Free planning tool

Battery runtime calculator

Estimate how long an inverter battery or portable power station could run a steady appliance load. Change the assumptions to match your equipment.

By Bankole Emmanuel · 22 September 2026 · An estimate, not a tested runtime

What the calculation means

With the starting figures, 900 Wh is available on the battery side. Supplying 119 W through a 90%-efficient conversion path draws about 132.2 W before any separate idle consumption. Dividing 900 by 132.2 gives about 6.8 hours.

If you already know the usable energy delivered at the AC socket from a reliable test, enter that Wh figure with both percentages set to 100 and idle draw set to zero. Applying the same losses twice would understate runtime.

Have amp-hours instead of watt-hours?

Multiply the battery bank’s nominal voltage by its amp-hour rating: Wh = V × Ah. A 12 V, 200 Ah bank is nominally 2,400 Wh. A 24 V, 200 Ah bank is 4,800 Wh. Use the whole bank’s rating; do not mix one battery’s voltage with the wrong bank capacity.

The Ah label alone is not enough to compare batteries. Use the manufacturer’s permitted discharge limits. Battery chemistry, discharge rate, age and temperature can all change the energy actually available.

A 200 Ah example

For a hypothetical 12 V, 200 Ah bank, assume 50% usable capacity, 90% conversion efficiency, a 100 W appliance load and no separately counted idle draw. The estimate is:

This is arithmetic under stated assumptions, not a claim that every 200 Ah battery lasts 10.8 hours. In particular, a simple energy model does not capture lead-acid discharge-rate effects.

What this tool does not decide

• Whether a device will start. Motor surge, waveform, voltage and socket limits need separate checks. Even a passed average-load check does not establish compatibility.
• A freezer’s exact runtime. Compressors cycle. Use measured average energy and separately check startup power.
• Solar recharge time. This model assumes no charging during discharge.
• Battery condition. Ageing, temperature, cutoff settings and rate-dependent capacity are not modelled.

For a new purchase, use the result as a planning estimate and ask the supplier for a runtime test under your intended load. For a complete example, read backup power for a laptop, router and fan.

Method and sources

The tool uses an energy-balance calculation with editable loss assumptions. For technical context, see Victron’s explanation of effective battery capacity and its inverter specifications, including zero-load power. The defaults are not specifications for those products.

Calculations run in your browser. This tool does not submit or store your entries. Report errors to hello@wattforhome.com.