An SI unit of power defined as the rate of energy output or consumption of one joule per second. The power consumed over a period of time is measured in watt-hours. The watt is named after Scottish engineer James Watt (1736–1819) who developed a steam engine to replace horses in coal pits. The term horsepower was used by Watt to demonstrate the equivalent power of his new machine, where one horsepower (hp) is equal to 746 watts.