Why can engine performance be determined by “horsepower”? Is it some kind of conversion to real horses? What kind?

Horse power was introduced by James Watt to be able to approximate the power of the steam engine. Simply put, by determining the horsepower of a horse and comparing it to a steam engine, he could say: "My steam engine will replace 10 horses". This way it was easy for everyone to imagine and was certainly well thought out in terms of marketing. The very definition of horse power was rather a very rough estimate (Watt based this estimate of how many pounds and to what height a pony, connected to a harness, would pull and estimated that the horse has 2x more power), in reality the average long-term horse power is roughly 10 times higher and the value stated by Watt corresponds rather to human performance. In the short term (in terms of energy, we say peak), the horse is able to achieve up to 100 times more power and a weightlifter's power is around 4 horses.

Since we want to determine power precisely and not by estimation, the Watt was defined using base units as the power with which we do 1 J of work in 1 s. The Watt's definition was problematic not only in that it was an estimate, but also in that the very definition of the pound and foot varies from country to country. So 1HP in England is not the same as in the US. From the definition of a Watt and the horse power estimate given, it was deduced that 1 horse power is roughly 750 Watts. This conversion (but in reverse — from Watts to Horsepower) is still used when describing engine power. But as mentioned above — 10 horsepower is realistically equal to 1 horse, so if someone claims to have 40 horses under the bonnet, then in reality there are only 4. Or using the Watt conversion of 8 ponies.