Horsepower has an interesting backstory. If you're a keen petrolhead, however, I'd advise you not to read it, as I have this week. Enjoy your blissful ignorance if you can. There are only three things you really need to know about it, in any case. First, that celebrated Scot James Watt didn't actually invent it, he just formalised it. Second, that it's more of an idea than a measurable product of an engine anyway - a bit of mathematics.