Horsepower also designated as hp, refers to the measurement of work that can be completed per minute by a machine or other mechanical device.
The term was coined by engineer James Watt (1763 - 1819). Watt was said to be working with ponies in a coal mine when he became interested in devising a way to measure the amount of work that could be achieved by the average horse. This would give him a viable measuring unit that had not previously existed.
Watt determined that the average pony could lift 22,000 foot pounds (27,116 N m) per minute. He increased the number by 50 percent and determined that horsepower should account for 33,000 foot pounds (44,742 N m) of work completed per minute. He came up with this measurement by calculating that one horse could raise 330 pounds (150 kg) of coal 100 feet (30.5 m) into the air per minute.
It served to reason that power should be measured in some way to give value to mechanical devices, and Watt found this to be an efficient way to help him sell his steam engines. Horsepower has been used as a form of measuring power by equipment manufacturers for the past 200 years.
Manufacturers have used techniques to make horsepower work to their benefit. In the 1960s, manufacturers engaged in a horsepower war, which saw them producing higher figures without testing them to ensure safety and durability. Insurers countered this by assessing higher insurance for what they deemed to be high risk.
The Royal Automobile Club adjusted the horsepower calculation in 1922. It was determined by multiplying the square of the cylinder diameter in inches by the number of cylinders used, and dividing the result by 2.5. However, this method ceased after 1947 in favor of more accurate formulas.