cycling AC units, adjusting thermostats, or discharging batteries to the grid.

A graph of net electricity demand that shows a deep midday dip (from solar overproduction) and a steep evening ramp (as solar drops off and demand peaks), shaped like a duck.

The point at which the cost of solar electricity equals or falls below the cost of grid-purchased electricity, making solar economically competitive without subsidies.

An electricity pricing structure where the same per-kWh rate applies regardless of usage amount or time of day. Simpler but less common than tiered or TOU rates.

The standard metric for comparing solar installation prices, calculated by dividing total system cost by its DC wattage. US average is $2.50-$3.50/W before incentives (2024).

The annualized rate of return on a solar investment accounting for all cash flows over the system's lifetime, including tax credits, energy savings, and maintenance costs.

The total value today of all future solar savings minus the initial investment, discounted at a chosen rate. A positive NPV means the investment is financially worthwhile.

The annual percentage increase in electricity prices over time. Historically 2-4% nationally, but varying significantly by state and utility.

Non-hardware expenses in a solar installation: permitting, inspection, interconnection fees, customer acquisition, labor, overhead, and profit margin. Soft costs are ~65% of US residential solar prices.

All solar system components beyond the panels themselves: inverter, racking, wiring, monitoring, electrical panel upgrades, and installation labor.

A financing product specifically designed for solar installations, typically offering 10-25 year terms. May be secured (using home equity) or unsecured, with rates from 3-9%.

A tax deduction that allows business solar system owners to deduct the cost of the system over time. Under MACRS, commercial solar depreciates over 5 years with possible bonus depreciation in year one.

A contract (similar to a PPA) where an energy services company installs efficiency or solar improvements and the customer pays from guaranteed energy savings. Common in commercial and municipal projects.

The complete cost of a solar system over its lifetime, including purchase price, financing costs, maintenance, inverter replacement, monitoring fees, and insurance — minus all incentives and energy savings.

A solar installation that is connected to the grid and includes battery storage, offering both grid-tied benefits (net metering, lower bills) and backup power during outages.

A battery configuration where the battery has its own inverter and connects on the AC side of the electrical panel. Allows adding a battery to an existing solar installation without replacing the solar inverter.

A battery configuration where the battery connects on the DC side of the solar system, sharing an inverter (hybrid inverter) with the panels. More efficient but requires compatible equipment.

Software and hardware that monitors and controls energy production, storage, and consumption in real time to optimize self-consumption, minimize costs, and manage battery dispatch.

A battery system sized to power an entire home during a grid outage, as opposed to backing up only essential/critical loads. Typically requires 2-4 batteries (30-50+ kWh).

Solar cells made from a single continuous crystal of silicon. They offer the highest efficiency (20-24%) among mainstream technologies and are identified by their uniform dark appearance.

Solar cells made from multiple silicon crystal fragments melted together. Slightly less efficient (17-20%) than monocrystalline but historically cheaper to manufacture.

Solar cells made by depositing thin layers of photovoltaic material (CdTe, CIGS, or amorphous silicon) on a substrate. Lower efficiency (11-18%) but flexible and lightweight.

An emerging photovoltaic technology using perovskite-structured compounds (typically methylammonium lead halides) as the light-absorbing layer. Lab efficiencies exceed 26%.

A multi-junction solar cell that stacks two or more absorber layers to capture different parts of the solar spectrum. Perovskite-silicon tandems are the most promising near-term technology.

An advanced silicon solar cell architecture that adds an ultra-thin oxide layer and doped polysilicon to reduce electron recombination at the contacts, improving efficiency to 24-26%.

A silicon solar cell design that adds a passivation layer to the rear surface, reducing electron recombination and boosting efficiency to 21-23%. Formerly the industry standard.

A solar cell that combines crystalline silicon with thin amorphous silicon layers, achieving high efficiency (24-26%) with excellent temperature coefficient and bifacial performance.

The rate at which a solar panel's power output changes with temperature, expressed as %/°C. A typical coefficient is -0.3% to -0.4% per °C — meaning output decreases as panels get hotter.

Solar cells integrated directly into building materials — roof tiles, facades, windows, or canopies — serving as both building envelope and electricity generator.

A safety feature that automatically disconnects a grid-tied solar system from the utility grid during a power outage, preventing the system from energizing downed power lines.

A safety system that detects unintended current flow to ground (often through damaged wiring or insulation) and disconnects the circuit to prevent fire or electrocution.

The U.S. standard for electrical installation safety (NFPA 70). Article 690 specifically covers solar PV systems, including wiring, grounding, labeling, and rapid shutdown requirements.

The local government entity (city, county, or township) responsible for permitting, inspecting, and approving solar installations and other electrical/building work.

Energy production lost due to dirt, dust, pollen, bird droppings, or other debris accumulating on solar panel surfaces and blocking sunlight.

When a solar inverter limits output because the DC input power from the panels exceeds the inverter's AC output rating. The excess energy is lost.

The ratio of a solar array's DC panel capacity to the inverter's AC output rating. Typical residential values range from 1.1 to 1.35.

A written guarantee from a solar installer specifying the minimum kWh of electricity the system will produce annually. If actual production falls short, the installer compensates the difference.

A lithium-ion battery chemistry (LiFePO₄) known for long cycle life (4,000-10,000 cycles), high safety (no thermal runaway), and lower energy density compared to NMC.

A lithium-ion battery chemistry (LiNiMnCoO₂) with higher energy density than LFP but shorter cycle life (1,500-3,000 cycles) and higher thermal runaway risk.

The number of complete charge-discharge cycles a battery can perform before its capacity drops below a specified threshold (typically 70-80% of original capacity).

The percentage of a battery's total capacity that is used during a discharge cycle. Most modern batteries allow 90-100% DoD.

The current charge level of a battery expressed as a percentage of its total capacity, similar to a fuel gauge. 100% SoC means fully charged; 0% means fully discharged.

An uncontrolled self-heating chain reaction in a battery that can lead to fire or explosion if the internal temperature exceeds a critical threshold.

The percentage of energy put into a battery that can be retrieved. Typical lithium-ion batteries have 90-95% round-trip efficiency — meaning 5-10% is lost as heat during charge/discharge.

An emerging battery technology using sodium ions instead of lithium. Lower energy density but uses abundant, cheap materials and performs well in cold temperatures.

A network of distributed energy resources (solar, batteries, EVs, smart appliances) coordinated by a central platform to provide grid services collectively, functioning like a power plant.

An electrical device that changes voltage levels. Step-up transformers increase voltage for long-distance transmission; step-down transformers decrease it for distribution and household use.

The process of maintaining the electric grid at exactly 60 Hz (in North America) by continuously balancing supply and demand in real time.

A localized energy system that can operate connected to the main grid or independently (islanded). Typically includes generation (solar), storage (batteries), and load management.

The intentional reduction of renewable energy output below what the resource could produce, typically because the grid cannot absorb the excess generation.

The Electric Reliability Council of Texas — the independent system operator that manages the Texas electric grid, which covers ~90% of the state and is largely isolated from other U.S. grids.

A regional transmission organization (RTO) coordinating the movement of wholesale electricity in 13 states and D.C., from Illinois to the mid-Atlantic coast. The largest power market in North America.

A market mechanism where power generators and storage systems are paid to be available to produce electricity during peak demand periods, ensuring sufficient supply exists to meet forecasted demand.

An export compensation mechanism where excess solar electricity sent to the grid is credited at a rate lower than the retail rate — typically wholesale, avoided cost, or a set export rate.

Services needed to maintain grid reliability beyond simply generating electricity — including frequency regulation, voltage support, spinning reserves, and black start capability.

When too much electricity needs to flow through a transmission line or path that doesn't have enough capacity, leading to rerouting, curtailment, or higher prices in the constrained area.

The minimum amount of electric power delivered or required over a given period at a constant rate. Historically provided by coal and nuclear plants that run continuously.

A power plant that runs only during periods of peak electricity demand. Typically natural gas combustion turbines that can start up quickly but have high per-kWh costs.

The process of restoring a power grid after a complete shutdown (blackout) without relying on external electricity. Requires generators or batteries that can start independently.

The ratio of heating or cooling output to electrical energy input for a heat pump. A COP of 3.0 means the system produces 3 kWh of heat for every 1 kWh of electricity consumed.

