Pick what you need to keep running and for how long to estimate the battery capacity (kWh), power (kW) and runtime you need.
Battery backup calculator
Pick what you want to keep running in an outage and for how long. We estimate the battery size, the power it must deliver, and how long a battery you are considering would last.
Assumptions and limits
- Appliance watts: Honda wattage guide, Champion wattage chart, ENERGY STAR product data (fridge 363 kWh/yr, upright freezer 438 kWh/yr, TV 85 W, ceiling fan 30 W, 8,000 Btu window AC 533 W), ResMed AirSense 11 user guide (CPAP 56 W typical, 73 W peak). LED bulb 9 W, router + modem 20 W and laptop 65 W are typical label values. Hours per day are examples: change them to match your home. Your appliance labels beat any chart.
- Usable energy: we count 85% of a battery’s rated kWh as reaching your appliances (battery limits and inverter losses; NREL’s NREL Annual Technology Baseline uses 85% round-trip efficiency for home batteries).
- Starting surge: we add the single biggest starting jump (starting minus running watts) to the total running watts, assuming motors start one at a time (Honda wattage guide, Generac sizing guide: starting is often 2–3× running). Where a chart gives no starting figure we assume 2× running.
- Size range: from the energy you need up to 25% more, our suggested margin for aging, cold weather and surprises.
- Battery classes: about 5 kWh (example: 3.84 kVA continuous), about 10 kWh (7.08 kVA) and about 13.5 kWh (11.5 kW) from maker data sheets of common home batteries. Models differ; the installer checks continuous and starting (LRA) ratings.
- Heating and cooling: central AC, heat pumps and electric heat use far more energy than the essentials. Medical devices: ask your doctor and the device maker about backup power.
- This is an estimate, not a design. A licensed installer must confirm sizing, wiring and permits.
Learn more in our Batteries guide, or see all calculators.