Watt-hour (Wh)
Modern equivalent
3,600 joules
One watt of power sustained for one hour, being one thousandth of a kilowatt-hour.
Where the kilowatt-hour suits household supply, the watt-hour suits portable power. Battery capacity for phones, laptops, power tools and electric bicycles is rated in watt-hours, and airline restrictions on lithium batteries are written in the same unit. A typical smartphone battery holds around 15 watt-hours, a laptop between 50 and 100.