The updated cooling efficiency metric for air conditioners and heat pumps, measured under 2023 DOE test procedures with higher external static pressure (more realistic conditions).

The updated heating efficiency metric for heat pumps under 2023 DOE test procedures. Represents total heating output (BTU) divided by total electricity input (Wh) over a heating season.

The efficiency rating for gas furnaces, boilers, and water heaters — the percentage of fuel energy that becomes useful heat. A 95% AFUE furnace converts 95% of gas energy to heat.

The standard efficiency metric for water heaters. It measures overall energy efficiency including standby losses. Heat pump water heaters achieve UEF of 2.5-4.0 vs. 0.6-0.7 for gas tank units.

A compressor that can modulate its speed continuously (using an inverter-driven motor) rather than cycling fully on and off. This allows the system to match output precisely to demand.

A ductless heat pump system with an outdoor compressor unit connected to one or more indoor air handlers (heads) mounted on walls or ceilings. Each head controls its own zone.

The industry-standard method (ACCA Manual J) for calculating a home's heating and cooling loads based on climate, insulation, windows, orientation, occupancy, and other factors.

A heat pump specifically designed to maintain high heating capacity at very low outdoor temperatures (typically rated to -13°F/-25°C or lower).

The fluid that circulates through a heat pump, absorbing and releasing heat as it changes between liquid and gas states. Modern systems use R-410A or the newer, lower-GWP R-32 and R-454B.

A heat pump that exchanges heat with the earth via underground loops filled with circulating fluid. The ground maintains a relatively constant temperature (50-60°F) year-round.

The electric vehicle charging connector standard (also known as J3400/SAE) originally developed by Tesla and now adopted industry-wide as the U.S. standard, replacing CCS1 for new vehicles.

EV charging using a standard 120-volt household outlet. Provides approximately 3-5 miles of range per hour of charging, or about 1.2-1.4 kW.

EV charging using a 240-volt circuit (same as a dryer or stove). Provides 12-60 miles of range per hour depending on the charger amperage (typically 20-80 amps).

High-power EV charging (50-350+ kW) that converts AC power to DC before delivering it directly to the battery, bypassing the vehicle's onboard charger for much faster charging.

Bidirectional technology that allows an EV battery to power a home during grid outages or for daily energy management, effectively using the car as a home battery.

Technology that allows EVs to discharge energy back to the utility grid, potentially earning revenue and providing grid services. Extends V2H beyond the home to the broader grid.

The federal Energy Efficient Home Improvement Credit (IRC §25C) providing 30% of the cost of qualifying energy efficiency upgrades: heat pumps ($2,000), insulation ($1,200), HPWH ($2,000), and more.

The federal Alternative Fuel Vehicle Refueling Property Credit (IRC §30C) providing 30% of the cost of installing EV charging equipment at a residence, up to $1,000.

The federal Residential Clean Energy Credit (IRC §25D) providing 30% of the cost of solar panels, battery storage, and other clean energy systems installed on a residence through 2032.

An IRA-funded point-of-sale rebate program for electrification upgrades: up to $8,000 for a heat pump, $1,750 for a HPWH, $4,000 for an electrical panel, and $14,000 total for LMI households.

An IRA-funded rebate program for whole-home energy efficiency improvements. Offers $2,000-$4,000 based on measured or modeled energy savings (typically 20-35% reduction).

When a tax credit exceeds your tax liability in a given year, the unused portion can be carried forward and applied to future tax years. The 25D clean energy credit allows unlimited carryforward.

A tradeable certificate representing 1 MWh (1,000 kWh) of solar electricity generation. In states with SREC markets, solar owners earn SRECs that can be sold for additional income.

An incentive paid based on the actual energy produced by a solar system over time (typically per kWh over several years), rather than a one-time upfront payment.

A fee charged by the utility to process the application for connecting a solar system to the grid. Ranges from $0 to $1,000+ depending on the utility and system size.

The standard metric for comparing solar system prices: total installed cost divided by system size in watts DC. National residential average: $2.50-$3.50/W before incentives (2025-2026).

The percentage of household income spent on home energy costs. The median U.S. household spends about 3%, while low-income households often spend 8-20%.

The interest rate used to calculate the present value of future cash flows. In solar economics, it reflects the homeowner's opportunity cost of capital — what return they'd earn elsewhere.

A state-level policy requiring utilities to source a specified percentage of their electricity from renewable sources by a target date. Over 30 states and DC have RPS policies.

A provision within an RPS requiring that a specific portion of the renewable target be met by solar energy, creating dedicated demand for solar generation.

The regulatory proceeding through which a utility requests approval from the state public utility commission (PUC) to change customer rates — typically to recover infrastructure investments.

A state-level regulatory body (also called PSC, BPU, or CPUC depending on the state) that oversees utility operations, approves rate changes, and sets interconnection rules.

The unbundling of electricity generation from transmission and distribution, allowing consumers to choose their electricity supplier in competitive markets. ~17 states have some form of deregulation.

The federal agency that regulates interstate electricity transmission, wholesale electricity markets, and natural gas pipelines. FERC does not regulate retail electricity rates.

A policy that sets energy performance targets for existing buildings, requiring improvements over time. Applied by city or state to commercial buildings and large multifamily.

A measure of thermal resistance — how well a material resists heat flow. Higher R-values mean better insulation. Recommended attic R-values range from R-38 to R-60 depending on climate zone.

The process of identifying and sealing unintentional air leaks in a building envelope — including gaps around pipes, wires, ducts, windows, and the attic hatch.

A diagnostic test that uses a calibrated fan mounted in an exterior doorway to depressurize the house and measure the total air leakage rate, expressed in CFM50 or ACH50.

The number of times the total air volume of a house is exchanged per hour when the house is depressurized to 50 Pascals (about 0.2 inches of water column) during a blower door test.

A professional assessment of a home's energy performance, including blower door testing, thermal imaging, duct leakage testing, insulation evaluation, and appliance efficiency review.

A U.S. EPA/DOE program that certifies products meeting energy efficiency standards above federal minimums. ENERGY STAR products use 10-50% less energy than standard models.

A scoring system from 0-150+ that rates a home's energy efficiency. A HERS Index of 100 equals the reference home; lower scores are more efficient. A net-zero home scores 0.

Heat transfer through building elements (like studs, concrete slabs, or window frames) that are more conductive than the surrounding insulation, creating "bridges" for heat to escape.

Air escaping from ductwork joints, connections, and holes before reaching conditioned rooms. The average U.S. duct system leaks 20-30% of airflow.

An electric cooking surface that uses electromagnetic induction to directly heat magnetic cookware, without heating the cooktop surface itself. 85-90% energy efficient vs. 32% for gas.

The total greenhouse gas emissions caused directly and indirectly by an individual, organization, event, or product, usually expressed in metric tons of CO₂ equivalent (tCO₂e).

The amount of CO₂ (or CO₂ equivalent) emitted per unit of electricity generated, typically in lbs CO₂/kWh or kg CO₂/MWh. It varies by region based on the generation mix.

The EPA's Emissions & Generation Resource Integrated Database — the definitive source for emissions data on U.S. power plants and electric grids, organized by region and subregion.

A measure of how much heat a greenhouse gas traps in the atmosphere relative to CO₂ over a specified period (usually 100 years). CO₂ has a GWP of 1; methane has a GWP of ~28-36.

The total greenhouse gas emissions generated during the manufacturing, transportation, installation, and end-of-life processing of a product — before it's ever turned on.

A building or home that produces as much energy as it consumes on a net annual basis, typically through a combination of efficiency measures and on-site renewable generation (solar).

A comprehensive analysis of a product's environmental impacts across its entire life cycle: raw material extraction, manufacturing, use, and disposal/recycling.

The portion of your electric bill covering the cost of maintaining poles, wires, transformers, and the local distribution system that physically delivers electricity to your home.

The portion of your electric bill covering the actual cost of generating or procuring electricity — the energy commodity itself. This is the portion solar offsets directly.

A flat monthly fee on your electric bill regardless of how much electricity you use. It covers meter reading, billing, and basic service. Typically $5-$20/month but some utilities are proposing $20-$50.

An electricity rate charged per kWh of consumption. The higher the volumetric rate, the more valuable solar is, because each kWh of solar production offsets a more expensive kWh of grid electricity.

An electricity rate structure where the price per kWh changes hourly based on wholesale market conditions. Offered by some utilities in deregulated markets.

