Battery runtime

Use an average over the operating cycle

A device that cycles on and off does not draw its peak current continuously. Include standby time and other loads when estimating the average. Keep startup demand separate: a battery may have enough stored energy but still be unable to supply a brief high-current event.

Why measured runtime may differ

The usable-capacity and efficiency inputs are allowances you supply. This arithmetic model does not predict aging, temperature effects, discharge-rate behavior, or the exact shutdown voltage of your equipment. Use representative discharge data when runtime matters to the design.

Relating amp-hours to watt-hours

Amp-hours describe charge, while watt-hours describe energy. Multiplying nominal voltage by amp-hour capacity gives an approximate nominal energy figure. A 12 V, 20 Ah battery corresponds to 240 Wh on that basis. At a steady 60 W load, ideal runtime is four hours before usable-capacity and efficiency allowances. This example is arithmetic, not a promise about a particular battery's discharge behavior.

Choose a load estimate that includes every device powered by the battery, including conversion equipment and standby consumers where applicable. A constant-power load may draw increasing current as battery voltage falls, so a single nominal-current assumption can miss that behavior. To compare batteries at different voltages, work in energy rather than amp-hours alone. The required-capacity mode can provide a starting size for a runtime target, but check the candidate battery's discharge curves and equipment cutoff conditions. Keep enough margin for the operating conditions instead of interpreting decimal runtime output as minute-by-minute accuracy.

Formula

Runtime = rated Ah × usable fraction × efficiency / load current.