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. Two common definitions in use today are the mechanical horsepower (or imperial horsepower), which is approximately 746 watts, and the metric horsepower, which is approximately 735.5 watts.
The term horsepower was invented by the 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 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 January 1, 2010, the use of horsepower in the EU is permitted only as a supplementary unit.
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.