An evaluation of a property's suitability for solar installation, including roof condition, orientation, shading, structural capacity, electrical panel, and utility interconnection.

Formal approval from the utility allowing a grid-connected solar system to be energized and begin exporting electricity. PTO is the final step after installation and inspection.

An assessment by a licensed structural engineer confirming that a roof can support the additional weight of solar panels and mounting hardware (typically 2-4 lbs/sq ft).

The process of determining how many solar panels to connect in series per inverter string based on voltage, temperature, and inverter operating window specifications.

Official authorization from local government (AHJ) to install a solar system. Typically requires construction, electrical, and sometimes structural drawings. Costs range from $0-$500+.

Replacing a home's electrical panel with a higher-capacity one (typically 100A to 200A) to accommodate additional electric loads like solar, battery, EV charger, or heat pump.

A community-organized group purchasing program where residents collectively negotiate lower solar prices from a pre-selected installer through bulk buying power.

A state or local institution that uses public funds to attract private investment in clean energy projects, offering low-interest loans, credit enhancements, and other financing tools.

An agreement where an energy service company (ESCO) guarantees a certain level of energy savings from efficiency improvements. If savings fall short, the ESCO covers the difference.

A residential development or neighborhood designed to produce as much clean energy as it consumes collectively, through shared solar, individual rooftop systems, and high-efficiency construction.

The energy output per unit of installed capacity over a period (typically one year). Expressed as kWh per kWp (kilowatt-peak). A good residential system in the U.S. achieves 1,200-1,800 kWh/kWp annually.

The percentage of a solar system's theoretical maximum output that is actually achieved, accounting for all real-world losses (temperature, shading, soiling, wiring, inverter). Typical: 75-85%.

Hardware and software that tracks your home's real-time electricity production (from solar), consumption (by load), and grid interaction. Common brands: Sense, Emporia, Enphase, SolarEdge.

A communication standard (ANSI/CTA-2045-B) that provides a standardized interface module for smart grid-enabled appliances, allowing utilities and aggregators to communicate with water heaters, HVAC, etc.

An open-source smart home connectivity standard backed by Apple, Google, Amazon, and Samsung. Matter-compatible devices work across all major ecosystems without vendor lock-in.

The ability of a solar + battery system to disconnect from the grid and operate independently, powering the home from stored and locally generated energy during an outage.

A contract where a third party owns the solar system on your roof and you purchase the electricity it produces at a set rate — typically lower than the utility rate, with annual escalators.

An inverter that manages both solar panels and battery storage in a single unit, handling DC-to-AC conversion for solar and bi-directional charging/discharging for the battery.

The total electricity (kWh) a solar system generates over a full year, accounting for all seasonal variations, weather, and system losses.

The maximum output of a solar system under Standard Test Conditions (STC: 1000 W/m² irradiance, 25°C cell temperature). This is the nominal system size quoted by installers.

The standardized laboratory conditions under which solar panels are rated: 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. Panel wattage ratings are always at STC.

The gold-standard certification body for solar installers. NABCEP-certified PV Installation Professionals have demonstrated advanced knowledge through examination and experience verification.

The backlog of energy projects (solar, wind, battery) waiting for utility approval to connect to the grid. As of 2024, over 2,000 GW of generation is waiting in U.S. queues.

The mounting hardware (rails, clamps, flashings, and brackets) that secures solar panels to a roof or ground structure. Key types: rail-mounted, rail-less, and ballasted (flat roof).

The compass direction a solar panel faces, expressed in degrees (0° = north, 90° = east, 180° = south, 270° = west). Due south (180°) is optimal for maximum annual production in the Northern Hemisphere.

The percentage of solar electricity that is consumed on-site by the household rather than exported to the grid. Without a battery: typically 25-40%. With a battery: 60-90%.

The degree to which a household meets its electricity needs from on-site generation and storage rather than grid purchases. 100% energy independence = off-grid capability.

The power of solar radiation received per unit area, measured in W/m² (watts per square meter). Standard peak irradiance at earth's surface is ~1,000 W/m² under clear skies at solar noon.

The total solar energy received on a surface over a period, typically measured in kWh/m²/day. Similar to peak sun hours but expressed as energy rather than equivalent hours.

The U.S. Department of Energy's primary laboratory for renewable energy and energy efficiency research. Located in Golden, Colorado. Provides definitive data on solar resource, costs, and technology.

The accumulated volume of energy projects awaiting grid connection approval. The U.S. backlog exceeds 2,600 GW as of 2024 — primarily wind, solar, and storage projects.

Colloquial reference to a wall-mounted residential battery storage system, often used specifically to refer to Tesla's Powerwall product (now in its third generation).

A dedicated circuit breaker in the electrical panel that receives power from the solar inverter. It connects the solar system to the home's electrical distribution system.

Moving electricity consumption from one time period to another — typically from expensive peak hours to cheaper off-peak hours — using timers, smart controls, or batteries.

Using batteries or load management to reduce maximum power demand during peak hours, lowering demand charges (for commercial) or maximizing TOU savings (for residential).

Large solar installations (typically 1 MW to 1 GW+) that sell electricity on the wholesale market or to utilities through power purchase agreements.

Any small-scale energy generation or storage connected at the distribution level rather than the transmission level. Includes rooftop solar, home batteries, EVs, and demand response.

A portmanteau of "producer" and "consumer" — a household or business that both produces and consumes electricity, typically through rooftop solar.

The process of splitting a heavy atomic nucleus (such as uranium-235 or plutonium-239) into two lighter nuclei, releasing a large amount of energy. This is the reaction that powers all current nuclear power plants.

The process of combining light atomic nuclei (typically hydrogen isotopes deuterium and tritium) to form heavier nuclei, releasing enormous energy. Fusion powers the sun but has not yet been achieved at commercial scale on Earth.

A naturally occurring radioactive element used as fuel in nuclear reactors. Natural uranium is 99.3% U-238 and 0.7% U-235. Most reactors require enrichment to 3-5% U-235.

The process of increasing the concentration of uranium-235 (the fissile isotope) in uranium fuel. Most commercial reactors need 3-5% enrichment (natural is 0.7%). Advanced reactors may need HALEU (5-20%).

Uranium enriched to between 5% and 20% U-235. Several advanced reactor designs (including many SMRs) require HALEU fuel, which is not yet commercially available at scale in the U.S.

A nuclear reactor with electrical output under 300 MW (vs. 1,000+ MW for conventional reactors), designed for factory fabrication and site assembly. Key designs include NuScale VOYGR, TerraPower Natrium, and X-energy Xe-100.

Nuclear reactor safety systems that rely on natural physical processes (gravity, convection, thermal radiation) rather than active components like pumps, valves, or human operators. Many SMR designs feature passive safety that prevents meltdown without any external intervention.

The U.S. federal agency responsible for licensing and regulating civilian nuclear power plants, reactors, materials, and waste. The NRC reviews and certifies reactor designs and issues operating licenses.

A donut-shaped (toroidal) device that uses powerful magnetic fields to confine hot plasma for nuclear fusion. The tokamak is the most developed approach to magnetic confinement fusion, used by ITER and SPARC.

The fourth state of matter — a superheated gas where atoms are stripped of their electrons. In fusion reactors, hydrogen isotopes are heated to 100+ million degrees Celsius, forming a plasma that enables fusion reactions.

In nuclear fission, a self-sustaining series of reactions where neutrons released by splitting one atom cause additional atoms to split, releasing more neutrons and energy. Reactors control this process; bombs let it run away.

Fuel assemblies that have been used in a reactor and are no longer efficient for sustaining a chain reaction. Spent fuel is highly radioactive and requires secure storage — initially in water pools, then in dry cask storage.

A power plant that uses natural gas in two stages: first burning it in a gas turbine, then using the waste heat to produce steam for a steam turbine. This two-stage process achieves 55-63% thermal efficiency — the highest of any thermal power plant.

A power plant that operates only during periods of peak electricity demand — typically natural gas simple-cycle turbines that can start up in 5-15 minutes. They run a few hundred hours per year and charge premium electricity rates.

A technique that injects high-pressure fluid into rock formations to create fractures, allowing oil and natural gas to flow to the wellbore. Combined with horizontal drilling, fracking unlocked vast shale oil and gas resources in the U.S.

