Why are car engines measured in horsepower? What does horsepower mean and how much horsepower does one horse generate?
— Motorhead, Escondido
Engines have been measured in horsepower since James Watt, who refined early steam engines, coined the term in the late 18th Century so that Josiah Q. Public IV, esq. could relate to the power of new-fangled machinery. His estimate that one horse could move 550 pounds of weight one foot in one second has proven to be pretty accurate, though horses can do ten times that much in short bursts of effort. The average human can usually generate a bit more than 1HP for a second or two. An elite athlete can make three times that much power.
Horsepower has never been a popular term. People were calling it “fallacious” in the early 19th Century and 90 years later predicting it had “seen its best days” since “as a scientific term it [had] been much abused.” Apparently, the neigh-sayers (ha!) were wrong, since we still use HP to measure a car’s power output. The watt, named after James Watt, would be a more useful term to use since it incorporates into SI units of measurement, but as long as auto manufacturers keep using the term, horsepower will stick around.