How many watts is one horsepower?

One horsepower is conventionally equal to 746 watts. This is the standard value for imperial or mechanical horsepower, the definition commonly intended in general science and engineering trivia.

Scottish engineer James Watt popularized horsepower in the late 18th century as a way to compare steam-engine output with the work performed by draft horses. The imperial definition is 550 foot-pounds per second. Converting that rate into SI units gives approximately 745.7 watts, conventionally rounded to 746 W.

The word horsepower does not describe just one universal unit. Metric horsepower, used in parts of Europe and elsewhere, is approximately 735.5 watts. Electrical horsepower is also commonly treated as 746 watts, while boiler horsepower is a different industrial measure of steam-generation capacity and is roughly 9,810 watts.

That variety explains why some references give 735 W or 745.7 W instead. For an ordinary question asking how many watts are in one horsepower, however, 746 W is the accepted rounded answer. A watt itself measures power: one watt equals one joule of energy transferred per second.