What is BHP? And how is it different to HP, PS and KWs?
There are many ways to measure the power output of a vehicle. Here we explain what the different metrics mean...
When researching a new or used car, it’s likely you will encounter its power output. However, rather confusingly, there are different ways to measure this, including brake horsepower (bhp), horsepower (hp), pferdestärke (PS) and kilowatts (kW).
So, here in this What Car?-style physics lesson, we explain what each unit means and how it’s measured.
What is brake horsepower?
Brake horsepower (bhp) is a traditional way of measuring how much power an engine produces. It’s slightly different to horsepower (hp), because hp is measured at the crankshaft of the engine, whereas bhp is measured at the driven wheels.
There’s always a slight difference between the two, with horsepower coming out slightly higher. This is because bhp takes into account frictional loss, whereas hp doesn’t. For example, a Volkswagen Polo with a 1.0-litre petrol engine produces 94bhp, but 95hp.
It’s worth noting the difference between the two increases with power output, so a BMW 3 Series M340i produces 369bhp, which equates to 374hp.
At What Car?, we always quote bhp to allow for accurate, consistent comparisons across our reviews.
How is bhp measured?
A dynamometer (also known as a dyno or rolling road) is used to measure an engine’s bhp. Specifically, it measures the rotational speed (revolutions per minute or RPMs) of an engine or electric motor, and ultimately the brake horsepower.
The way it works is it places a load on the engine or drivetrain of a vehicle and it then evaluates how the engine or motor reacts when it’s building speed. The most common type of dynamometer for cars is a chassis dyno, which places the vehicle’s driven wheels on weighted rollers that create resistance to measure the power.
What is PS?
Pferdestärke (PS) is the German translation for horsepower, and is the metric measurement of hp. This means it’s slightly different to hp: one PS is equal to 0.9863 of one hp. Most European car manufacturers quote PS.
How do you convert bhp to PS?
As a rule of thumb, you can knock off one PS for every 100PS to reach a rough bhp figure. If you want to be completely accurate, multiply the PS figure by 0.9864 to reach the bhp total, or bhp by 1.0139 to get back to PS.
What is a kW?
A kilowatt (kW) is a measurement of power commonly used for electric cars, but it can be used for internal combustion engine cars as well. It’s slightly different to bhp and hp, because it measures how fast the car can use energy to drive the wheels.
With an electric car charger, it can also be used to measure the charging power, which indicates how fast the car's battery can be charged.
A European Union (EU) Directive tried to replace the use of PS with kW in the 1990s. However, few motorists had any immediate understanding of kW ratings, so car makers mostly stuck with PS. That said, some manufacturers of electric cars have now started to quote kW figures.
What is a kWh?
A kilowatt hour (kWh) is different to a kW, because the former measures the amount of energy available to use. The capacity of an electric car’s battery is measured in kWh, for example.
How do you convert bhp to kW?
One bhp is equivalent to 745.7 Watts, so multiply bhp by 0.7457 to get a kW figure, or kW by 1.341 to do the sum in reverse.
What is torque?
Torque is the measurement of the engine’s pulling power, so the more of it you have, the easier it is to pull a heavy load at a consistent speed. This is different to bhp, hp and PS, where a higher figure helps you accelerate faster and to an increased top speed.
Diesel engines typically produce more torque than petrol engines (plus they deliver it from lower revs), so they’re generally better for towing.
Electric cars often produce a lot of torque, too. However, there’s a key difference with the way an electric motor delivers power compared with an internal combustion engine, because it doesn’t need to build revs to build power. Instead, it delivers its power and torque almost instantly, which is why some electric cars can accelerate so quickly; the Tesla Model 3 Performance, for example, can get from 0-60mph in just 2.9sec.
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