Natural gas trapped within fine-grained sedimentary rock (shale) formations. Previously considered unrecoverable, shale gas became accessible through hydraulic fracturing and horizontal drilling — the "shale revolution" of the 2000s-2010s.

A drilling technique where the wellbore turns from vertical to horizontal, running laterally through a thin rock formation for thousands of feet. This accesses far more of the resource than a vertical well alone.

Natural gas cooled to -162°C (-260°F), at which point it becomes a liquid occupying 1/600th of its gaseous volume. LNG is shipped in specialized tankers to markets that pipelines cannot reach.

A unit of energy commonly used to price natural gas. One MMBtu equals approximately 1,000 cubic feet of natural gas. U.S. natural gas prices are typically quoted in dollars per MMBtu.

A unit of heat energy equal to 100,000 BTU. Natural gas bills for residential customers are typically measured in therms. One therm is roughly equivalent to 100 cubic feet (1 Ccf) of natural gas.

A measure of power plant efficiency — the amount of fuel energy (in BTU) needed to produce one kilowatt-hour of electricity. Lower heat rates mean higher efficiency. A typical CCGT has a heat rate of about 6,400 BTU/kWh.

Technologies that capture CO2 emissions from power plants or industrial facilities before they reach the atmosphere, then transport and store the CO2 permanently underground in geological formations.

A federal tax credit for capturing and permanently sequestering CO2. The IRA increased the credit to $85/ton for geological storage and $180/ton for direct air capture. Available for projects that capture specified minimum amounts annually.

Technology that captures CO2 directly from ambient air (as opposed to from a smokestack). DAC requires significant energy and currently costs $400-$1,000+ per ton of CO2 removed, but costs are expected to decline with scale.

The primary component of natural gas (CH4). Methane is a potent greenhouse gas — about 80x more warming than CO2 over a 20-year period. Methane leakage from natural gas infrastructure is a significant climate concern.

A structure that impounds water in a reservoir, releasing it through turbines to generate electricity. The energy comes from the gravitational potential of water stored at elevation.

A grid-scale energy storage system using two reservoirs at different elevations. Excess electricity pumps water uphill; when power is needed, water flows downhill through turbines. About 22 GW of capacity exists in the U.S.

A type of hydroelectric facility that generates power from the natural flow and elevation drop of a river, without creating a significant reservoir. Output varies with river conditions.

A pipe or channel that carries water from a dam's reservoir to the turbines below. The penstock converts the water's potential energy (from height) to kinetic energy (from velocity) as it descends.

A structure built alongside a dam that allows fish (especially salmon and other migratory species) to pass upstream to spawning grounds. Fish ladders consist of ascending pools that fish jump between.

A geothermal power system that creates artificial reservoirs by drilling into hot dry rock and fracturing it, then circulating water through the fractures to extract heat. EGS removes the need for natural underground water and permeability.

A geothermal (or waste heat recovery) power plant where hot water from the earth heats a secondary working fluid with a lower boiling point through a heat exchanger. The secondary fluid vaporizes to drive a turbine. The geothermal water never touches the turbine and is reinjected.

Hydrogen produced by splitting water (electrolysis) using renewable electricity. The process produces zero direct greenhouse gas emissions. Currently costs $4-$8/kg vs. $1-$2/kg for conventional (gray) hydrogen.

Hydrogen produced from natural gas via steam methane reforming (SMR) without capturing the resulting CO2 emissions. This is how 95% of hydrogen is produced today, emitting about 10 kg of CO2 per kg of H2.

Hydrogen produced from natural gas with carbon capture and storage (CCS) applied to capture 85-95% of the CO2 emissions. Costs between gray and green hydrogen.

The process of using electricity to split water (H2O) into hydrogen (H2) and oxygen (O2). The main electrolyzer types are PEM (proton exchange membrane), alkaline, and solid oxide (SOEC).

An electrochemical device that converts hydrogen and oxygen directly into electricity, water, and heat — the reverse of electrolysis. Fuel cells are 40-60% electrically efficient with no combustion emissions.

The IRA's Clean Hydrogen Production Tax Credit, providing up to $3.00 per kilogram for hydrogen produced with very low lifecycle CO2 emissions (less than 0.45 kg CO2/kg H2). Lower credits available for less-clean production methods.

An alcohol fuel (C2H5OH) typically produced from corn in the U.S. Blended into nearly all gasoline sold in America at 10% (E10). The U.S. produces about 16 billion gallons per year — the world's largest producer.

A renewable diesel substitute made from vegetable oils (primarily soybean), used cooking oil, or animal fats through a chemical process called transesterification. Used in blends (B5-B20) with petroleum diesel.

Methane captured from organic waste sources (landfills, dairy farms, wastewater treatment) and cleaned to pipeline quality. Chemically equivalent to fossil natural gas, usable in any gas appliance or pipeline.

A gas mixture (primarily methane and CO2) produced by the anaerobic decomposition of organic matter. Sources include landfills, wastewater treatment plants, livestock manure, and food waste digesters.

A federal program (established 2005, expanded 2007) that requires transportation fuel sold in the U.S. to contain specified volumes of renewable fuel. Mandates roughly 20 billion gallons/year of biofuel blending.

Jet fuel produced from renewable or waste sources (biomass, used cooking oil, municipal waste, green hydrogen + CO2) that achieves significant lifecycle CO2 reductions compared to petroleum jet fuel. A drop-in replacement for conventional jet fuel.

The amount of energy stored per unit of weight (gravimetric, Wh/kg) or volume (volumetric, Wh/L). Gasoline has about 12,500 Wh/kg; lithium-ion batteries have about 250-300 Wh/kg — a 40-50x gap.

The average total cost of building, fueling, and operating a power plant per unit of electricity produced over its lifetime, expressed in $/MWh. LCOE enables apples-to-apples comparison across different generation technologies.

Power plants that can be turned on, off, or adjusted in output on demand by grid operators. Natural gas, hydropower, nuclear, geothermal, and batteries are dispatchable. Wind and solar are "variable" (weather-dependent).

The minimum level of electricity demand on the grid, typically met by power plants that run continuously at low cost — traditionally coal and nuclear. The concept is evolving as variable renewables and storage change grid operations.

The sequence in which power plants are dispatched to meet electricity demand, ranked from cheapest to most expensive operating cost. Renewables and nuclear (near-zero fuel cost) dispatch first; expensive peaker plants dispatch last.

The ability of a power plant to restart independently without relying on external electricity from the grid. Hydroelectric dams and some gas turbines have black start capability — essential for recovering from grid-wide blackouts.

The global shift from fossil fuels (coal, oil, natural gas) toward cleaner energy sources (solar, wind, nuclear, storage, efficiency, electrification). Driven by economics, climate policy, technology advancement, and energy security.

The world's largest government-owned emergency petroleum stockpile, stored in salt caverns along the U.S. Gulf Coast. Current capacity: ~714 million barrels; current inventory: ~370-400 million barrels.

The total raw energy consumed by a country before conversion losses — including all fuels for transportation, electricity generation, industrial processes, and heating. The U.S. consumes about 100 quadrillion BTU (quads) of primary energy annually.

A unit of energy equal to one quadrillion (10^15) BTU. Used for national and global energy accounting. U.S. primary energy consumption is approximately 100 quads per year.

The percentage of fuel energy that a thermal power plant converts into electricity. The rest is waste heat. Typical values: coal 33-37%, gas CCGT 55-63%, nuclear 32-34%, gas simple cycle 30-40%.

Earthquakes caused or triggered by human activities, including wastewater injection from oil/gas operations, hydraulic fracturing, geothermal energy production, and reservoir filling. Most are small (below magnitude 3) but some have exceeded magnitude 5.

A traditional unit of heat energy, approximately the energy needed to heat one pound of water by one degree Fahrenheit. One kWh equals about 3,412 BTU. Used to rate furnaces, water heaters, and heating/cooling systems.

The base unit of electric current, measuring how many electrons flow through a conductor per second. Household circuits are typically rated at 15, 20, or 30 amps.

A unit of electrical pressure or potential difference. US residential power is typically 120V (standard outlets) or 240V (large appliances, EV chargers). Solar panels produce 30-50V each.

One million watts, used to measure the capacity of power plants and utility-scale energy installations. A typical US home uses about 1.2 kW on average, so 1 MW can serve roughly 800 homes.

