Home Battery Backup Runtime Calculator
Estimate how long usable battery-side DC energy could support a steady average AC load after allowing for inverter losses and energy held in reserve.
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View all saved project estimatesWhat does the home battery runtime calculator calculate?
Enter usable battery-side DC capacity, average supported AC load, inverter efficiency, and the reserve you want left unused. The calculator converts available battery energy into delivered AC energy and divides it by average load. If a product specifies AC-delivered usable energy, enter that value with 100% conversion efficiency to avoid counting losses twice.
Battery runtime from delivered energy
The reserve removes energy intentionally held back, and inverter efficiency reduces the remainder to estimated delivered AC energy. Runtime is delivered kilowatt-hours divided by average kilowatts. The equation assumes a steady supported load and does not confirm surge capability, circuit coverage, battery power limits, or automatic transfer behavior.
How to measure for an accurate estimate
- Use usable battery-side DC energy before inverter losses; if the product publishes usable AC-delivered energy instead, enter that figure and set inverter efficiency to 100%.
- List only outage-supported devices, record their operating watts, and estimate duty cycles for cycling loads such as refrigerators.
- Use a plug-in meter, energy monitor, or manufacturer data to replace nameplate guesses with realistic average demand.
- Calculate separate essential-load and expanded-load scenarios because a few large appliances can change runtime substantially.
Essential loads on a 13.5 kWh battery
A home battery provides 13.5 kWh of usable battery-side DC energy. The selected AC outage loads average 800 watts, inverter efficiency is 92%, and the owner keeps a 10% energy reserve.
After reserve and conversion losses, about 11.18 kWh reaches the loads. At a steady 800-watt average, the planning runtime is approximately 13.97 hours.Before you purchase material
- Compare both usable energy in kWh and continuous and surge power in kW; runtime alone does not establish equipment compatibility.
- Ask qualified providers to confirm supported circuits, equipment listing, operating temperature, warranty terms, and local approvals.
- Discuss medical or life-safety loads with the equipment provider and emergency-planning professionals instead of relying on a calculator.
- Treat promised runtime carefully when a quote omits reserve settings, conversion losses, standby consumption, or assumed appliance duty cycles.
Common questions
Should I use gross or usable battery capacity?
Use usable battery-side DC energy for the relevant operating mode, not gross cell capacity. If the manufacturer instead publishes usable AC-delivered energy after conversion, enter that figure and set inverter efficiency to 100%. Obtain clarification when the specification does not identify which side of the inverter it measures.
How should I estimate the average home backup load?
List supported devices, note operating watts, and account for how often cycling appliances actually run. A refrigerator, for example, does not draw its running power continuously. Metered historical data is better than adding every nameplate watt as though all devices operate constantly.
Why might a battery shut down before the estimated runtime?
A motor startup surge or simultaneous load can exceed the battery or inverter power limit even when energy remains. Temperature, aging, standby consumption, reserve settings, communications, protected circuits, and manufacturer controls can also end support earlier than this steady-load energy estimate suggests.
Does this result show whether a battery can run my whole home?
No. Runtime addresses energy duration at an entered average load. Whole-home suitability also depends on continuous power, surge power, equipment compatibility, selected circuits, operating temperature, installation design, local requirements, and household priorities. Have qualified providers evaluate those factors for the specific property.
Sources and reference standards
- U.S. Department of Energy — Solar-Plus-Storage 101Federal introduction to battery storage capacity, solar-plus-storage operation, resilience, and planning limitations.
- National Laboratory of the Rockies — Storage Futures StudyNational-laboratory research on energy-storage duration, power, capacity, use cases, and grid integration.
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