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Battery Runtime Calculator

Estimate how long a battery can supply a constant load. Enter its energy in Wh/kWh or capacity in Ah with voltage, then choose your own discharge and efficiency assumptions.

Idealized estimate, not a guaranteed runtime. Actual results vary with load changes, chemistry, discharge rate, temperature, battery age, voltage sag, inverter losses and cutoff behavior. Enter realistic depth-of-discharge and efficiency values from product documentation. Do not use this to size or install electrical systems; follow manufacturer instructions and local rules.

How do you calculate battery runtime?

Convert battery capacity to watt-hours, multiply by the depth-of-discharge and efficiency percentages, then divide usable watt-hours by the load in watts. For example, 100 Wh ÷ 10 W = 10 ideal hours before losses.

Frequently asked questions

How do you calculate battery runtime?

Convert battery capacity to watt-hours, multiply by the depth-of-discharge and efficiency percentages, then divide usable watt-hours by the load in watts. For example, 100 Wh ÷ 10 W = 10 ideal hours before losses.

Are my battery or load values uploaded?

No. The calculation runs locally in your browser; inputs are not sent to a server or saved by this tool.

How do I convert amp-hours to watt-hours?

Multiply amp-hours by nominal voltage: Wh = Ah × V. This is an energy estimate; the usable amount depends on discharge limits and real system losses.

Will the battery last exactly this many hours?

No. It is an idealized estimate for a constant load. Real runtime depends on battery chemistry, discharge rate, temperature, age, voltage sag, system losses and low-voltage cutoff.