Estimate only. Real runtime changes with how often appliances cycle, motor start-up surges and whether solar recharges the battery during the outage.
How the runtime is calculated
- Energy available: number of batteries × usable capacity × efficiency. The default 85% is the round-trip efficiency in NREL's Annual Technology Baseline; using it for discharge is conservative.
- Runtime: that energy ÷ the power of what you keep on.
The calculator also checks power: if your load is more than the batteries' continuous power, they cannot run it at all, whatever their capacity. The defaults, 12.5 kWh and 5 kW per battery, are the ATB's representative residential battery.
How to make the battery last longer
Keep only essentials on: refrigerator, lights, internet, phone charging and medical devices. Electric heating, air conditioning, water heaters and ovens drain a battery fastest. With solar panels, a battery can recharge during the day if the system is designed to keep working in an outage.
To size a new system for the hours you need, use the home battery backup calculator.