One billion watts or 1,000 megawatts. Used to describe national energy capacity. The US has about 1,300 GW of total installed generating capacity.

One thousand kilowatt-hours, used for wholesale electricity pricing and utility-scale energy production metrics. The average US home uses about 10.5 MWh per year.

The SI unit of energy. One watt-second equals one joule. One kWh equals 3.6 million joules. Rarely used in consumer energy contexts but common in physics and engineering.

A measure of air conditioning and heat pump capacity equal to 12,000 BTU per hour. A typical home needs 2-5 tons of cooling capacity depending on climate and square footage.

A computer simulation of a building that predicts energy consumption based on climate data, insulation levels, HVAC equipment, occupancy patterns, and appliance usage. Tools like EnergyPlus and BEopt are used by auditors.

A phased strategy to replace fossil fuel appliances (gas furnace, water heater, stove, clothes dryer) with high-efficiency electric alternatives (heat pumps, induction), often coordinated with solar and panel upgrades.

A calculation of the total electrical demand of a home or building, accounting for all circuits, appliances, and future additions like EV chargers or heat pumps. Required for panel upgrade decisions.

An assessment of whether a property is suitable for solar panels, considering roof condition, orientation, shading, structural capacity, local regulations, and utility interconnection requirements.

Designing a home energy system to maintain critical loads (refrigeration, medical equipment, lighting, communications) during extended power outages using battery storage, generators, or microgrids.

A timeline for reducing a household or community carbon emissions to net zero, typically by switching to renewable electricity, electrifying heating and transport, and improving building efficiency.

A review of all available electricity rate plans from your utility to find the tariff structure that minimizes your bill given your usage patterns, solar production, and battery storage.

A DOE-developed rating from 1 to 10 that assesses a home energy efficiency based on the building envelope, heating/cooling systems, and hot water system. Similar to a miles-per-gallon rating for vehicles.

A review by a licensed inspector to verify that electrical work meets National Electrical Code (NEC) requirements. Required before utility interconnection of solar systems and after panel upgrades.

Digging a narrow channel to bury electrical conduit, typically for ground-mount solar arrays, detached garage EV charger wiring, or battery storage systems installed away from the main panel.

A hole drilled through the roof to attach solar panel mounting hardware. Properly flashed penetrations use waterproof sealant and metal flashing to prevent leaks for the life of the system.

A solar racking system that uses heavy blocks (concrete or steel) to hold panels in place without penetrating the roof. Common on flat commercial roofs and some residential flat roofs.

A solar array installed on a metal frame structure anchored to the ground rather than a rooftop. Allows optimal tilt and orientation, and avoids roof condition issues.

Solar panels mounted on a covered parking structure, providing both electricity generation and vehicle shade. Popular for homes with poor roof orientation and for commercial properties.

The process of verifying that a newly installed energy system (solar, battery, HVAC) operates correctly and meets design specifications before handoff to the owner.

A secondary electrical panel fed from the main panel, used to distribute circuits to a specific area (garage, workshop, addition). Solar systems and EV chargers are sometimes connected via subpanels.

A program where a local government purchases electricity on behalf of residents, often choosing a greener energy mix than the default utility, while the utility still handles delivery.

A utility rate option that allows customers to pay for electricity sourced from specific renewable energy projects, often with a price premium or discount locked in for 10-20 years.

A loan program where energy efficiency or solar upgrades are repaid through a line item on your monthly utility bill, often with no upfront cost and favorable interest rates.

A community-organized group purchasing program for solar installations, where bulk buying power negotiates lower per-watt prices for all participants. Often facilitated by nonprofits or local governments.

State or utility programs that provide free or heavily subsidized solar installations to qualifying low-income households. Examples include California SOMAH, Illinois Solar for All, and federal programs under the IRA.

A utility program that pays customers to reduce electricity use during peak demand periods, usually through smart thermostat adjustments, water heater cycling, or battery discharge scheduling.

Federal and state programs providing point-of-sale rebates for energy efficiency upgrades (insulation, heat pumps, electrical panels, windows). The IRA authorized up to $14,000 per household through HEEHRA.

Rate structures that replace traditional 1:1 net metering, typically crediting solar exports at a lower rate based on time-of-day, grid value, or avoided cost. Examples include California NEM 3.0.

Two financing models for solar: a lease means a company owns the panels and you pay monthly rent; a loan means you own the panels and make loan payments, keeping the tax credit and all incentives.

A mortgage product that allows borrowers to finance energy improvements into their home loan at the time of purchase or refinance, based on projected energy savings reducing future expenses.

An IRS-approved accounting method to accelerate depreciation of solar systems on commercial or rental properties, allowing owners to deduct a larger portion of the cost in early years.

The IRA provision allowing entities to sell clean energy tax credits to unrelated taxpayers for cash. Enables project developers without sufficient tax liability to monetize credits.

A fixed-income security issued to raise capital specifically for climate and environmental projects, including renewable energy installations, energy efficiency retrofits, and grid modernization.

Additional percentage points on top of the base 30% ITC for projects meeting specific criteria: low-income community (+10-20%), domestic content (+10%), energy community (+10%), under 5 MW (+10%).

A rate structure where electricity prices spike dramatically (5-10x normal) during a few designated critical peak hours per year, typically the hottest summer afternoons. Customers receive advance notice.

A tiered pricing structure where the per-kWh rate increases as usage rises through defined blocks. The first block (baseline) is cheapest; higher tiers penalize heavy consumption.

A rate structure that includes a charge based on your highest 15-minute average power draw (kW) during the billing period, in addition to volumetric kWh charges.

The price at which generators sell electricity to utilities and retailers in regional markets (PJM, ERCOT, CAISO, etc.), typically $20-80/MWh. Not directly paid by residential customers.

The per-kWh credit received for electricity exported from a home solar system to the grid. May equal retail rate (net metering), avoided cost (net billing), or a time-varying amount.

A flat monthly fee on your electricity bill regardless of how much energy you use. Covers utility infrastructure costs. Typically $5-30 for residential customers but rising in many states.

An electricity meter certified to accuracy standards required for financial transactions, such as net metering credits, community solar allocation, or performance-based incentive payments.

A regulatory proceeding where a utility requests permission from the Public Utility Commission to change its rates, rate structure, or cost recovery mechanisms. Typically occurs every 2-4 years.

The historical and projected rate at which electricity prices increase over time. US electricity prices have risen about 2-3% annually over the past 20 years, though this varies by state.

Simple payback counts years until cumulative savings equal cost. Discounted payback adjusts future savings for the time value of money, producing a longer but more accurate payback period.

The total lifetime cost of a battery system divided by total energy it delivers over its life, expressed in $/kWh. Accounts for purchase price, installation, degradation, replacement, and round-trip efficiency losses.

The effective per-kWh value of solar electricity to the homeowner, accounting for self-consumption at retail rates, exports at lower rates, demand charge reduction, and avoided rate escalation.

Combining multiple financial incentives (federal ITC, state rebate, SREC income, utility rebate, property tax exemption) on a single project to maximize total savings.

The cost a utility avoids by not having to generate or purchase electricity because a customer-sited resource (like rooftop solar) provides it instead. Used to set export rates in net billing tariffs.

The total all-in price of a solar system divided by its nameplate DC capacity. US residential solar averages $2.50-3.50/W installed in 2026, down from $7+/W a decade ago.

An estimate of the economic damage from emitting one additional ton of CO2, including impacts on agriculture, health, property, and ecosystems. The EPA estimate is approximately $51 per ton (2020 dollars).

The amount of CO2 emitted per kWh of electricity generated by the local grid, varying by region and time of day based on the generation mix. Ranges from 0.1 lb/kWh (hydro-heavy) to 1.8 lb/kWh (coal-heavy).

The CO2 emissions rate of the next power plant that would turn on (or off) in response to a change in demand. Often higher than the average grid emissions factor because peaker plants are usually gas-fired.

The process of recovering valuable materials (silicon, silver, copper, aluminum, glass) from decommissioned solar panels. The EU mandates solar recycling; US federal regulation is still developing.

The area of land required per unit of energy produced, measured in acres per MW or square meters per kW. Solar farms need about 5-7 acres per MW; wind farms need 30-60 acres per MW (but most land remains usable).

A credit representing one metric ton of CO2 equivalent avoided or removed from the atmosphere, purchased to compensate for emissions elsewhere. Quality varies widely by project type and verification standard.

