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

James Wilson
Aug 13, 2019

If there is one thing the motor industry has made about as clear as mud, it is the meaning behind the term 'horsepower' (hp). Most people have seen the hp abbreviation when looking into cars, but chances are you will have seen bhp, PS and kW, too. So what does it all mean?

Horsepower is the power output of an engine. Put in layman's terms this means how much energy an engine can produce in a given period of time. A car producing a lot of hp can send a lot of energy to the wheels helping drive you down the road faster.

You would expect then, the more horsepower a vehicle has the quicker it will go. In reality, it depends just as much on how much the car weighs. The heavier the car is, the more energy you need to get it moving. A Challenger 2 tank has around 1,200hp, yet will struggle to crack 40mph. A garden variety Ford Fiesta typically has around 100hp but will sit at 70mph (and then some) all day long.

 

Realistically, 75 to 200hp is ample for standard day-to-day driving. In most cases, smaller cars such as the Seat Mii will sit at the lower end of the power scale and larger vehicles such as the Kia Optima will sit at the higher end.

Move into the realms of supercars or even hypercars and the power figures soon get ridiculous. For example, a modern-day Porsche 911 has around 380hp (a Mk1 911 has had around 128hp) and will go from 0-62mph in 4.2 seconds.

Why is there so much confusion around horsepower?

The confusion between hp, bhp (brake horsepower), PS and kW arises because hp and bhp aren’t strictly interchangeable while everything else is. You can convert from hp into PS into kW and back again just fine. Likewise, you can convert bhp into PS into kW and back with no problems. You cannot, however, go from hp to bhp.

The reason being, while hp is the power output of an engine, bhp is the power output of a car at its wheels. The difference between the two is any loss of energy through a car’s drivetrain, such as friction in a gearbox.

Similarly, adding things such as four-wheel drive can have a profound impact on the difference between hp and bhp. You can apply a rule of thumb for converting hp to bhp, but as every car is different the only way to know both values exactly is measure them.

Kilowatts or kWs are used to describe power output in general, you could take a kW measurement at the wheel, the engine or if you really fancied it, halfway between the two. Commonly though, kW refers to an engine’s output just like hp. Cynically speaking, quoting hp, PS or kW is an easy way for manufacturers to make their cars look more powerful as the number will always be marginally bigger than the bhp measurement.

PS vs hp

PS is shorthand for Pferdestärke, which is horsepower in German. It's a 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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