CentralNotice From Wikipedia, the free encyclopedia Jump to: navigation , search For The Chemical Brothers song, see Horse Power (song) . For the machine also known as a "horse power", see Horse engine . "hp" redirects here. For the technology corporation, see Hewlett-Packard . For other uses, see HP (disambiguation) . A team of six horses mowing hay in Lancaster County, Pennsylvania Horsepower ( hp ) is a unit of measurement of power (the rate at which work is done). There are many different standards and types of horsepower. The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses . It was later expanded to include the output power of other types of piston engines , as well as turbines , electric motors and other machinery. [ 1 ] [ 2 ] The definition of the unit varied b...