The power output of your car’s engine can be measured in a variety of different ways, but the most common is in horsepower.
At a quick glance, you might think that horsepower means exactly that: the power of a horse. But does one horsepower really equal the power of one horse?
What is horsepower?
Horsepower is a unit of measurement for power, which is the rate at which work is done. If you imagine ‘work’ as pushing a weight up a hill, then ‘power’ is how fast you do it.
The term was originally coined by Scottish engineer James Watt to compare the output of steam engines with the power of work horses. It was later expanded to include the power outputs of piston engines like those found in your car, plus turbines, electric motors and other machinery.
Does one horsepower equal one horse?
Not quite. It’s a common misconception that one horsepower is equal to the peak power production of a horse, which is capable of a maximum of around 14.9 horsepower. By comparison, a human being is capable of approximately five horsepower at peak power production.
Instead, Watt designated horsepower to be equivalent to the amount of power that a horse can sustain for an extended period of time. However, there are many different variations of horsepower.
How can horsepower have different definitions?
Unlike the measurement of, say, time, the exact definition of horsepower can vary depending on regional variations and the specific machine whose power is being measured.
Mechanical horsepower, also known as imperial horsepower, is the measurement invented by James Watts and is approximately equivalent to 745.7 watts of power. Pferdestarke, also known as PS or metric horsepower, is a similar measurement developed in Germany that’s equivalent to 735.5 watts.
Horsepower for electric motors is equivalent to 746 watts, while boiler horsepower, which is used to measure the output of steam boilers is more different still, with one boiler horsepower roughly equivalent to 9,810 watts.
Anything else I should know?
When it comes to your car, in the UK the most commonly used measure of engine power is brake horsepower, or bhp.
While your engine may produce, as an example, 100 horsepower, the various mechanical parts of your car’s driveline will mean that not all of that power will eventually reach your wheels.