A safety device that detects dangerous electrical arcs (sparks) in wiring and disconnects the circuit. Required by NEC in most living spaces and now in solar systems to prevent rooftop fires.

An algorithm in solar inverters and charge controllers that constantly adjusts voltage and current extraction to keep panels operating at their maximum power output as conditions change.

The configuration of how solar panels are wired in series and parallel to match inverter voltage and current requirements. Proper string design maximizes production and prevents equipment damage.

The watts generated per unit area of a solar panel, calculated as panel wattage divided by area. Higher efficiency panels have greater power density, meaning fewer panels for the same output.

A small electronic component inside solar panels that allows current to flow around shaded or damaged cells, preventing them from becoming resistive loads that heat up and reduce output.

Connecting electrical equipment to the earth to provide a safe path for fault current, preventing electric shock and allowing protective devices to operate correctly.

An electrical device on utility poles or in ground-level boxes that steps voltage down from distribution level (4-35 kV) to residential service (120/240V). Solar export capacity may be limited by local transformer ratings.

Electricity consumed by electronic devices when they are turned off but still plugged in (standby mode). The average US home wastes 5-10% of electricity on phantom loads — about $100/year.

Electronic circuits that convert and control electrical power, including inverters, charge controllers, power optimizers, and EV charging circuits. The technology enabling the clean energy transition.

A utility meter that records both electricity consumed from the grid and electricity exported to the grid, required for net metering and net billing customers with solar or battery systems.

The additional energy produced by bifacial solar panels from light reflected onto their rear surface. Typical bifacial gain is 5-15% depending on ground reflectivity (albedo), mounting height, and tilt angle.

The operating temperature of solar cells under sunlight, typically 20-40 degrees Celsius above ambient temperature. Higher cell temperatures reduce voltage and power output.

Automated systems in grid-tied inverters that detect when the utility grid has lost power and immediately disconnect the solar system to prevent energizing downed power lines.

The percentage of time a power system (solar, battery, generator) is operational and available to produce energy. Well-maintained residential solar achieves 98-99% availability over its lifetime.

The annual energy production per kilowatt of installed solar capacity, measuring how effectively a system converts nameplate capacity into actual production. US values range from 1,000-1,800 kWh/kWp depending on location.

The ratio of actual energy produced by a solar system to its theoretical maximum if operating at nameplate power for all daylight hours. Accounts for real-world losses from temperature, shading, soiling, and inverter efficiency.

The gradual decline in solar panel output over time, typically 0.3-0.5% per year for modern panels. After 25 years, panels still produce 87-92% of their original rated power.

How efficiently a solar cell converts light at different wavelengths. Monocrystalline cells respond best to near-infrared; thin-film and perovskite cells capture a broader spectrum.

The total lifespan of a battery regardless of usage, determined by chemical aging. Even an unused lithium-ion battery degrades over time, especially at high temperatures or extreme states of charge.

State-level rules governing how solar and battery systems connect to the utility grid, including application requirements, timelines, fees, insurance, and technical standards (IEEE 1547).

Minimum energy efficiency standards for new construction and major renovations, set by states/cities based on IECC or ASHRAE 90.1. Some states now require solar-ready or solar-equipped new homes.

State laws that prevent HOAs, municipalities, or other entities from prohibiting or unreasonably restricting residential solar installations. Over 30 states have some form of solar access law.

A state or federal policy requiring utilities to source an increasing percentage of electricity from clean energy sources (not just renewables — may include nuclear, CCS). More inclusive than an RPS.

A fee levied on fossil fuel producers or users based on the CO2 emissions of their fuel. Sets a direct price signal without capping total emissions. Not currently implemented at US federal level.

An IRA-defined geographic area with brownfields, retired coal mines/plants, or high fossil fuel employment, qualifying clean energy projects built there for a 10% bonus tax credit.

An IRA provision granting a 10% ITC bonus for solar projects using panels and components manufactured or assembled in the United States, as defined by percentage of cost thresholds.

An integrated home battery product combining battery cells, inverter, charge controller, and battery management system in a single enclosure. Examples: Tesla Powerwall, Enphase IQ Battery, SonnenCore.

A ventilation system that exchanges stale indoor air for fresh outdoor air while recovering 70-80% of the energy (heat and moisture) from the outgoing air stream. Essential for tight, efficient homes.

A water heater that uses a heat pump to extract warmth from surrounding air rather than burning gas or using resistive electric elements. Uses 2-3x less energy than a standard electric water heater.

A Wi-Fi-connected thermostat that learns occupancy patterns, optimizes temperature schedules, responds to demand response signals, and can be controlled remotely. Saves 10-15% on heating/cooling.

A large fan installed in the ceiling/attic that pulls cool evening air through open windows and exhausts hot air from the attic. Can reduce air conditioning use by 50-90% in dry climates.

A type of heat pump consisting of an outdoor condenser and one or more indoor air handlers connected by refrigerant lines rather than ductwork. Each indoor unit independently controls one zone.

A roofing product that integrates photovoltaic cells into shingle-shaped modules, replacing conventional roofing while generating electricity. Examples: Tesla Solar Roof, GAF Energy Timberline Solar.

A rechargeable lithium-ion battery pack with built-in inverter and outlets, ranging from 500Wh to 3,000Wh+. Used for emergency backup, camping, and powering small loads during outages.

A vehicle propelled entirely by one or more electric motors, powered by a rechargeable battery pack. Includes battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs).

Materials in a building (concrete, brick, tile, water) that absorb and store heat, moderating temperature swings. Homes with high thermal mass stay cooler in summer days and retain warmth longer at night.

A standby or portable engine-driven generator that produces electricity during power outages, typically fueled by natural gas, propane, or gasoline. Rated in watts (e.g. 7,500W portable, 22,000W whole-home).

Light-emitting diode bulbs that use 75% less energy and last 25 times longer than incandescent bulbs. A full home LED conversion saves $100-200/year in electricity.

A thin polymer film applied to window glass that blocks infrared heat while allowing visible light. Can reduce solar heat gain by 40-70%, lowering cooling costs without darkening rooms.

An integrated installation combining solar panels with battery storage, allowing self-consumption of solar energy, time-of-use arbitrage, and backup power during outages.

Software that coordinates solar production, battery storage, EV charging, smart appliances, and grid signals to minimize energy costs and maximize self-consumption automatically.

The practice of co-locating agricultural activity (crops, grazing, pollinator habitat) with solar panel installations. Panels provide shade and reduce water evaporation; agriculture maintains land productivity.

A centralized system that controls and monitors a building mechanical, electrical, lighting, and security systems. In residences, smart home platforms (Home Assistant, Google Home) serve this role.

A community-scale thermal energy network that distributes heating or cooling from a central plant to multiple buildings through underground pipes, achieving higher efficiency than individual building systems.

A system where neighbors can buy and sell excess solar electricity directly to each other using blockchain or utility-mediated platforms, bypassing traditional wholesale markets.

A systematic review of past electricity bills to identify billing errors, suboptimal rate plans, unexpected usage spikes, and opportunities for savings through rate switching or demand reduction.

The steps required to obtain local government approval for a solar or battery installation, typically including plan submittal, structural review, electrical review, and zoning verification.

An annual settlement date when your utility calculates whether you have a net credit or balance from 12 months of net metering. Excess credits may roll over, be paid out at avoided cost, or expire.

The practice of obtaining 3+ quotes from different solar installers and comparing them on price, equipment quality, warranty terms, and company reputation before making a decision.

Real-time tracking of solar production, battery status, and energy consumption using manufacturer apps (Enphase, SolarEdge, Tesla) or third-party platforms (Solar Analytics, PVOutput).

The steps to file a manufacturer warranty claim for defective solar panels, inverters, or batteries: document the issue, contact manufacturer, obtain RMA number, arrange replacement, and handle reinstallation labor.

A dollar-for-dollar reduction in federal income taxes equal to 30% of the total installed cost of a residential solar system (through 2032). Steps down to 26% in 2033 and 22% in 2034.

The IRA extended the 30% ITC to standalone battery storage systems installed after January 1, 2023. Previously, batteries only qualified if installed with solar. Capacity must be 3 kWh or more.

A cash payment from your electric utility for installing qualifying energy equipment (solar, battery, heat pump, smart thermostat). Amounts and availability vary widely by utility.

