Watt

A watt is a standard unit of power. It is equal to one joule of energy per second. For electrical power, one watt is equal to one ampere of current per second.

History of Watt

A watt was formally defined at an international conference in 1889. It is named after James Watt (1736-1819), a Scottish inventor famous for his steam engines that powered the Industrial Revolution. The symbol for watts is a capital letter W.

Measuring Watts

A watt is measured using a wattmeter. An analogue wattmeter uses a pair of fixed current coils and a moving voltage coil connected to a needle. A digital wattmeter measures voltage and current many times a second and calculates the power usage.

Kilowatts, Megawatts and Watt-Hours

A kilowatt (kW) is one thousand watts, and a megawatt (MW) is one million watts. A light globe uses between 50 and 100 watts, a car engine uses about 25 kilowatts, and a power station produces over 500 megawatts.

A watt-hour is a confusing term because it is not a unit of power, like the watt. It is a measure of energy and is used by utility companies to price their energy. Electricity bills show how many kilowatt-hours of energy was used and the price per kilowatt-hour. For example, a 100W light globe run for ten hours will use one kilowatt-hour of energy.