What is horsepower?

Terms like horsepower, brake horsepower, PS and kW are used an awful lot, so we thought it'd be useful to know what they all mean

Simon Ostler James Wilson
Sep 20, 2021

If you’ve ever talked about cars, read a car-related article or watched a motoring television program then you have probably heard the term ‘horsepower’ come up on a very frequent basis. It is also a term that will likely make an appearance if you are looking at purchasing a new or used car, but what does it actually mean?

There are a number of ways to refer to the power output of a car's engine or motor, so it can sometimes be less clear than ideal. For that reason, we've unravelled the mystery of horsepower and laid it bare right here.

Horsepower (hp) is the metric used to indicate the power produced by a car's engine - the higher the number, the more power is sent to the wheels and, in theory, the faster it will go.

Other elements also come into play that affect a car’s performance, such as the weight. A heavier car will need more horsepower to get moving - if two cars both produce 200hp, but one weighs 500kg more, the lighter car will accelerate quicker.

Then there's the issue of how that power is measured. Not everyone takes the reading in the same way, so it's likely you'll find a car that comes with more than one power figure. It's hardly what you call simple, so let's get to it, read on for more info on horsepower.

Why are there different ways of measuring engine power?

The power output of a car can be measured in several ways. Horsepower measures the amount of power produced by the engine, but then you also have brake horsepower (bhp) which only measures the power that reaches the wheels - this will always be less and as such these two are not interchangeable.

Other measures of power that are becoming more commonly used are Pferdestrke (PS) and Kilowatts (kW). These also measure the actual power produced by the engine, but where PS will generally produce a near-identical number to hp, kW is an entirely different metric altogether - 75kW is roughly equal to 100hp.

Where possible, if you are comparing two cars, try to match up like-for-like metrics to find the most accurate comparison.

Kilowatts can be used to describe power output in general. You could take a kW measurement from the wheels, or the engine depending on your fancy. It is more common for kWs to refer to engine power though.

PS vs hp

PS is the most popular measurement for car manufacturers based in Germany, but also throughout much of Europe.

Its popularity is falling though, as the EU’s preferred unit of power is now kW. While it may seem frustrating that the EU should pick such a unit, it makes sense in the long run thanks to electric vehicles (EVs). Electrical power is most commonly quoted in kW, so as EVs become more popular it stands to point that car manufacturers should sell electric cars with units that consumers already understand.

Bhp vs hp

As mentioned above, hp and bhp differ in that they are measured in different places. The power output of an engine is simply hp. Power measured at the wheels is bhp. An easy way to help remember this is that brakes are found inside the wheels of cars hence that is where brake horsepower is measured.

Horsepower vs torque

Car manufacturers often quote hp for the headline figures, but scratch a little deeper into a car’s performance and soon you will come across torque. Don’t panic, this is not another unit of power. Usually measured in Nm (newton metre) or lb/ft (pound-foot), torque is a unit of how much turning force a car can produce.

This can be measured at the engine or the wheels (although there is no helpful adage of ‘brake’ at the beginning to differentiate the two). Torque is responsible for how quickly your car accelerates, the more torque, the easier it is to get your wheels turning.

You may see a torque figure followed by an engine speed, for example 1,500-2,500rpm. This means peak torque delivery is around these revs, so it's better to change up a gear if the revs are too high. Electric cars differ here, though, because torque is delivered instantly - there's no need to build the revs up.

The history of hp

The term horsepower dates back to the black and white times when a man named James Watt (as in the Watt in kilowatt) supposedly used horses to define how quickly he could remove materials from a mine. Over time the term stuck, but various alternatives have cropped up (such as kW and PS), as has confusion!

 

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