A bill credit received for solar electricity exported to the grid under a net metering policy. At full retail net metering, each exported kWh earns a credit equal to the retail electricity rate.

A state law exempting the purchase of solar energy equipment from state and local sales tax. Over 25 states provide full or partial solar sales tax exemptions.

A solar cell architecture using phosphorus-doped silicon (negative charge carriers). N-type cells (TOPCon, HJT) offer higher efficiency, lower degradation, and better shade tolerance than traditional P-type PERC cells.

A battery storage system with a built-in inverter that connects to the home AC electrical panel. Can be added to any existing solar system without modifying the solar inverter.

The ability to shift or reduce electricity consumption in response to grid conditions, price signals, or renewable energy availability. Enabled by smart appliances, batteries, EV chargers, and water heaters.

The transition from fossil fuel-powered systems to electric alternatives: gas furnaces to heat pumps, gas stoves to induction, gasoline cars to EVs. A key strategy for reducing building carbon emissions.

Improving a building envelope through air sealing, insulation, window upgrades, and duct sealing to reduce energy waste. The most cost-effective first step before any clean energy investment.

Upgrading the electrical grid with smart meters, sensors, automated switches, and two-way communication to handle distributed energy resources, reduce outages, and enable dynamic pricing.

A state policy requiring utilities to procure or incentivize a specified amount of energy storage capacity. California, New York, New Jersey, and Oregon have storage mandates.

Switching from fossil fuels to electricity specifically where doing so reduces total energy use, total emissions, or total cost compared to the fossil-fuel alternative.

A condition where households spend more than 6-10% of income on energy bills (vs the national average of ~3%). Disproportionately affects low-income, elderly, and minority communities.

Techniques to reduce or reshape a building electricity demand profile: scheduling high-draw appliances during off-peak hours, pre-cooling/heating, and using battery storage to flatten demand peaks.

A DOE certification for homes so energy-efficient that a standard solar system can offset all or nearly all annual energy consumption. Requires high-performance envelope, HVAC, lighting, and appliances.

A new home built with pre-run conduit from the electrical panel to the roof, a panel with spare breaker capacity, southern roof area free of penetrations, and structural capacity for panels.

A lithium battery chemistry (LiFePO4) known for safety, longevity, and thermal stability. Used in most residential battery systems. Lower energy density than NMC but 5,000-10,000 cycle rating.

An HVAC configuration pairing an electric heat pump with a gas furnace, automatically switching to gas when outdoor temperatures drop below the heat pump economic crossover point.

When solar panels produce more DC power than the inverter can convert to AC, the excess is clipped (wasted). Intentional with high DC/AC ratios to maximize morning/evening production at the cost of midday losses.

When a grid operator instructs generators (including rooftop solar) to reduce output because supply exceeds demand. Increasing in regions with high solar penetration during spring midday hours.

A private electrical distribution network within a building complex (apartments, shopping centers) where a central buyer purchases wholesale electricity and resells it to occupants at a negotiated rate.

Disconnecting completely from the utility grid and meeting all energy needs with self-owned generation (solar, battery, generator). Currently rare and expensive but declining in cost.

A one-time charge by the utility to review and approve the connection of a customer-sited generation system (solar, battery) to the grid. Fees range from $0 to $1,000+ depending on system size and state.

The process of safely removing a solar or battery system at end of life: disconnecting from grid, removing panels and racking, filling roof penetrations, recycling or disposing of components.

The cooling output of an air conditioner divided by its electrical input at a specific outdoor temperature (usually 95 degrees F). Higher EER means more efficient cooling. Being replaced by SEER2.

Inflated savings projections from solar installers that assume unrealistic energy escalation rates (5-7% vs actual 2-3%), cherry-picked production estimates, or ignore degradation and maintenance costs.

A factor accounting for climate, latitude, altitude, and local weather patterns that adjusts national average solar production estimates to your specific location. Ranges from 0.6 (Seattle) to 1.3 (Phoenix).

A utility-owned or community-owned battery storage system serving multiple homes, typically installed on distribution transformers. Provides shared backup and grid services without individual home batteries.

A market failure where the person who pays for an energy upgrade (landlord) is not the person who benefits from lower energy bills (tenant), or vice versa, discouraging investment.

Using energy storage or backup generation to guarantee that a renewable energy source can deliver a consistent, dispatchable power output regardless of weather or time of day.

Varying electricity transmission charges based on grid congestion, where high-demand pathways cost more. Locational marginal pricing (LMP) reflects congestion in wholesale markets.

The variation in solar output across seasons due to changing sun angle, day length, and weather patterns. US systems typically produce 20-40% of annual energy in summer vs 10-15% in winter.

Software-based allocation of solar production across multiple meters or units within a building, used for community solar and multi-family properties where physical net metering is impractical.

State legislation protecting homeowners right to install and operate solar energy systems by limiting restrictions from HOAs, local governments, and neighbors regarding aesthetics, placement, and shade.

A professional certification from the North American Board of Certified Energy Practitioners validating that a solar installer has met education, experience, and examination requirements.

An inverter that manages both solar panels and battery storage in a single unit, handling DC-to-AC conversion for solar, battery charging/discharging, and grid interaction.

An electronic device that regulates the voltage and current from solar panels to a battery bank, preventing overcharging and deep discharge. Essential for off-grid and battery-based systems.

An electrical enclosure where multiple solar panel strings are combined into a single higher-current circuit before feeding into the inverter. Contains fuses or breakers for each string.

Equipment that quickly reduces solar panel voltage to safe levels (under 80V within 30 seconds) when the system is shut off, as required by NEC 2017+ section 690.12.

An electrical device that switches a home between utility grid power and backup power (generator or battery). Prevents backfeed that could endanger utility workers.

An electrical panel with built-in monitoring, circuit-level control, and automated load management. Examples include Span, Lumin, and Schneider Square D Energy Center.

An electric cooking surface that uses magnetic fields to heat cookware directly rather than heating a burner element. Boils water 50% faster than gas, with 90% energy efficiency vs 40% for gas.

A ventilation fan installed in the roof or gable that is powered by a small integrated solar panel, reducing attic temperatures by 20-40 degrees F and lowering cooling costs.

A safety system that detects current leaking to ground through unintended paths (damaged insulation, water intrusion) and disconnects the circuit to prevent shock or fire.

Calculating the number of solar panels to wire in series per string to match the inverter voltage window, accounting for temperature effects on panel voltage throughout the year.

The ratio of total solar panel DC nameplate capacity to inverter AC output capacity. A typical residential system has a DC/AC ratio of 1.1-1.3, meaning more panel watts than inverter watts.

The aluminum or steel mounting structure that attaches solar panels to roofs, ground frames, or pole mounts. Racking must withstand local wind and snow loads per building codes.

A device that connects solar inverters and batteries to the internet, enabling remote monitoring via manufacturer apps. Most modern inverters include built-in Wi-Fi; older systems need a separate gateway.

A solar cell manufacturing technique that creates heavily doped regions under metal contacts and lightly doped regions elsewhere, improving both conductivity and light absorption.

A thin layer applied to solar cell surfaces (typically silicon nitride) that reduces light reflection from 30-35% to under 3%, allowing more photons to enter the cell.

The rear protective layer of a solar panel, typically white or black polymer film (Tedlar, TPT). Provides electrical insulation, moisture protection, and UV resistance.

An enclosure on the back of a solar panel containing bypass diodes and electrical connections where the panel output cables originate.

Energy production lost due to dirt, dust, pollen, bird droppings, and other debris accumulating on solar panel surfaces. Typically 2-5% annually, but up to 25% in dusty or agricultural areas.

Energy lost when panels in a string have different output levels due to manufacturing variation, uneven shading, soiling, or degradation. The weakest panel limits the entire string.

Energy lost as heat in the electrical cables connecting solar panels to the inverter and from the inverter to the electrical panel. Typically 1-3% depending on wire gauge and run length.

Reduction in solar output caused by shadows from trees, chimneys, neighboring buildings, or other obstructions falling on panels. Even partial shading of one cell can disproportionately reduce string output.

The reflectivity of the ground surface beneath or around solar panels, expressed as a fraction from 0 (absorbs all light) to 1 (reflects all light). Snow has high albedo (0.8); grass is low (0.2).

A graph showing inverter conversion efficiency at different power loading levels. Most inverters peak at 97-99% efficiency around 30-50% of rated capacity, dropping at very low and very high loads.

