What size battery for an overnight outage?
A practical essential-load example for one night of backup power. It separates usable capacity from rated capacity.
Technical feedback wanted: if the load mix, reserve margin, or depth-of-discharge assumption looks off, copy the result summary or share link and send a correction.
Essential loads
| Load | Watts | Hours | Energy |
|---|---|---|---|
| Refrigerator average | 120W | 12h | 1.4 kWh |
| Lights | 80W | 6h | 0.5 kWh |
| Router and devices | 50W | 12h | 0.6 kWh |
| TV / small electronics | 150W | 3h | 0.5 kWh |
| Margin for startup and losses | - | - | 2.0 kWh |
Planning total: about 5 kWh delivered to loads. At 92% inverter efficiency, 90% depth of discharge, and a 10% reserve, the calculator suggests about 5.4 kWh usable and 6.7 kWh nominal before site constraints.
Why usable and rated capacity differ
A battery advertised as 10 kWh does not always mean 10 kWh should be used in every outage. Chemistry, allowed depth of discharge, inverter losses, temperature, and reserve settings all affect usable energy.
Open the editable calculator
This preloads a 0.625 kW critical load for 8 hours, LiFePO4 chemistry, 90% depth of discharge, and a 10% reserve.
Open prefilled battery calculatorYou can edit the inputs, then copy or share the result if you want another technical opinion.