Watt-hour (Wh)

A watt-hour is the basic unit of measuring electrical energy. Energy is power used over time. 

The watt-hour is a unit of energy equal to one watt of power sustained for one hour. For example, a 15-watt light bulb left on for one hour will consume 15 watt-hours of energy. If it is left on for two hours, it will consume 15 watts x 2 hours = 30 watt-hours. 

Because a watt-hour is a relatively small amount of power, units that reflect multiples of watt-hours are more commonly used in the energy industry including the kilowatt-hour (1,000 watt-hours), megawatt-hour (1,000,000 watt-hours), and gigawatt-hour (1,000,000,000 watt-hours). The kilowatt-hour (kWh) is commonly used for billing usage by end-use customers, while the megawatt-hour (MWh) is commonly used to describe the energy output of generators and is also used in trading wholesale energy.