What Is Horsepower?

Definition

Horsepower (hp) is a unit of power — the rate at which work is done or energy is transferred. Power is not the same as force or energy; it measures how quickly energy moves. One watt equals one joule per second, and horsepower is simply a larger, more relatable package of the same idea.

The unit was defined by the Scottish engineer James Watt in the 1780s. Watt observed draft horses turning a mill and estimated the work one horse could sustain. He settled on the figure of 33,000 foot-pounds per minute — equivalent to 550 foot-pounds every second — and called that amount of power one horsepower.

That original definition still anchors the modern mechanical horsepower used for car and engine ratings: 1 hp = 745.699 watts. Because different industries adopted slightly different reference figures, several flavours of horsepower exist today.

Types of Horsepower

Not all horsepower is identical. The four definitions below are the ones you are most likely to encounter on a spec sheet, and they differ by a few watts each.

Metric horsepower is often labelled PS (from the German Pferdestärke) or CV (French cheval-vapeur). It is about 1.4% smaller than mechanical hp, which is why a European engine quoted in PS shows a slightly higher number than the same engine quoted in hp.

Horsepower to Watts/Kilowatts

To convert mechanical horsepower to kilowatts, multiply by 0.7457. To reverse it, multiply kilowatts by 1.341. The table shows common reference points.

Where You See It

Horsepower remains the headline figure for anything with an engine or motor. Car and motorcycle makers quote it to describe peak engine output, and enthusiasts use it to compare performance across models.

You will also find it on lawn mowers, outboard boat motors, generators, air compressors, and industrial pumps. Electric tools and HVAC equipment often list horsepower alongside watts. To move between any of these units instantly, use the power converter.

History

When James Watt began selling his improved steam engines, his customers were buyers who still relied on horses to drive pumps and mills. Watt needed a way to express how many animals each engine could replace, so he framed his product in terms his market already understood: horses. The unit was as much a marketing tool as an engineering one, and it worked — it let a buyer reason that a 10-horsepower engine did the work of ten horses without the feed, stabling, or rest.

The unit outlived the steam era and the horse it was named after. The SI unit of power, the watt, was later named in Watt’s honour, and 1 horsepower now equals 745.699 watts by definition. Two centuries on, horsepower still sells engines — proof that a relatable unit can outlast the technology that inspired it. Compare any rating yourself with the power converter.