Runtime + compatibility calculator

Battery backup runtime calculator

Estimate usable energy, expected runtime, required battery size, continuous-output headroom, and startup-surge headroom from your own numbers.

Shows assumptionsIncludes efficiencyChecks surgeNo price required
How to calculate battery backup runtime
Planning tool

Battery backup runtime calculator

Estimate usable energy, expected runtime, the nominal capacity needed for your target hours, and whether the inverter ratings appear compatible with the load. Use measured device wattage whenever possible.

Enter your system and load

The defaults model a 1,000Wh LiFePO4 power station carrying a 100W average load.

Presets are examples, not equipment specifications. Replace them with measured numbers.
Nominal watt-hours shown by the manufacturer.
Account for battery-management cutoff and chemistry limits.
AC inverter losses make delivered energy lower than battery capacity.
Energy intentionally left unused for uncertainty or battery care.
Use average operating load for runtime—not only startup watts.
Used to estimate the nominal battery capacity needed.
The sustained AC output limit.
Relevant for compressors, pumps, motors, heaters, and power supplies.
Confirm both the number and how long the manufacturer allows it.

Estimated result

6h 56m

about 6.93 hours at a steady 100W load

Net usable energy769.5Wh
Capacity for target1,155Wh
Continuous headroom+1,700W
Surge headroom+1,900W
The entered running and surge loads are below the inverter ratings. Confirm voltage, waveform, connector, transfer behavior, and manufacturer instructions before use.
  • This is a planning estimate, not a guaranteed runtime.
  • Cycling loads and temperature can materially change the result.

What the calculator is doing

Net usable Wh = nominal Wh × usable battery % × conversion efficiency % × (1 − reserve %)
Runtime estimate

Net usable watt-hours divided by the average load in watts.

Required capacity

Target hours × average watts, adjusted upward for usable capacity, efficiency, and reserve.

Power compatibility

Running watts are compared with continuous output; startup watts are compared with surge output.

Important limitation: Batteries and appliances do not behave like perfect fixed loads. Refrigerators defrost and cycle, pumps start and stop, computers change power states, heating accessories draw extra power, batteries lose capacity with age and temperature, and surge ratings may apply for only a short interval. Measure the actual equipment and perform a supervised compatibility test.

Use the result to choose the right guide

Best Picks hub

Choose the right system category and use-case guide.

Open guide →

Runtime method

Understand watt-hours, loss assumptions, cycling, and reserve.

Open guide →

UPS sizing

Compare VA, watts, waveform, transfer time, and shutdown needs.

Open guide →

Runtime calculator questions

Should I use running watts or startup watts for runtime?

Use average running watts for the energy calculation. Enter startup or surge watts separately to check whether the inverter may start the load.

Why is usable energy lower than the advertised watt-hours?

Battery-management limits, inverter conversion, standby consumption, reserve, temperature, and battery age reduce the energy delivered to an AC load.

How should I estimate a refrigerator?

For runtime, a 24-hour energy measurement or the appliance’s annual kWh rating is better than using compressor running watts continuously. For inverter compatibility, also check compressor startup surge.

Does a compatible watt rating guarantee the device will work?

No. Voltage, frequency, waveform, grounding, connector, transfer time, operating mode, surge duration, and manufacturer restrictions can still make the combination unsuitable.

Calculator methodology reviewed August 6, 2026. Results are estimates and not electrical, medical, or installation advice.

Decision chart Estimated runtime at a 45W CPAP load Starting estimate using 80% usable energy. Real runtime changes with surge, cycling, temperature, inverter draw, and battery condition.
300Wh battery 5.3 hr
500Wh battery 8.9 hr
1,000Wh battery 17.8 hr
2,000Wh battery 35.6 hr
3,600Wh battery 64 hr
Decision chart Estimated runtime at a 100W cycling load Starting estimate using 80% usable energy. Real runtime changes with surge, cycling, temperature, inverter draw, and battery condition.
300Wh battery 2.4 hr
500Wh battery 4 hr
1,000Wh battery 8 hr
2,000Wh battery 16 hr
3,600Wh battery 28.8 hr
After the calculation

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