Home Battery Backup Runtime Calculator

Estimate how long a battery could support an average essential load before reaching your chosen reserve.

Usable battery capacity (kWh) Average backed-up load (kW) Reserve (%) Number of batteries

Formula

Available energy = usable kWh × battery count × (1 − reserve). Runtime = available energy ÷ average load.

Why average load matters

A refrigerator, lights and internet may average far less power than central air conditioning, electric heat or an electric water heater. Peak-power capability is also separate from energy capacity.

Solar recharge is not included

This simple runtime estimate assumes no solar recharge during the outage. Actual solar-plus-storage systems can behave differently depending on weather, system design and backup controls.

Next step

Compare Home Battery vs Generator and review Solar Battery Cost.