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Horsepower (HP) is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.[1] Horsepower was originally defined to compare the output of steam engines with the power of draft horses. The unit was widely adopted to measure the output of piston engines, 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.

The definition of the horsepower also has varied between different applications:

  • 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 735.499 watts.
  • 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 conversion rate 0.735 kW for 1 HP
  • 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.[2][3]
  • The Royal Automobile Club (RAC) horsepower or British tax horsepower is an estimate based on several engine dimensions.


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