How to size a home inverter battery for a power cut
Estimate backup energy and inverter output from appliance watts, outage hours, usable battery capacity and conversion losses.
Start with the appliances you will actually run
Write down the appliances that must stay on, their rated input in watts, and how long each one will run during a cut. Read the nameplate or manual instead of relying on a generic online wattage: a ceiling fan, refrigerator or pump can vary by model and operating condition.
Add watts × quantity for the simultaneous load. Separately add watts × quantity × hours for energy. A 75 W fan running for 5 hours uses 375 Wh; two such fans use 750 Wh. One kW is 1,000 W, and one kWh is 1,000 Wh.
Allow for battery and inverter losses
The appliances use AC energy, while a battery stores DC energy. A first-pass nominal battery estimate is: outage energy ÷ inverter efficiency ÷ usable battery fraction. The usable fraction is the portion of rated battery energy you intend to draw; it is not always the same as the manufacturer’s maximum depth of discharge.
Example: 1.0 kWh of AC load, an assumed 85% inverter efficiency and 80% usable fraction gives 1 ÷ (0.85 × 0.80) ≈ 1.47 kWh nominal. This is an estimate, not a product recommendation. Battery capacity can also depend on temperature, age, discharge rate, chemistry and the manufacturer’s test conditions.
Check power and surge separately from energy
Battery kWh answers “for how long?” Inverter output answers “how much at once?” Add the watts of appliances likely to run together. Compare that load with the inverter’s continuous output in watts and VA, using the power factor stated by the manufacturer.
Motors and compressors can draw more power while starting than while running. Check the appliance starting requirement and inverter surge rating before connecting a refrigerator, pump or air conditioner. The planner on this site gives an initial estimate; it cannot verify a particular model’s surge compatibility.
Convert watt-hours to amp-hours carefully
For a DC battery, nominal Ah ≈ nominal Wh ÷ nominal volts. For example, 1,470 Wh ÷ 12 V is about 123 Ah. This conversion does not mean that every 12 V, 123 Ah battery will deliver 1.47 kWh to AC appliances. The usable limit, discharge rate and conversion losses still apply.
Keep voltage consistent across the battery bank and inverter. Never mix batteries or alter wiring based only on an online calculation; follow the equipment instructions and use a qualified installer.
Use the estimate as a shopping shortlist
Check the battery datasheet for nominal energy, allowed depth of discharge, maximum charge and discharge power, cycle conditions and warranty. Check the inverter datasheet for continuous watts, VA, surge duration, efficiency and compatible battery voltage. A well-sized system has to satisfy both energy and power requirements.
Enter your own appliance list in the backup planner and adjust its assumptions. Then compare the result with the exact manufacturer specifications.
References
These guides give planning estimates and general safety practices, not engineering advice for a specific installation. Check appliance nameplates and manufacturer documentation, and use a qualified electrician for anything beyond a plug-in appliance.