Estimate how long a battery will run a load, from its capacity in watt-hours (or amp-hours and voltage), the load in watts and an efficiency allowance.
Runtime is estimated by multiplying the battery capacity in watt-hours by the efficiency percentage, then dividing by the load in watts. When capacity is given in amp-hours, watt-hours are first worked out as amp-hours multiplied by the nominal voltage. The result is an estimate: real runtime varies with load, temperature, battery age and cut-off behaviour, and many batteries should not be drained completely, which shortens usable runtime further. The default efficiency of 85 percent is a typical allowance for conversion losses and can be edited.
Why is the real runtime shorter than the estimate? The estimate assumes a steady load and the stated efficiency. Real runtime varies with load, temperature, battery age and cut-off behaviour, and batteries kept above a minimum charge deliver less than their full rated capacity.
How are amp-hours converted to watt-hours? Watt-hours equal amp-hours multiplied by the nominal voltage. A 10 Ah battery at 12 V stores about 120 Wh before efficiency losses.