Battery Capacity and Runtime Calculator

Separate energy from power

Battery kWh is stored energy; load watts are power. Divide available AC watt-hours by load watts for hours.

A 5 kWh battery capacity is energy, not a promise of 5 kW output. That is why continuous load has its own watts field. For a fridge, fan and lights operated together, sum their running power or measure an average across the intended backup period.

Enter relevant DoD and efficiency

Use the manufacturer’s permitted discharge range and inverter efficiency at the intended load. This tool does not select electrical equipment or design wiring.

At 80% DoD, 5 kWh yields 4 kWh on the battery side; 90% discharge efficiency leaves 3.6 kWh AC. Enter percentages as percentages; the engine divides each by 100. If a datasheet reports round-trip rather than discharge efficiency, identify the relevant discharge component before using it here.

Worked default example

A 500 W load for 8 hours needs 4 kWh of AC energy. With example values of 80% DoD and 90% inverter efficiency, the rated capacity needed is about 5.56 kWh. In comparison mode, an example 5 kWh battery would run that load for about 7.2 hours.

The 7.2-hour result assumes a steady 500 W load. The 5.56 kWh requirement is mathematical; it is not a selected commercial pack size or an allowance for aging.

Check real power limits

Even with enough energy, the inverter may lack continuous or starting power. Loads change and batteries age. Check specifications and consult a qualified installer.

Motor loads may surge at startup. Inverters can have continuous and short-duration power limits separate from energy capacity. Read battery and inverter specifications together, including their approved connection method. A qualified installer must check electrical protection and compatibility before actual use.

For variable loads, model at least essential-only and expanded-use cases. Required capacity rises directly with load watts and target hours, but a real battery can have an output-power ceiling distinct from its stored energy. Check both kWh and W before selecting equipment.