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.〔("Horsepower" ), ''Encyclopedia Britannica Online''. Retrieved 2012-06-24.〕〔("International System of Units" (SI) ), ''Encyclopedia Britannica Online''. Retrieved 2012-06-24.〕 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 permitted only as a supplementary unit.〔("Directive 2009/3/EC of the European Parliament and of the Council of 11 March 2009" ), ''Official Journal of the European Union''. Retrieved 2013-02-15.〕
==Definitions of term==
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 or 98.6% of an ''imperial mechanical horsepower''.
* 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.〔("Horsepower" ), ''Sizes.com''. Retrieved 2009-02-05〕〔("Metric Horsepower" ), ''Sizes.com''. Retrieved 2012-06-24.〕〔"Conversion Factors", page F-313 of ''Handbook of Chemistry and Physics'', 58th Edition, CRC Press Inc., Cleveland, Ohio ISBN 0-8493-0458-X, 1977, Robert C. Weast (ed.)〕
* The ''boiler horsepower'' is used for rating steam boilers and is equivalent to 34.5 pounds (about 15.6 kg) of water evaporated per hour at 212 degrees Fahrenheit (100 degrees Celsius), or 9809.5 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.
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