Kilowatt-Peak (kWp)
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.
Why It Matters
kWp is the system size you see in quotes (e.g., "8 kW system"). Real-world output is always lower than kWp due to temperature, angles, and shading — typically 75-85% of nameplate.
Related Terms
Watt (W)
A unit of power equal to one joule per second. Solar panels are rated in watts (W) or kilowatts (kW = 1,000W).
Specific Power (kWh/kWp)
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.
Performance Ratio (PR)
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%.