Home Battery Backup Calculator
Estimate how many hours a home battery will cover a chosen average load during an outage. AC-rated usable energy is used as-is; DC battery energy is reduced by inverter loss.
Results
Backup runtime: 27 h
That's about 1.13 days
AC energy delivered to the house: 13.5 kWh
Same battery on the other load tiers
- Essentials — fridge, lights, router (~350 W): 38.6 h
- Essentials + electronics (~600 W): 22.5 h
- Half-house / light HVAC (~1,000 W): 13.5 h
- Whole-house average (~1,250 W): 10.8 h
How it works
Hours are AC-side energy divided by the average watts you leave on. An AC-rated usable figure is already at the house. A DC battery figure is multiplied by 0.90 first — the same default inverter efficiency as the battery-bank and inverter-size tools.
hours = usable_kwh × 1000 × inverter_factor / avg_load_w
inverter_factor = 1.00 if energy_basis is ac, 0.90 if dc
days = hours / 24
usable_kwh- Usable kilowatt-hours left in the battery. Do not type a nameplate figure that includes reserved or unusable capacity.
energy_basis- ac if the kWh figure is already at the inverter output; dc if it is battery or cell energy.
inverter_factor- inverter_factor is 1.00 for AC-rated energy, 0.90 for DC battery energy. Not a free-form input — pick the energy rating.
avg_load_w- Average AC watts during the outage. A fridge that cycles is much less than its 725 W running figure.
hours- usable_kwh × 1,000 × inverter_factor ÷ avg_load_w.
days- hours ÷ 24.
Assumptions
- The 13.5 kWh default is Tesla Powerwall 3 nominal battery energy, stated as 13.5 kWh AC on the official datasheet (25°C, beginning of life). Type the usable figure from your spec sheet; a 10 kWh nameplate lead-acid bank is not 10 kWh usable.
- Inverter efficiency is 90% (C1 / C7 default) and is applied only when the capacity is a DC battery figure. Powerwall 3 and other hybrid units that publish kWh AC already include that conversion — applying 0.90 again would understate runtime by 10%.
- Average load is not peak load. Whether a fridge compressor or a well pump will start is an inverter surge question — use the inverter-size tool.
- Essentials 350 W = DOE fridge 725 W × 8/24 h plus a 100 W lights/router/phone allowance. 600 W and 1,000 W add electronics and a cycling HVAC slice. 1,250 W is EIA's 10,791 kWh/year U.S. household figure ÷ 8,760 h, rounded.
- A window air conditioner or space heater on the backup panel can jump the average into the kilowatts. The result is a constant-load estimate, not a minute-by-minute house model.
FAQ
How long will a Powerwall run a refrigerator?
A cycling fridge at the DOE hint is about 242 W average. Powerwall 3's 13.5 kWh is already AC, so about 56 hours — just the fridge. Add lights, a router, and phones (the 350 W essentials preset) and the same battery is about 39 hours.
Can a home battery run my AC during an outage?
A window unit that averages 1,000 W on the half-house preset drains 13.5 kWh AC in 13.5 hours. A central system is often several kilowatts while the compressor is on and will empty a single Powerwall-class battery in a few hours. Size the inverter for surge separately.
What loads should I put on the backup panel?
Fridge, a few lights, the router, phone chargers, and medical devices first. Leave the dryer, oven, and most HVAC off the backup panel unless you have the kWh and the inverter surge to match. The essentials preset is that short list averaged.
Sources
- Tesla Powerwall 3 datasheet — Nominal Battery Energy 13.5 kWh AC (25°C, beginning of life, 3.3 kW charge/discharge; captured 2026-09)
- EIA FAQ — How much electricity does an American home use? (10,791 kWh/year, 2022 residential customer)
- U.S. DOE Energy Saver — Estimating appliance and home electronic energy use (fridge running watts and cycling hint)
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