Horsepower, PS and kilowatts

There are two horsepowers

This is the single fact that explains most of the confusion, and almost nothing states it openly.

Mechanical horsepower — the one used in Britain and the United States, sometimes written bhp — is defined as 550 foot-pounds per second, which works out to about 745.7 watts. Metric horsepower is defined differently: 75 kilogram-force metres per second, which is about 735.5 watts. It is written PS in Germany, CV in France, Italy and Spain, ch elsewhere, and pk in the Netherlands.

The gap is 1.4 percent, and it runs in a direction that flatters the metric figure. Because each metric horse is slightly smaller, a given engine produces more of them. An engine rated 300 hp is 304 PS. One rated 200 hp is 203 PS. Nothing about the engine changed; only the size of the unit did.

That is why a car can be advertised with two different power figures in two markets and both be honest. It is also why a conversion that silently assumes one definition will be out by a percent and a half — small enough to look like rounding, large enough to be wrong.

Kilowatts are the official number

In the European Union and much of the world that follows its conventions, the kilowatt is the legal unit for engine power. It is what appears on a vehicle registration document, what tax bands and insurance categories are calculated from, and what the paperwork means when it disagrees with the brochure.

The watt is SI, so it needs no definition of its own beyond the metre, kilogram and second, and it does not care which horse anyone had in mind. That is exactly why regulators chose it — a unit with two competing definitions is a poor basis for a tax band.

Manufacturers keep quoting horsepower alongside it because that is what buyers understand, and in most European markets the marketing figure is PS rather than hp. So a car sold in Germany is commonly described as, say, 150 kW / 204 PS, and the same car reaches an American brochure as 201 hp. All three describe one engine.

When two sources disagree, the kilowatt figure is the one to trust. It is the number on the official document and the only one of the three that cannot be quietly restated in a different definition.

Electric cars have quietly inverted this. An electric motor is specified in kilowatts by the people who design and build it, because that is the unit the battery, the inverter and the charging connector are already measured in — so the kilowatt figure is the real one and the horsepower number in the brochure is the translation, rather than the other way round.

That makes the arithmetic unusually easy to sanity-check, since the same unit appears twice on the same spec sheet meaning two different things. A car with a 250 kW motor and a 77 kWh battery is quoting a rate and a quantity: kilowatts is how fast energy moves, kilowatt-hours is how much of it there is. Charging speed is quoted in kilowatts for exactly the same reason, which is why a 150 kW charger and a 150 kW motor are describing comparable rates of energy flow in opposite directions.

Where the power was measured matters more than the unit

A conversion error costs you one and a half percent. Measuring at a different point in the drivetrain costs you fifteen or more, so this is the larger effect by a wide margin.

Brake horsepower is measured at the crankshaft, with the engine on a test bench. Wheel horsepower is measured at the wheels on a rolling road, after the gearbox, differential and driveshafts have each taken their share. The loss is commonly around 15 percent for a rear-wheel-drive car and more for all-wheel drive, because there is more machinery in the way.

This is why a car advertised at 300 hp can show 255 at the wheels and nothing be wrong. The two numbers are answers to different questions, and neither is the manufacturer being dishonest — but comparing a brochure figure against a dyno printout is comparing a crank number with a wheel number, which will always look like a shortfall.

Test standards add another layer. Modern figures are net — measured with the alternator, water pump and exhaust system attached, as the engine actually runs in the car. American figures before the early 1970s were commonly gross, measured with those accessories stripped off, which produced numbers that were substantially higher and are not comparable to anything quoted today. A classic car whose original brochure claims a startling output is usually quoting gross.

Power is not the number that decides how it feels

Worth saying, because the conversion question is usually a proxy for a comparison question.

Power describes the rate at which work is done, and for a car it mostly determines what happens at the top end — how hard it pulls once it is already moving quickly, and what it will eventually reach. It is a poor predictor of the thing most people actually notice.

Torque and where in the rev range it arrives is closer to what a car feels like from a standstill or when overtaking. Two engines with identical peak power can behave completely differently if one delivers its torque from low revs and the other needs to be worked hard to find it. This is much of why a diesel and a petrol engine of the same stated output feel unrelated to drive.

And weight matters as much as either. Power-to-weight ratio, usually quoted in kilowatts or horsepower per tonne, compares far more usefully across cars than power alone: a light car with modest output routinely outperforms a heavy one with more. If the reason for converting a figure is to decide whether one car is quicker than another, dividing by kerb weight will tell you more than the conversion itself.

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