SolarMathKit

Battery runtime

Solar battery runtime calculator

Estimate how long an existing battery or bank can support a steady load. AC calculations include inverter loss and idle draw; DC calculations do not pretend an inverter is present.

Runs in your browserEditable assumptionsPlanning estimate

Inputs

Estimate available runtime

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

Planning estimate

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.

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

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: This calculator is for planning. Check voltage limits, current limits, temperature corrections, protection devices, cable sizing, and installation rules against equipment manuals and a qualified professional.