Around 1789, Scottish scientist and engineer James Watt first used the term "horsepower" to show how many horses the steam engines he invented could replace.
As you know, the old horsepower unit (735.5 watts in conventional units) is actually much more than the average horse can develop for any length of time. And yet, when does a horse create such power? It turns out that in most European countries, including Russia, horsepower is defined as 75 kgf m / s, that is, as the power sufficient to lift a 75 kg load to a height of 1 meter in 1 second. In this case, 1 liter. from. is exactly 735, 49875 watts, which is sometimes called metric horsepower. Currently, horsepower is formally withdrawn from use in Russia, but it is still used to calculate the transport tax and is still very widespread in the automotive environment.
In October 1960, when at the XI General Conference on Weights and Measures, the unified International System of Units SI was adopted. In this system, power is expressed in watts in honor of the same James Watt.
It should be noted that human fantasy was not limited to "ordinary" horse powers. In addition to traditional hp (horse power), "boiler" horsepower - bhp, "electric" - ehp, "metric" - mhp and "water" - whp are in circulation in the UK, Canada and the USA. But the British thought this was not enough: they also came up with British horsepower (hpUK), which is 1.014 of the generally accepted one (and therefore 1 hpUK = 746 watts, not 736).