Although it may seem safe to assume that one horsepower is the output a horse is capable of creating at any one time, that is incorrect. In fact, the maximum output of a horse can be up to 15 horsepower,[2] and the maximum output of a human is a bit more than a single horsepower. For extreme athletes, this output can be even higher with Tour de France riders outputting around 1.2 horsepower for around 15 seconds, and just under 0.9 horsepower for a minute.[3] https://energyeducation.ca/encyclopedia/Horsepower
I must now once again question the nature of reality.
the maximum output of a horse can be up to 15 horsepower,
That's the problem. The unit was not developed on the maximum power a horse could put out. It was intended to be what a typical horse could continuously sustain throughout the work day.
Also why switching horses was a thing: a fully rested horse could run at a higher hp, then change horses and the new one could keep outputting thevhigher up whilecthe previous one rested.
Like switching rechargeable batteries, only the battery was the horse.
It's just playing with fractions and linear extrapolation. Horsepower has a time denominator. If you measure how fast I can run (not fast) in .1 second intervals, then take the highest number and extrapolate that to miles per hour it will seem impressive.
Horsepower is averaged based on an extended time period, how much power a horse can put out on average while constantly working. They can't do 15 hp on a constant basis, they can only do it for a relatively brief period of time, so their average is 1 hp. A 15 hp engine can run at 15 hp for a much longer period of time, which a horse can't do. If the engine was hypothetically capable of working consistently without ever breaking down, it would be able to run at 15 hp indefinitely. But even with the machine's lifespan in mind, it can still run for years at the same output, which is impossible for a horse.
1hp is the average power over time for a horse. That 15hp number is peak. There's like a whole thing with lifting a set weight up and seeing how far the horse traveled and whatnot to get to that number which is sorta interesting but it ends in hp is sorta flawed and we mostly use it wrong but still it's neat.
1 horsepower is supposed to be the rate a shitty old timey horse can work over the course of a whole day. Also it was created as a way to market steam engines to replace horses as a source of mechanical power so there was an incentive to lowball the horse.
There's a difference between maximum power and maximum continuous power. It's like your car engine; it might be rated for hundreds of horsepower, but most of the time cruising down the highway it might be making 20 or so just to keep you loafing along.
Frederick Taylor did an exact study on this sort of thing for multiple manual tasks back in the late 1800s/early 1900s.
Of course, he also advocated for higher wages and shorter working hours for those who were capable of accomplishing more, but that part gets conveniently forgotten about 🤷♀️
"The folks at Art of Engineering calculated duckpower by dividing the mass of the waterfowl by the mass of a horse, then applied Kleiber's law. In short, they determined that one horsepower is equivalent to 131.2 duckpower."