Horsepower is simply a measure of power, which itself is a measurement of the rate at which work is done. Essentially, it is the way of determining how much energy is transferred in a set period of time. It is calculated by dividing the amount of work done by the time taken to do it. In the case of James Watt, he studied the amount of work that a horse was able to do in an hour and then attempted to accurately portray this in a way that was easy to understand.
Watt observed horses working in a variety of conditions, and eventually came to the conclusion that they were able to perform 33,000 foot-pounds of work in a single minute. This is roughly equivalent to a horse being able to pull a weight of 330 pounds along 100 feet or a horse pushing a mill wheel fully around 144 times by exerting a force of 180 pounds. By using the calculation for power, Watt determined that a single horse was able to produce 32,572 foot-pounds per minute, which he then rounded to 33,000 foot-pounds per minute for simplicity.
So, a steam engine that is able to do 33,000 foot-pounds of work each minute is said to have one horsepower. The unit soon caught on, and was used to work out how powerful each engine was in comparison to the output of a horse.