A solar + battery configuration where the battery has its own inverter separate from the solar inverter. Solar DC is converted to AC, then back to DC to charge the battery.

A solar + battery configuration where solar panels charge the battery directly through a shared hybrid inverter or charge controller, without converting to AC first.

Technology allowing an electric vehicle battery to power a home during outages or peak pricing periods. The EV acts as a mobile battery, typically providing 40-100 kWh of storage.

Buying electricity when it is cheap (off-peak or midday solar surplus) and using or selling it when prices are high (peak evening hours). Battery storage enables residential energy arbitrage.

A consumer who also produces energy, typically through rooftop solar. Prosumers both consume grid electricity and export surplus generation, blurring the line between consumer and producer.

Non-hardware expenses in a solar installation: permitting, inspection, interconnection application, sales commission, design, overhead, and profit margin. Soft costs are 50-65% of total US residential solar cost.

The ratio of energy delivered by a system over its lifetime to the energy required to build, install, and maintain it. Solar panels have an EROI of 10-25, meaning they generate 10-25x the energy used to make them.

The interest rate used to calculate the present value of future cash flows. In solar economics, a higher discount rate reduces the calculated value of long-term savings, making projects look less attractive.

The complete lifetime cost of an energy system including purchase, installation, maintenance, degradation, inverter replacement, and eventual decommissioning over a 25-30 year period.

A state mandate requiring utilities to generate or purchase a specified percentage of electricity from renewable sources by a target date. 30+ states and DC have an RPS.

A state regulatory body that oversees electric, gas, and water utilities, approving rate changes, service quality standards, and policies like net metering. Also called PSC in some states.

The restructuring of electricity markets to separate generation from distribution, allowing consumers to choose their electricity supplier while the utility handles delivery.

The regulatory framework governing how solar customers are credited for electricity they export to the grid. Policies vary dramatically by state, ranging from full retail credit to avoided cost only.

An on-site evaluation of a property for solar installation, examining roof condition, orientation, tilt, shading, structural capacity, electrical panel, and utility interconnection requirements.

The formal request submitted to your utility for permission to connect a solar or battery system to the grid. Includes system specifications, single-line diagrams, and equipment datasheets.

A contractual commitment from a solar installer guaranteeing minimum annual energy production, with financial compensation if the system consistently underperforms.

A tradeable certificate representing 1 MWh of solar electricity generation. In states with SREC markets (NJ, MA, DC, MD, PA, OH, IL), solar owners earn SRECs they can sell for additional income.

The Modified Accelerated Cost Recovery System allows commercial and rental property solar owners to depreciate the system cost over 5 years, with bonus depreciation enabling 80-100% first-year write-off.

A state law that excludes the added value of a solar installation from property tax assessments. Over 35 states provide full or partial solar property tax exemptions.

A federal tax credit of up to $7,500 for new qualifying electric vehicles purchased in 2023-2032 under IRA section 30D. Income and MSRP caps apply. Assembly and battery sourcing rules restrict eligibility.

A federal tax credit of up to $4,000 (or 30% of sale price, whichever is less) for qualifying used electric vehicles purchased from a dealer. Income cap: $75,000 AGI single / $150,000 joint.

A federal tax credit of up to $2,000 per year for qualifying heat pump installations (air source and geothermal) under IRA section 25C. Covers the cost of the heat pump plus installation labor.

The total greenhouse gas emissions produced during the manufacturing, transportation, installation, and end-of-life processing of a product. Solar panels have embodied carbon of 20-50 g CO2/kWh over their lifetime.

The volume of water consumed per unit of electricity generated. Coal and nuclear use 400-700 gallons per MWh for cooling; solar PV uses near-zero water during operation.

Unintended releases of greenhouse gases during extraction, processing, and distribution of fossil fuels. Methane leaks from natural gas systems are a major source of fugitive emissions.

A personal or secured loan specifically for financing a solar installation. Terms typically range from 10-25 years with interest rates of 3-8%. The homeowner owns the system and keeps all incentives.

An agreement where a company installs and owns solar panels on your roof, and you pay a fixed monthly lease payment for the electricity they produce. Lease terms are typically 20-25 years.

A public or quasi-public financial institution that uses limited public funds to leverage private investment in clean energy projects through loans, credit enhancements, and co-investment.

If your federal tax liability is less than your solar tax credit amount, you can carry the unused portion forward to future tax years. There is no limit on how many years you can carry forward.

The amount of electricity usage at the lowest tier rate under an inclining block rate structure, designed to cover basic needs at an affordable price. Set by the utility based on climate zone and season.

The portion of your electricity bill covering the cost of transmitting and distributing power from generators to your home. Includes transmission, distribution, and infrastructure fees.

An electricity rate that changes hourly based on wholesale market conditions, reflecting the actual cost of generating power at each moment. Prices can spike during extreme demand events.

The ACCA-standard method for calculating heating and cooling loads of a building based on climate, insulation, windows, orientation, occupancy, and infiltration. Required for proper HVAC sizing.

A separate electrical sub-panel containing only the circuits you want powered during an outage (refrigerator, lights, internet, medical equipment). Simplifies battery system sizing and cost.

A planned allocation of energy consumption across building systems (heating, cooling, lighting, appliances, hot water) designed to meet a specific performance target like net zero or ENERGY STAR.

An evaluation of a roof condition, age, material, orientation, and structural capacity to determine suitability for solar installation and remaining useful life.

The Electric Reliability Council of Texas, the grid operator managing 90% of Texas electric load. ERCOT operates an isolated grid not connected to the Eastern or Western interconnections.

A graph of net electricity demand (total demand minus solar production) that dips in midday when solar produces heavily, then ramps steeply in the evening as solar drops and demand rises.

A power plant that operates only during periods of peak electricity demand, typically natural gas turbines that can start up quickly but have higher per-kWh costs and emissions.

The sequence in which power plants are dispatched to meet demand, ranked from lowest to highest marginal cost. Renewables (near-zero marginal cost) dispatch first; gas peakers dispatch last.

The US standard (NFPA 70) governing safe electrical installation practices, updated every three years. NEC Article 690 specifically covers solar PV systems; Article 706 covers energy storage.

The national standard for interconnecting distributed energy resources (solar, battery, generators) with the electric grid, specifying voltage regulation, frequency response, and islanding detection requirements.

A certification from Underwriters Laboratories indicating that a product has been tested and meets safety standards. Solar panels need UL 61730; inverters need UL 1741; batteries need UL 9540.

The California Energy Commission weighted efficiency rating for solar inverters, averaging efficiency across six power levels weighted for typical operating conditions. More realistic than peak efficiency claims.

The laboratory conditions under which solar panels are rated: 1,000 W/m2 irradiance, 25 degrees C cell temperature, AM1.5 spectrum. Real-world conditions rarely match STC exactly.

PVUSA Test Conditions rating — a more realistic solar panel power rating using 1,000 W/m2 irradiance but 45 degrees C cell temperature (vs 25 degrees C for STC). PTC is typically 88-92% of STC.

Solar energy production estimates that account for the gradual decline in panel output over time, typically modeled at 0.3-0.5% per year from the first-year baseline.

The ratio of actual energy produced over a period to the theoretical maximum if the system operated at full rated capacity continuously. US residential solar has a 15-25% capacity factor depending on location.

A review by a local building inspector verifying that a solar or electrical installation meets code requirements. Typically required before the utility grants permission to operate.

The historical and projected annual increase in electricity prices. US residential rates have averaged 2-4% annual increases over the past 20 years.

The portion of electrical power that oscillates between source and load without doing useful work, measured in volt-amperes reactive (VAR). Required to maintain voltage levels on AC power systems.

The percentage of household income spent on energy bills. Low-income households often face energy burdens of 8-20%, compared to 3% for median-income households.

The systematic process of verifying that all components of a solar or energy system are installed correctly, configured properly, and performing to specification before handoff to the owner.

When solar panel output exceeds the inverter maximum AC capacity, the inverter limits (clips) the output to its rated power. Energy above the clip threshold is lost.

The minimum clear space required between solar panels and roof edges, ridges, valleys, and fire access pathways, as mandated by local fire codes and the International Fire Code (IFC).

The cost a utility avoids by not having to generate, transmit, or distribute electricity because a customer solar system is producing power instead. Often used as the basis for solar export compensation.