Science & Technology

Horsepower Doesn't Mean What You Think It Means

The Tesla S P90D has 259 horsepower behind its front axle, and 503 horsepower in the rear. Just imagine — the power of more than 700 horses in an electric car. Except, that's not true at all. One horse doesn't equal one horsepower. In fact, it was never meant to.

Power ... Unlimited (Horse) Power!

Let's just rip this misconception off like a band-aid: working at peak performance, a horse can produce about 14.9 horsepower, and a human can produce about 5. Assuming you're not some sort of superhero with the strength of five horses, it should be pretty clear from that fact that one horsepower doesn't equal the power of one horse. And actually, if you think about it, it would be a little weird if it was. After all, not all horses are created equal, so that Tesla might only have 500 or so Belgian draft horses but thousands and thousands of Thumbelinas.

When we learned that a horse can produce way more than one horsepower, we had a lot of theories about why. Some of us assumed that it was because horses back then were smaller or weaker. But nope, the measurement was never meant to line up with the energy that a horse could actually produce. And when the more we learned about the reasoning for coming up with the unit, the more it made sense.

The Meaning of HP

So what is a horsepower? In the US, it's 33,000 foot-pounds-per-minute. (Metric countries, hold your horses. We'll get to you soon.) That means that if you hook up a 33-pound bucket to a horse exerting one horsepower, it can get dragged 1,000 feet in a minute. Increase the weight to 330 pounds, and the horse can only go 100 feet; bring it down to 1 pound and that horse is going to move 33,000 feet in a minute (theoretically). The unit was invented by James Watt (as in, the guy a "watt" is named after), after the amount of work the average horse could be relied on to do. After all, you're not going to be running your horse at top capacity all day long — you can't, realistically. So yeah, a horse can produce 15 horsepower and a human can produce 5, but neither of them are going to be able to keep up that pace for long.

Of course, with a measurement this antiquated, you can expect that it hasn't even been touched by the metric-imperial battle. Not so! Countries that use the metric system do the same calculations James Watt laid out back in the day, but instead of foot-pounds, they calculate meter-kilograms. That means that the German pferdestärke (PS) and the Italian cavalli vapore (CV) are both measures of horsepower, but they're 1.4 percent less than the horsepower measurements you get in the US. And you thought Celsius was confusing.

Why Engines are Commonly Measured in Horsepower

Written by Reuben Westmaas February 8, 2018

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