What Horsepower Actually Measures

One horsepower is 745.7 watts. Or 746 watts. Or 735.5 watts. All three are in current use, they all get called horsepower, and the difference is large enough to change a car's advertised output by several units.

Watt invented it as a sales tool

James Watt was selling steam engines to mine owners who currently used horses. He needed a unit those buyers already understood, so he measured what a mill horse could do turning a wheel and rounded it: 33,000 foot-pounds per minute, or 550 foot-pounds per second.

That is the mechanical horsepower, and converting it to SI gives 745.6999 watts. There is nothing natural about the number — it exists because a marketing problem in the 1780s needed a solution.

Real horses, incidentally, can substantially exceed one horsepower in short bursts. Watt was measuring sustained work, which is what mattered to someone replacing a horse that had to keep going all day.

The three horsepowers

Name	Watts	Where it is used
Mechanical (hp)	745.70	UK and US engines, general use
Electric (hp)	746 exactly	US motor nameplates
Metric (PS, CV, pk)	735.50	European car specifications

Metric horsepower was defined as 75 kilogram-force metres per second — the same idea as Watt's, worked out in metric units, landing about 1.4% lower.

This is why the same car appears with two different power figures depending which market is advertising it. A car quoted at 150 PS in Germany is 148 hp in Britain. Nothing has changed but the unit, and neither figure is wrong.

Convert in either direction with the horsepower to watts converter or watts to horsepower.

Why electric cars sound weaker than they are

Electric motors are rated in kilowatts, because that is the honest SI unit for power and because the same number relates directly to the battery and the charger.

A 150 kW motor is 201 hp. Quoted in kilowatts it sounds like a kettle; quoted in horsepower it sounds like a hot hatch. Same motor.

Kilowatts	Horsepower	Typical
0.75 kW	1 hp	Workshop extractor, small pump
3 kW	4 hp	Generator, pressure washer
75 kW	101 hp	Small city car
150 kW	201 hp	Warm hatchback, mid-range EV
500 kW	671 hp	Performance EV

Power is not the thing you feel

Horsepower is a rate of doing work. Torque is the twisting force available at a given moment. The two are related by engine speed:

Power = torque × rotational speed

An engine producing enormous torque at low revs and an engine producing modest torque at very high revs can quote identical horsepower and feel nothing alike to drive. The first pulls from idle; the second has to be revved to do anything.

This is the substance behind the usual complaint that a car "has the numbers but feels flat" — peak horsepower is a single point on a curve, and the shape of that curve is what you actually experience. Manufacturers quote the peak because it is one number and it is the biggest one.

Electric motors sidestep the argument entirely by producing near-maximum torque from zero revs, which is why a modest EV feels quick from a standstill and why comparing it to a petrol car on horsepower alone tells you very little.