What a watt measures
Power is the rate of doing work: one watt is one joule of energy every second. A kilowatt is simply a thousand of them. Horsepower is older and stranger. When James Watt was selling steam engines in the 1780s he needed a number his customers understood, so he rated his machines against the horses they replaced and settled on 550 foot-pounds of work per second. Continental Europe later built a metric version, the Pferdestarke or PS, defined as lifting 75 kilograms one metre in one second. The two horses are nearly the same size, but not quite.
Both horsepower definitions are exact, because the pound, the foot and standard gravity are all exactly defined: mechanical horsepower is 745.699872 watts and PS is 735.49875 watts. PS is about 1.4 percent smaller, which matters more than it sounds when the number on the brochure is the whole point.
hp, bhp and PS on car spec sheets
European manufacturers traditionally quote PS, the old German DIN horsepower, while British motoring habits favour bhp. The b stands for brake, the dynamometer that measured the engine, so bhp is mechanical horsepower by another name rather than a different sized unit. French cv and Italian cavalli are PS again in local dress. Because PS is the smaller unit, the same engine always carries a slightly bigger PS number: a 150 PS hatchback is 148 bhp, and a 110 kW motor is about 150 PS. The European Union's official unit is the kilowatt, which is why every spec sheet now lists kW first and why electric cars are pushing kilowatts into everyday car talk. When you compare two cars, make sure both figures are in the same flavour of horsepower, or convert both to kW and be done with it.
Worked examples
kW or kWh, power or energy
The kilowatt trips people up most when it meets the kilowatt-hour on an electricity bill. A kilowatt is a rate, like speed; a kilowatt-hour is an amount, like distance. Run a 3 kW kettle for ten minutes and you have used 3 × 10 ÷ 60 = 0.5 kWh of energy. Charge an electric car on a 7 kW home wallbox for eight hours and 56 kWh has gone into the battery. Your supplier bills you for the kWh, the amount, not the kW, the rate, so a powerful appliance used briefly can cost less than a feeble one left running all day. If you want to put pounds against those numbers, our energy running cost calculator does exactly that, and the fuel cost calculator covers the petrol equivalent. For the rest of the car maths, the speed converter and MPG converter are the natural companions to this page.
Frequently asked questions
What is the difference between hp and PS?
They are two different horsepower definitions. Mechanical horsepower (hp) is 550 foot-pounds per second, which works out at 745.7 watts. PS, metric horsepower, is 75 kilogram-force metres per second, or 735.5 watts. PS is about 1.4 percent smaller, so the same engine always shows a slightly bigger number in PS than in hp.
What is 100 kW in horsepower?
100 kW is 134.1 hp, or 136.0 PS. To convert kilowatts to horsepower, multiply by 1.341 for mechanical hp or by 1.360 for PS. Going the other way, 100 hp is 74.6 kW and 100 PS is 73.5 kW.
Why do car adverts use PS instead of hp?
PS is the traditional continental European unit, standardised in Germany as the DIN horsepower, and most European manufacturers still quote it. UK adverts mix PS and bhp freely, which muddies comparisons because the PS figure is always slightly higher for the same car. The official EU unit is actually the kilowatt, which is why spec sheets list kW alongside.
Is bhp the same as hp?
For everyday purposes, yes. The b stands for brake, meaning the power was measured at the engine's output shaft with a brake dynamometer rather than calculated on paper. It describes how the figure was measured, not a different sized unit, so bhp converts exactly like mechanical hp in this tool.
Are these conversions exact?
Yes, apart from display rounding. Mechanical horsepower is defined exactly through the international pound and foot, giving 745.699872 watts, and PS is defined as exactly 735.49875 watts. The tool uses the full factors internally and rounds only for display.