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 30, 2019

The term 'horsepower' can be a source of confusion and if you're looking into buying a new car it's likely that you'll come across it. But what does it actually mean? Well, there are numerous ways to refer to the power output of a car, so it's sometimes less clear than it ought to be. For that reason we've put together everything you need to know about horsepower right here.

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

However, in reality, there are other elements such as the weight of the car that will also affect a car's outright speed. 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.

 

Why are there different ways of measuring engine power?

The power output of a car's engine 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 becoming more commonly used are Pferdestrke (PS) and Kilowatts (kW). These are mostly interchangeable with horsepower, as they too measure the actual power produced by the engine.

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 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 consumers 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 yet 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.

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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