While it is true that the maximum output of a horse is around 15 horsepower, when you average the output of a horse over the course of a work day it ends up being around a horsepower. Watt defined this amount as “the amount of work required from a horse to pull 150 pounds out of a hole that was 220 feet deep”.
I learned a things:
Thank God it wasn’t something arbitrary like pulling a 100 pounds out of a hole that was 250 feet deep.