While today it may appear both a strange and outdated measure, the horsepower (hp) originates from a practical historical problem. In much the same manner as other thermodynamic units, terms and sign conventions, the horsepower traces its roots back to the early industrial period and the advent of steam power. While Scottish inventor James Watt was not the first to compare the output of a steam engine to that of a horse, he is credited with formulating the modern definition of the horsepower.
Late in the 18th century, Watt was exploring different ways to market his new improved steam engine. Though not the first viable machine of its type, Watt’s innovative design offered a significant advantage in thermal efficiency compared to its contemporaries by adding a condenser to an existing design. This addition eliminated the need to repeatedly heat and cool the main cylinder of the engine. It was relatively easy to show the advantage of his engine over its mechanical competitors by demonstrating its reduced fuel consumption. However, in order to broaden his market, Watt wanted to illustrate the advantages of steam power to a wider audience in hopes of convincing those who had not yet adopted the technology.
In order to understand the marketing ploy that spawned the term horsepower, it is important to realize that many of the tasks early steam engines were performing had, up until that point been performed by horses. For this reason, Watt derived a method by which the output of his engines could be readily compared to that of a horse. This allowed a potential buyer to understand approximately how many of their horses could be replaced by a steam engine of a given size. While accounts differ on the exact circumstances under which Watt originally derived the value for his comparison, he recorded the now accepted value in 1782 while working with a client to provide a steam engine to power a mill.
Watt observed a team of draft horses driving the mill as they walked in a 24 ft diameter circle at a rate of 2 ½ revolutions per minute. He then estimated that each horse was pulling with a force of 180 lbf (pounds-force) while operating the mill. It is not known how Watt arrived at this figure, but the value thus derived has persisted. Using these values, along with some liberal rounding, he concluded that a horse could continuously produce a power output of approximately 33,000 ft·lbf/min (foot-pounds force per minute).
This somewhat arbitrary value derived over 200 years ago has endured. While the International System of Units (SI) uses the watt, named for the very same James Watt, as the standard unit of power, both US customary and imperial units still use the horsepower. One horsepower is equivalent to approximately 746 watts. While many engineers quite sensibly tend towards the use of SI units wherever possible, the automotive industry still provides power output figures in terms of horsepower. This even includes electric vehicles, whose familial relation to the long lineage of thermal engines found in typical automobiles is dubious at best. For example, the Tesla Model S Plaid’s three electric motors offer a combined peak power output of 1,020 hp. Despite the multitude of technical advancements that have arisen in the intervening time, we still measure power output in terms of the horses that were observed and analyzed by James Watt over two centuries ago.