What unit measures electrical power?

The watt is the SI unit of electrical power.

A watt measures the rate at which energy is transferred or converted. In an electrical circuit, power can be calculated as voltage multiplied by current: P = VI. Thus, a device operating at one volt and one ampere uses one watt.

The unit honors Scottish inventor and engineer James Watt, whose improvements to steam engines helped establish the importance of measuring mechanical power. A watt is not the same as a joule: a joule measures energy, while a watt measures energy per second. Household appliances are commonly rated in watts or kilowatts, with one kilowatt equal to 1,000 watts.