The watt-hour (Wh) is a unit of energy commonly used to measure the capacity of batteries and electrical systems. It represents the amount of energy expended when a device operates at a power of one watt for one hour. For example, a battery with a capacity of 1000 Wh can deliver one watt of power for 1000 hours or 100 watts for 10 hours. The watt-hour is a key metric in evaluating the energy storage and consumption of devices like electric vehicles, solar panels, and home energy systems.