HP, BHP, PS, kW and the rest

What’s in a Power Unit?

One of the first things people want to know about a car is its power rating. The want to know how fast it will go and in how short a time they might make it from A to B. So, how exactly are these power ratings articulated?

Herein lies the problem. It seems that automakers around the world have been united by their enduring disagreement on which unit of measurement works best. Below is a selection of units currently in use:

• Horsepower (HP) – arguably the most common and best-known and often measured using a dynamometer
• Brake-horsepower (BHP) – a HP unit that takes into account minus friction-based energy loss
• Pferdestärke (PS) – a unit close to HP commonly used in Europe
• Kilowatt (kW) – the favoured unit of the physicist
• Cheval-vapeur (CV) – French equivalent of horsepower

First, what are the similarities between these units?

The obvious thing that binds all the units together regardless of their provenance or popularity is that they measure the same thing — power. Though they appear on the surface to house all kinds of differences, there are some quite striking similarities:

Horsepower and brake-horsepower and obviously the closest in meaning. Where horsepower is a rather “raw” unit, brake-horsepower merely tries to improve on accuracy by factoring in the energy loss of an engine caused by friction. That was the idea, at least, but nowadays it seems that the good-old “horsepower” ratings that you see actually take this loss into account anyway.

PS is used in Germany and was created as part of an attempt to make the unacceptably random-seeming horsepower unit more metric. The two units have ended up being quite close, in fact, with 1 PS being equivalent to 0.98HP. The cheval-vapeur (CV) is also very close to HP, with 1CV equalling 0.99CV.

What about the important differences?

When it comes to difference, we have to bring in the physicists’ favourite – the kilowatt (kW). While the automakers scramble about with their different units which all have different names but are within a hair’s breadth of each other in reality, the scientists prefer to use something a bit more standard.

The kW is that standard unit. A single unit of horsepower is the same as 0.74kW, which means that when you convert a car’s power into kilowatts, you do end up with a smaller number. This is anathema to some automakers, clearly, who want their power ratings to sound as high as possible.

This difference potentially explains why European automakers like using PS, since that yields a bigger number even than horsepower. A car with 400PS rating would only be 392hp. There’s something quite appealing about that bigger number.

Why do we even need this power rating?

To some drivers it may seem arbitrary and meaningless. If we’re interested in the car’s comfort, safety features or gas mileage, then the PS or horsepower number might not be interesting. To others, however, it is an essential.

Those with a need for speed will be very keen to know this number, since it indicates how fast the car will go; whether it’s a performance machine or not. They may also want to gain some idea of what it will feel like to drive. The high-hp cars are undoubtedly more exciting for those who seek the thrill.

On the other hand, insurance companies are also interested in power ratings to combine in their formula for calculating premiums. A high-hp car is likely to be seen as riskier, therefore they are very interested to know.

Power units in the age of electric

In the end, perhaps kW will have the last laugh. As more cars turn electric, the kWh rating of the car’s battery is more important than ever to drivers. In the age of the EV, driving distance on a single charge is the new priority of many. Will our beloved power ratings fade into obscurity? Regardless, it is still valuable to know the differences and similarities between them.