Battery runtime calculator

Preliminary estimate

Inputs

Calculation note

Runtime assumes a steady load. Use measured load data and manufacturer battery/inverter limits for project review.

Method

Delivered Wh = usable kWh x 1000 x (1 - reserve percent / 100) x inverter efficiency

Runtime hours = delivered Wh / load W

Runtime days = runtime hours / 24

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Formula variables

  • Usable kWh is the energy available before reserve and inverter efficiency are applied.
  • Load W is the steady connected load used for the estimate.
  • Reserve percent is held back from the runtime calculation.
  • Inverter efficiency is the entered delivery efficiency percentage.

Assumptions

  • Runtime is calculated from usable energy after reserve and inverter efficiency are applied.
  • Load is treated as steady watts for the duration of the estimate.
  • Actual runtime varies with surge loads, standby loads, battery temperature, battery age, inverter curve, and BMS limits.
  • This calculator does not select battery equipment, inverter size, protection, conductors, or energy-storage installation requirements.

Examples

Small backup load. A 5 kWh usable battery feeding a 500 W load at 90 percent inverter efficiency with 15 percent reserve estimates about 7.65 hours runtime.

Load-list workflow. Build or measure the steady load first, then use the runtime result as a planning screen before manufacturer review.