Battery Backup Runtime Calculator
This tool lets you choose common devices used during an outage. It keeps three power numbers separate. Average watts affect runtime. Running watts affect the inverter's continuous load. Starting surge tells you whether motors can turn on.
1Enter the battery
2Choose outage loads
The compressor and pump presets use separate values for average use, normal running, and startup.
Estimated backup runtime
48.6 hours
2.0 days at the estimated average load.
Usable AC energy
10.9 kWh
Average load
225 W
This includes the normal on-and-off cycles of refrigerators and sump pumps.
Running load
535 W
Estimated motor surge
955 W
Measure essential devices when possible. Refrigerator and pump use changes with heat, incoming water, age, and thermostat settings. Battery limits, cold weather, aging, inverter standby use, and repeated motor starts can shorten real runtime.
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Quick planning answer
Usable AC energy equals rated battery kWh times allowed discharge and inverter efficiency. A 13.5 kWh battery with 90% allowed discharge and 90% efficiency gives about 10.9 kWh to your devices. At an average load of 300 W, that is about 36 hours before allowing for temperature, age, and extra reserve settings.
Measure the devices you need during an outage
A refrigerator label may show the highest current, but the compressor runs only part of the hour. A plug-in watt meter used for 24 hours gives a better average. Smart plugs can help measure electronics. Use properly rated equipment for motors and medical devices.
Sump-pump use changes with the weather. The average may be near zero on a dry day and very high during the storm that caused the outage. Count pump cycles when water is entering quickly. Keep another form of protection if a flood would cause serious damage.
Starting watts can be the first limit
Compressors and pumps use more power for a short time when they start. If the inverter shuts off during startup, having plenty of stored kWh will not help. Check continuous output, how long the inverter can handle a motor surge, and whether several automatic devices may restart together.
- Test refrigerator and freezer temperature with normal door opening.
- Test a sump or well pump starting while the refrigerator is already running.
- Remember internet equipment, medical devices, controls, and blowers on gas appliances.
- Consider automatic load controls and clearly label the backup-load panel.
Real runtime will vary
Battery age, cold weather, inverter standby use, cell balancing, reserve settings, and conversion losses reduce the energy available at outlets. Hot weather also makes a refrigerator run longer. Use this estimate to compare plans, and keep a reserve for an outage that lasts longer than expected.
Solar can extend runtime only if the system can charge during an outage and the weather provides enough power after serving active loads. A generator can provide another charging source, but it also needs fuel, safe ventilation, proper connections, and maintenance.
Questions people ask before buying
How do I calculate battery backup runtime?
Multiply rated battery energy by allowed depth of discharge and inverter efficiency. Then divide the usable watt-hours by the average load in watts.
Why is average load lower than running load?
Refrigerators, freezers, and pumps turn on and off. Running watts matter while they are on. Average watts better show the energy they use over many hours.
What is surge power?
A motor may use several times its normal running power for a short time when it starts. The inverter must handle that surge without shutting off, even when other devices are already running.
Can one home battery run central air conditioning?
Some can. You must check compressor running power, locked-rotor starting power, other loads running at the same time, soft-start support, inverter limits, and the runtime you want.
Method, sources and limits
The device presets include planning values for average power, running power, and motor starting power. Average watts for refrigerators, freezers, and sump pumps include the time they are normally off. Use the custom fields to add measured devices or replace the presets.
Usable AC energy equals rated kWh × depth of discharge × inverter efficiency. Runtime equals usable watt-hours divided by average watts. The tool checks running power and surge power separately against the inverter ratings you enter.
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