Watt-hour (Wh)

Amp-hours tell you how much current a battery can push for how long. But a pack at 14.4 volts and a pack at 36 volts with the same amp-hours hold very different amounts of energy. Watt-hours fix that by combining the two numbers.

A pack marked 14.4 volts and 2.6 amp-hours holds about 37 watt-hours. Feed that to a machine drawing 40 watts and you get a bit under an hour, minus the energy lost as heat.

Airlines count in watt-hours too. US rules let you carry batteries up to 100Wh in the cabin without asking, and ban anything over 160Wh.