A Quick Note on Horsepower

If you want to appreciate the impact of energy and power in your everyday life, I can’t recommend the book Energy and Civilization by Vaclav Smil highly enough.

For me, the book also helped me understand more about my family history and provided a window into their daily lives, especially his discussion on horsepower.

I come from a long line of farmers in Central Wisconsin. Both of my grandpas eventually bought tractors for the farm work, but when they were kids, almost all of the work that wasn’t done by humans was done using horses. True to his name, my Grandpa Phil loved horses and used them on the farm as an adult.

Horsepower (hp) is a unit of power, measuring the rate at which work is done, and it quantifies the output of mechanical devices, especially engines. The term gained steam—no pun intended—in the late 18th century when James Watt developed important improvements to the steam engine and needed a way to market his invention in a unit that his potential customers could easily understand: horsepower.

Watt’s horsepower measured the amount of work one horse could steadily perform throughout the course of an entire day on a sustained basis, not the maximum power a horse could provide in short bursts, which can be as high as 15 horsepower.

A human can work at a steady rate of about 0.13 horsepower, meaning a horse can perform about 7.5 times more work than a human over the course of a day. This means my Grandpa Phil was able to do about 15 times more work over the course of the day, thanks to his team.

It’s one thing to read about horsepower and its impact on productivity, but if you want to feel the difference between horsepower and no horsepower, try using a rotary blade push mower to mow a quarter-acre yard —like I foolishly did when we first bought our house— and then do the same thing with the gas-powered lawnmower.

Once was enough for the rotary mower, but it was not lost on me that the small, gas lawnmower I purchased has a four-horsepower engine, which means the small machine in my garage does twice as much work as my grandpa’s horses could or the equivalent sustained output of 30 men.

The lawnmower also helps explain the