What is horsepower?

Horsepower is a unit of power representing the power exerted by a horse when it pulls an object. James Watt, the inventor of the steam engine, defined the term horsepower after determining through careful measurements that a horse is typically capable of a power rate of 550 foot-pounds per second. This means that a horse, harnessed to an appropriate machine, such as a plough or wagon, can lift 550 pounds at the rate of one foot per second. Today the standard unit of power is named for Watt, and one horsepower is equal to approximately 745.6999 watts. Outside the United States, the English word “horsepower” is often used to mean the metric horsepower, a slightly smaller unit.

The horsepower number used to describe engine power literally relates to the strength of an average horse. The man in this picture is using a four-horsepower plow!