Along with the industrial revolution, came a multitude of mechanisms used to "harness" energy sources such as water power and steam power. This necessitated some uniform means of describing how powerful these machines could be. The most ubiquitous source of power in those agrarian days, was the horse. This was the most logical base of reference. So along came "horsepower" (also noted as two words, "horse power").
But how much power is horse power? What most people don't understand is that time is an element of the equation. Here, we find the definition tucked into the corner of a page of the November 6, 1856 issue of the Maine Farmer:
"The power of a horse is understood to be that which will elevate a weight of 33,000 pounds the height of one foot in a minute of time, equal to about 90 pounds at the rate of four miles an hour."
But who devised this definition? It was James Watt of course...the guy who invented the steam engine. His need for a way to explain a "quantity" of power was urgent. How would he promote his machines, if he couldn't present how much power they provided? Watt needed a way to explain a steam engine's capability in a way to which people could relate. So he did some tests, and established "the power of a horse."
Later, when Watt's steam engines came to be used to generate elecricity, we needed a means of explaining how much energy was produced. Along came the watt (as in "60-watt light bulb"), a unit named after James Watt. But The Heirloom Orchardist doesn't have the horsepower to explain how that term came about.