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title: Home Battery Sizing Guide description: How to determine the right battery capacity for your home — from essential backup to whole-home coverage and TOU optimization. summary: How to determine the right battery capacity for your home — from essential backup to whole-home coverage and TOU optimization. category: battery difficulty: Intermediate updated: 2026-02-09 tags: ["battery", "sizing", "backup", "storage", "capacity"] relatedTools: ["/tools/battery-runtime", "/tools/solar-sizing"] faqs:

• question: How many kWh of battery do I need? answer: "It depends on your goal. For essential backup (fridge, lights, WiFi, phones) during an 8-12 hour outage, 10-13 kWh is sufficient. For comfortable backup including some AC, 20-26 kWh. For whole-home backup during a multi-day outage, 30-40+ kWh. For TOU optimization without backup concerns, 10-15 kWh covers most evening peak usage."
• question: How many Tesla Powerwalls do I need? answer: One Powerwall 3 (13.5 kWh) handles essential backup for 8-12 hours. Two Powerwalls (27 kWh) provide comfortable backup with some AC for a full day. Three (40.5 kWh) approach whole-home backup for extended outages. For TOU optimization, one Powerwall is typically sufficient for an average home.
• question: Can a home battery run my air conditioning? answer: "Yes, but AC is the biggest energy consumer in most homes. A central AC unit draws 3-5 kW, meaning a 13.5 kWh battery could run it for only 3-4 hours. For extended backup with AC, you need 26+ kWh of battery capacity — or use a smaller window unit (0.5-1.5 kW) which a single battery can sustain much longer."
• question: Should I get a battery if I have solar? answer: "It depends on your state's net metering policy and your goals. If your state has strong net metering (full retail credit for exports), a battery's financial benefit is mainly TOU arbitrage and backup power. If your state has weak net metering (like California NEM 3.0), a battery is almost essential to capture full value from your solar system."
• question: How long will a battery power my home during an outage? answer: "A rough formula: runtime (hours) = usable battery capacity (kWh) ÷ average load (kW). If you have a 13.5 kWh battery and run only essentials at 1.5 kW, that's about 9 hours. If solar is also producing, the battery recharges during the day, potentially providing indefinite power for modest loads in sunny weather."

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Home Battery Sizing Guide

Getting the right battery size means matching capacity to your actual needs — not oversizing (wasting money) or undersizing (inadequate backup). This guide walks through a practical sizing approach.

Step 1: Define Your Goal

Battery sizing starts with why you want a battery:

| Goal | Typical Size Needed | |------|-------------------| | Essential backup (8-12 hours) | 10–13 kWh | | Comfortable backup (12-24 hours) | 20–26 kWh | | Whole-home backup (24-48+ hours) | 30–40+ kWh | | TOU rate optimization | 10–15 kWh | | Solar self-consumption | 10–15 kWh | | Off-grid living | 40–80+ kWh |

Most homeowners fall into the essential backup + TOU optimization category, which 10–15 kWh handles well.

Step 2: Calculate Your Load

What You're Powering Matters More Than Your Total Bill

Your average home usage (say, 30 kWh/day) isn't what you need to size for during backup. What matters is the specific loads you want to keep running:

| Appliance | Power Draw (watts) | Daily Energy (kWh) | |-----------|-------------------|-------------------| | Refrigerator | 100–200 | 1.5–3.0 | | LED lights (10 bulbs) | 100 | 1.0–2.0 | | WiFi router + modem | 20 | 0.5 | | Phone/laptop charging | 50–100 | 0.5–1.0 | | TV | 80–150 | 0.5–1.5 | | Ceiling fans (3) | 150–225 | 2.0–4.0 | | Window AC unit | 500–1,500 | 4.0–12.0 | | Central AC | 3,000–5,000 | 15.0–30.0 | | Electric stove (per hour) | 2,000–5,000 | 2.0–5.0 | | Clothes dryer (per load) | 3,000–5,000 | 3.0–5.0 | | Well pump | 750–1,500 | 1.0–3.0 | | Garage door opener | 500 | 0.1 | | Medical equipment (CPAP) | 30–60 | 0.3–0.5 |

Essential Loads (Tier 1)

Fridge + lights + WiFi + phones ≈ 1.0–1.5 kW average draw, or about 5–7 kWh per 12-hour outage. A single 13.5 kWh battery handles this easily.

Comfortable Loads (Tier 2)

Add TV, fans, and a window AC ≈ 2.0–3.0 kW average, or about 12–18 kWh per 12 hours. Two battery units or a 20+ kWh system.

Whole-Home (Tier 3)

Central AC, cooking, laundry ≈ 5.0–8.0 kW average, or about 30–48 kWh per 12 hours. Three or more battery units.

Step 3: Factor in Solar Recharging

If you have solar panels, your battery recharges during the day during an outage. This dramatically extends runtime:

Example:

• 13.5 kWh battery, 8 kW solar array
• Essential loads: 1.5 kW average
• Sunny day: solar produces 35–40 kWh

The solar panels produce far more than essentials need. The battery absorbs excess during the day and covers the nighttime. In good weather, this setup can provide indefinite essential backup.

In cloudy/rainy weather, expect ~25% of normal solar production — still enough to partially recharge and extend runtime significantly.

Step 4: Consider Power (kW) Not Just Energy (kWh)

Batteries have two ratings:

• Energy capacity (kWh): How much total energy is stored
• Power output (kW): How much power can be delivered at once

If your peak demand is 7 kW (AC compressor starting + fridge + lights), but your battery can only output 5 kW, the system will shut down or shed loads.

| Battery | Energy (kWh) | Continuous Power (kW) | Peak Power (kW) | |---------|-------------|----------------------|-----------------| | Tesla Powerwall 3 | 13.5 | 11.5 | 12.0 | | Enphase IQ 5P | 5.0 | 3.84 | 7.68 | | Franklin WH aPower2 | 15.0 | 10.0 | 12.0 | | Generac PWRcell | 9–18 | 4.5–9.0 | 7.6 |

For homes that need to run a central AC unit (3–5 kW startup surge), make sure the battery's peak power rating can handle it.

Step 5: Stack If Needed

Most battery systems are modular — you can install multiple units:

| Units | Total kWh | Total kW | Best For | |-------|----------|----------|----------| | 1 | 10–15 | 5–11 | Essential backup + TOU | | 2 | 20–30 | 10–22 | Comfortable backup + AC | | 3+ | 30–45+ | 15–33+ | Whole-home / off-grid |

Quick Sizing Formula

For a rough estimate:

Battery kWh = Average load (kW) × Hours of backup needed

Then add a 20% buffer for inefficiency (batteries lose ~8-10% in the charge/discharge cycle, and loads are never perfectly smooth).

Example: Want 12 hours of backup at 2 kW average draw:

2 kW × 12 hours × 1.2 (buffer) = 28.8 kWh

Two battery units (27 kWh) would be the right ballpark.