Horsepower (hp) is the name of several units of measurement of power, the rate at which work is done. The most common conversion factor, especially for electrical power, is 1 hp = 746 watts. 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. The definition of the unit varied between geographical regions. Most countries now use the SI unit watt for measurement of power. With the implementation of the EU Directive 80/181/EEC on January 1, 2010, the use of horsepower in the EU is only permitted as supplementary unit.

Units called "horsepower" have differing definitions:

  • The mechanical horsepower, also known as imperial horsepower, of exactly 550 foot-pounds per   second is approximately equivalent to 745.7 watts.
  • The metric horsepower of 75 kgf-m per second is approximately equivalent to 735.5 watts.
  • The Pferdestärke PS (German translation of horsepower) is a name for a group of similar power measurements used in Germany around the end of the 19th century, all of about one metric horsepower in size.
  • The boiler horsepower is used for rating steam boilers and is equivalent to 34.5 pounds of water evaporated per hour at 212 degrees Fahrenheit, or 9,809.5 watts.
  •  One horsepower for rating electric motors is equal to 746 watts.
  • Continental European electric motors used to have dual ratings, using a conversion rate of 0.735 kW for 1 hp
  • British Royal Automobile Club (RAC) horsepower is one of the tax horsepower systems adopted around Europe which make an estimate based on several engine dimensions.