Energy Calculators

Home battery backup sizing

Estimate the battery capacity you need to keep essential loads running during an outage.

Running a refrigerator, router, LED lights, device charging and a furnace fan (example loads below) for one day uses about 5.3 kWh, which needs roughly 6.6 kWh of battery capacity at 90% usable capacity and 90% inverter efficiency.

Essential loads
LoadWattsHours/day

For appliances that cycle (fridges, furnace fans), enter running hours, not hours plugged in. Example loads are editorial; replace them with yours.

days
%
%
kWh
Battery capacity needed
6.6 kWh
Battery units
1of 13.5 kWh
Energy used
5.3 kWh5.34 kWh/day
Peak power if all run at once
0.79 kW

Formula: energy = Σ (watts × hours per day) ÷ 1,000 × days; capacity = energy ÷ (usable share × inverter efficiency); peak power = Σ watts. Motors (fridges, pumps, fans) draw extra power for a moment when starting; check the battery’s surge rating.

Inputs used: loads: 5; days: 1; usable: 90%; inverter: 90%.

These are estimates, not financial advice.

Worked examples

Scenario (example loads)EnergyCapacity neededPeak power
Example essentials (list above), 1 day5.3 kWh6.6 kWh0.79 kW
Example essentials, 3 days16.0 kWh19.8 kWh0.79 kW
Essentials + 1,000 W well pump × 1 h + 2,000 W window AC × 6 h, 1 day18.3 kWh22.6 kWh3.79 kW

How we calculate

energy = Σ (watts × hours per day) ÷ 1,000 × days; capacity = energy ÷ (usable share × inverter efficiency); peak power = Σ watts. Example loads and battery parameters are editorial defaults listed on the methodology page.

Frequently asked questions

How big a battery do I need for a power outage?

Add up the watt-hours of everything you want to run (watts × hours per day), multiply by the number of days, and divide by the share of the battery you can use and the inverter efficiency. The calculator does this for any list of loads.

Can a home battery run central air or an electric furnace?

Large heating and cooling loads use far more energy than essentials, and they need a high continuous and surge power rating. Size for them separately and check the battery’s kW rating, not only its kWh capacity.

Do solar panels recharge the battery during an outage?

Only if the system is designed to operate off-grid during an outage. Many grid-tied solar systems shut down when the grid fails. Ask your installer whether your system supports backup operation.

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