This estimator calculates how long a battery can support a selected rooftop solar load after accounting for usable depth of discharge and discharge efficiency. It also shows usable stored energy and the battery capacity required for a chosen target duration.
Use it for preliminary backup planning, self-consumption design, or comparing battery sizes. The result assumes a steady load; appliances with startup surges, changing demand, temperature effects, and battery power limits require additional design checks.
1. Enter battery capacity
Use total nameplate energy capacity in kilowatt-hours.
2. Set usable discharge
Enter the share of nameplate capacity allowed to discharge during the event.
3. Account for efficiency
Use expected battery-to-load discharge efficiency.
4. Enter supported load
Use the average power demand of the circuits or building loads the battery will serve.
5. Compare with a target
Enter desired backup hours to see the required nameplate capacity and margin.
Usable delivered energy = Battery capacity × Depth of discharge × Discharge efficiency
Storage duration = Usable delivered energy ÷ Average load
Required battery capacity = Target hours × Average load ÷ (Depth of discharge × Efficiency)
The main result is the estimated number of hours the entered battery can support the steady average load.
The estimate does not verify inverter power, surge capability, reserve settings, battery degradation, or solar generation during the backup period.
Given: 13.5 kWh battery, 90% depth of discharge, 92% efficiency, and 1.8 kW average load.
Calculation: Usable energy = 13.5 × 0.90 × 0.92 = 11.178 kWh. Duration = 11.178 ÷ 1.8 = 6.21 hours.
Result: Estimated backup duration is 6.21 hours.
Reserve limits and conversion losses mean not all stored nameplate energy reaches the supported loads.
This calculator does not add ongoing solar generation. For daylight backup, model solar production and load over time separately.
Peak load is important for inverter sizing, but duration is normally estimated from average energy demand. Check both power and energy constraints.
Enter the expected future usable nameplate capacity rather than the new-battery rating to estimate duration after degradation.
The entered battery is smaller than the nameplate capacity required to meet the target duration under the selected assumptions.