Sometimes the units we use are downright bizarre.

The metric system makes all sorts of sense, but, in the United States, we’ve decided to stick with standards that include the width of human feet (foot), the width of human thumbs (inch), the amount of land a yoke of oxen could plow in a day (acre), the distance from a king’s nose to his thumb (yard), and a thousand paces (mile).

But whose foot? And whose thumb? And which oxen? Which king? In this case, likely King Henry I. Was the distance from his nose to thumb vastly different from most people? Do we know for sure how long his unit was? Whose pace reckoned the mile? Some of the specific measurements even have multiple possible origins. For example, the inch might be the width of a thumb or the length of three barleycorns, a measurement of the size of a grain of barley. 

Another artifact from the imprecise past is horsepower. Still, in the 21st century, manufacturers tout the horsepower measurements of cars. Electric cars can produce more than 1,000 horsepower. The Ferrari SF90 can pump out 986. On the other end of the spectrum, a 1.8 liter Toyota Corolla features the power of 132 horses. Even the weakest automobile certainly seems to have the potency of a lot of equines. But what exactly is horsepower?

This realistic photograph shows the precise distance from King Henry I's nose to his thumb

The development of horsepower as a unit started in the 18th century. At the time, horses ruled the mechanical world of work. Scottish engineer James Watt developed a steam engine in 1765, an improvement over contemporary technology that went a long way to initiating the Industrial Revolution. But, in a world of horse titans, how could one convince the masses that your steam engine is more efficient than four-legged powerhouses?

Watt embraced the term “horsepower” to compare his engines to the real thing. He was certain that steam power was superior to physical, but could he prove it? How much power did a horse actually possess?

Watt worked on a way to extract hard numbers. After some experiments, he estimated that one horse could turn a mill wheel with a 12-foot radius at a rate of 2.4 rotations per minute. At a mine, using the wheel as a pump, this speed meant the horse could lift 550 pounds of water one foot in one second. So, one horsepower is around 33,000 foot-pounds per minute.

In this image, James Watt would have an extremely short yard measurement

Of course, this unit is a generality. Not all horses are equal.

As stated in the name, horsepower is a unit of measurement of power, which has a scientific definition. Power is the rate at which work is completed. In physics, work is the energy transferred to or from an object via the application of force along a displacement. Today, we use the watt as the SI unit of power. One unit of mechanical horsepower equates to approximately 746 watts.

This comparison is not really helpful to the discerning student of power. What can one horse or 746 watts achieve?

A visualization of the original definition of a horsepower - graphic by Sgbeer and Martinvl

The camera light on a cellphone runs at 1 watt. A typical LED bulb requires 7 watts to run. Charging a laptop pulls 30-60 watts. Televisions operate on power between 30 and 100 watts. A drill runs on power between 500 and 900 watts. A microwave can vary between 600 and 1,200 watts. So, horses can generate quite a bit of power.

What about variation between horses, though? Was Watt’s original equine expending maximum energy or merely completing the job? A study published in Nature reported that a horse can beget as much as 14.9 horsepower over a few seconds. They also noted that Watt’s measurement of horsepower is roughly in line with the rate at which a horse can sustain work in an agricultural setting.

What about humans? At our peak, we can pump out about 1.2 horsepower; on a sustainable level, we can produce just one-tenth of what a horse can. High-level athletes can drive 2.5 horsepower for brief periods and up to 0.35 hp for longer spans. During Usain Bolt’s world record performance of the 100-meter dash, in which he finished the race in 9.58 seconds, he generated a maximum of 3.5 horsepower in the first second of the undertaking.

The true power of a horse is love - photo by Peter.shaman

Has any of this investigation allowed you to better visualize how mighty a car is when its makers tout the horsepower? The Ferrari SF90 can perform work at a rate commensurate to a horse lifting 550 pounds of water 986 feet in one second. That would be one robust horse. Of course, no one really wants to lift water with a Ferrari.

When it comes to work and horsepower, the units are a bit difficult to visualize. Bigger is more powerful, of course, but what the figures represent is not necessarily straightforward. Perhaps we would be better served if we ditched horsepower and the metric watt, instead utilizing a new unit based on Usain Bolt.

One bolt is equal to the power required to run a 100-meter dash in 9.58 seconds. That’s a unit I can conceptualize!

Further Reading and Exploration

How Much Horsepower Does a Horse Have? – Voltage Lab

How Horsepower Works – How Stuff Works

James Watt And The Revolution Of Horsepower – The Chronicle of the Horse

Horsepower from a horse – Nature

Scientists model ‘extraordinary’ performance of Bolt – Phys Org

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