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| Horsepower | |
|---|---|
 | |
| General information | |
| Unit of | power |
| Symbol | hp |
Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the imperial horsepower as in "hp" or "bhp" which is about 745.7 watts, and the metric horsepower as in "cv" or "PS" which is approximately 735.5 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 power-generating machinery such as piston engines, turbines, and electric motors.[1][2] The definition of the unit varied among 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 1 January 2010,[3] the use of horsepower in the EU is permitted only as a supplementary unit.
- ^ "Horsepower", Encyclopædia Britannica Online. Retrieved 2012-06-24.
- ^ "International System of Units" (SI), Encyclopædia Britannica Online. Retrieved 2012-06-24.
- ^ "Directive 2009/3/EC of the European Parliament and of the Council of 11 March 2009". Official Journal of the European Union. 7 May 2009.