Watt-Hour Calculator

A watt-hour (Wh) is a unit of energy: one watt of power drawn for one hour. It is the honest way to compare batteries, because it folds both voltage and capacity into a single number. Two batteries can both say “5,000,” but a 5,000 milliamp-hour phone cell and a 5,000 milliamp-hour 36-volt tool pack hold wildly different amounts of energy.

There are two common ways to land on watt-hours. For a battery, multiply its voltage in volts (V) by its capacity in amp-hours (Ah): Wh = V times Ah. Watch the units here, because most phones and power banks print capacity in milliamp-hours (mAh), and amp-hours are mAh divided by 1,000. A 10,000 mAh power bank is 10 Ah, and at its cell voltage of about 3.7 V that is roughly 37 Wh, not 10,000 of anything. For a device, multiply its power draw in watts by how many hours it runs: Wh = watts times hours. Divide by 1,000 to get kilowatt-hours (kWh), the unit your electricity bill uses.

Some real anchors: a phone battery is around 12 Wh, a laptop 50 to 90 Wh, a large portable power station 500 to 2,000 Wh, and an electric car around 60,000 Wh (60 kWh). One number worth knowing before you fly: most airlines cap spare lithium batteries in carry-on at 100 Wh without approval, which is why a 27,000 mAh power bank often gets waved through and a 30,000 mAh one does not. Add your electricity price per kilowatt-hour and this calculator also shows what a full charge costs.