Horsepower was a sales pitch to help mine owners replace horses
A 25-horsepower engine, Watt told owners, could replace 75 horses.
My First Million · Rory Sutherland: completely legal marketing hacks that always workSutherland says horsepower was made to sell steam engines, not to do science.
Rory Sutherland, the author of Alchemy, tells the story as an example that adoption matters as much as invention. Watt sold steam engines to mine owners to replace horses that walked in circles to drain water so miners could work without drowning.
Talk of boiler capacity or piston length left buyers cold, Sutherland says. They had one question: how many horses could they stop feeding if they bought an engine.
So Watt invented horsepower as what Sutherland calls a marketing unit. A 25-horsepower engine, Watt told owners, could replace 75 horses because it worked around the clock while horses worked in shifts.
Owners could scribble feed and care costs "on the back of an envelope," Sutherland says, and conclude they'd take two.
Sutherland notes it does not carry a scientist's name because it was chosen for selling.
Questions people ask
How much power is one horsepower, really?
One mechanical horsepower is defined as 746 watts, enough to lift 550 pounds one foot in one second. The metric equivalent, the watt, is now standard in science and electric vehicles.
Why did a 25-horsepower engine replace 75 horses, not 25?
According to Sutherland, Watt claimed horses worked in shifts while his engine could run day and night. If horses worked roughly three shifts to cover a day, 25 horsepower times three shifts equals 75 horses replaced. The specific three-to-one math is Sutherland's retelling of Watt's sales pitch.
Do we still use horsepower today?
Yes, in cars, lawnmowers, boats, and other engines, especially in the United States and Britain. Electric cars and scientific work more often use watts or kilowatts, named for Watt himself, but horsepower remains common in advertising and everyday talk.