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horsepower (hp)

The horsepower (hp) is a unit in the foot-pound-second ( fps ) or English system, sometimes used to express the rate at which mechanical energy is expended. It was originally defined as 550 foot-pounds per second (ft-lb/s).

A power level of 1 hp is approximately equivalent to 746 watt s (W) or 0.746 kilowatt s (kW). To convert from horsepower to watts, multiply by 746. To convert from watts to horsepower, multiply by 0.00134. To convert from horsepower to kilowatts, multiply by 0.746. To convert from kilowatts to horsepower, multiply by 1.34.

While the horsepower, the watt, and the kilowatt are all reducible to the same dimensional units (they each represent a certain rate of energy expenditure, or power), the horsepower is rarely used to express power in any form other than mechanical. You will likely get raised eyebrows if you talk about a 1-hp microwave oven, just as you would feel uncomfortable talking about a 37-kW outboard motor.

Also see power , International System of Units ( SI ), and Table of Physical Units .

This was last updated in March 2011

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So it means HP is for mechanical?
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If you are talking about electric motors in the U.S. you use horsepower.  Motors are converting electrical power to mechanical power, therefore you use horsepower because it is a form of mechanical power. If you are talking about something other than mechanical power you use watts.  So you toaster or microwave or christmas tree lights consume electrical power expressed in watts.  
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hi :)
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