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Battery runtime

Battery runtime calculator for Ah, Wh and watts

Estimate how long an existing LiFePO4, AGM, or other battery bank can support a load. Enter capacity in Ah or Wh and load in watts or kW. AC calculations include inverter loss and idle draw; DC calculations do not pretend an inverter is present.

Estimate available runtime

How is the battery capacity specified?
Load type
Advanced assumptionsEditable planning values

Your planning estimate

Local calculation

Enter your values, then calculate. Results will show the formula inputs and rounding used.

Formula used

runtime hours = nominal Wh × DoD × battery efficiency ÷ effective load W

For AC loads, effective battery-side power is load watts divided by inverter efficiency, plus inverter idle draw. For direct DC loads, the entered load watts are used without inverter loss.

See the full assumptions and rounding policy.

Maintenance and verification

Maintained by: SolarMathKit Formula version: 1.3.0 Last reviewed:

The calculator calls the shared, tested TypeScript calculation modules documented in the methodology. Automated tests cover published examples, invalid inputs, rounding, and result invariants.

SolarMathKit maintains this implementation. The review is an internal technical check, not an independent engineering certification.

Worked example: 12V 100Ah battery and a 100W DC load

Use 80% depth of discharge and 95% battery efficiency.

  1. Nominal energy: 12V × 100Ah = 1,200 Wh.
  2. Usable energy: 1,200 × 0.80 × 0.95 = 912 Wh.
  3. Runtime: 912 Wh ÷ 100W = 9.12 hours.
Result: The planning estimate is about 9 hours 7 minutes. A comparable AC load will run for less time once inverter losses are included.

Common mistakes to avoid

  • Dividing rated Wh by load watts without a usable-capacity factor.
  • Applying inverter efficiency to a direct DC load.
  • Leaving inverter idle draw out of a low-power AC scenario.
  • Treating a refrigerator nameplate wattage as a continuous measured average.

Questions people ask

Can I calculate runtime from battery Ah and load watts?

Yes. The calculator converts Ah and voltage to Wh, applies usable-capacity and efficiency factors, then divides by the battery-side load in watts.

Why can real runtime be shorter?

Temperature, battery age, discharge rate, inverter behavior, and appliance duty cycles can all lower runtime.

Does series or parallel change total energy?

With the same number of matching batteries, either arrangement keeps total Wh the same. Series raises voltage; parallel raises Ah.

Can I use advertised usable kWh?

This form expects nominal energy before depth-of-discharge and battery-efficiency adjustments. If you only have a verified usable figure, set the assumptions carefully to avoid subtracting the same loss twice.

Before you build: Check voltage, current, temperature, protection, cable sizing and installation rules with the equipment manuals and a qualified professional.