Why Modern Engines Still Measure Power Output In Horsepower
It all really comes down to habit. Once people get used to doing things one way — like using the word "horsepower" to refer to an engine's output — it can be exceedingly difficult to get them to change. And to be sure, people have been using horsepower to measure an engine's work for hundreds of years.
Folks had been referring to how much work a horse could do for even longer than that, but in the 1780s, James Watt began using the term to help quantify how much better his steam engine was than others' early models. Remember, Watt didn't literally invent the steam engine; that was Thomas Newcomen who created a steam-powered engine to help pump water out of mines in the early 1700s.
But an early business arrangement paid Watt (and his partner, Matthew Boulton) based on how much more work their engine could do compared to Newcomen's. To simplify that comparison, Watt gave the word "horsepower" a specific meaning based on his belief an actual horse could lift 33,000 pounds one foot in one minute. So at its core, horsepower was a word developed as a marketing tool.
Horsepower has really only stuck as a term for engines, although how much you really need is up for debate. Watts are used to measure power in many other applications, following the pattern set by units like amps, ohms, and volts, all of which come from inventors' names: André-Marie Ampère, George Simon Ohm, and Alessandro Volta.
The evolution of horsepower marketing
By the time the Benz Patent Motor Car — generally accepted as the world's first automobile — hit the road, the term was in wide enough use to be part of an early news story. As reported in a June 4, 1886 article in the Neue Badische Landeszeitung, the Benz engine, "which has a cylinder width of nine centimeters and is mounted on springs above the axle between the two wheels at the rear, develops almost one horsepower despite its rather delicate appearance." The powerplant's position in the chassis puts the Benz on our list of rear-engine cars that stood the test of time as well.
Like Watt, early automakers were also quick to use the measurement to set their products apart from the competition. The first advertisement for the Ford Model T, for example, came in 1908 and prominently mentioned its 20-hp four-cylinder engine. Plymouth had "82 horsepower rarin' to go" in its 1935 lineup, and 128 hp — more than one horsepower for every inch of wheelbase — was a top selling point for the 1940 Hudson Eight, per Consumer Guide Automotive. Once the 1949 Oldsmobile Rocket 88 kicked off the muscle-car horsepower wars, albeit with a mere 135 ponies, the stage was set for the very word "horsepower" to become part of American culture — even though, truth be told, a typical horse can actually deliver about 15 of Watt's horsepower.
Other ways to measure power
Horsepower is not the only old word in use today with roots that no longer seem to make sense. Consider the word "high" in "highway." It doesn't have to do with the altitude of the road, but hearkens back to when "high" meant something like "main" — in other words, the original highway was a main road.
Moreover, other terms have made inroads on horsepower, including the one named for Watt himself. William Siemens, who was part of the family behind the international engineering and technology company still in business today, suggested using Watt's name for a unit of electrical power in 1886, at a meeting of the British Association for the Advancement of Science. It was adopted globally in 1960 as an official part of the International System of Units. This is a key reason certain parts of the world actually don't use horsepower when talking about modern engines.
The International System of Units is less formally known as the metric system, so many metrically-minded countries use watts — or kilowatts — to measure an engine's output. Some use PS as well, but that's an abbreviation of the German word "pferdestärke," which is a direct translation of horsepower – so it goes back to Watt, too. Using kilowatts to describe EVs makes sense because, as mentioned, watts are specifically a measure of electric power. It also was a great name for an EV from the early Crap Era, when electric cars were at their nadir.
