Follow @rpmgo

What’s the difference between kW, HP, BHP, PS or WHP?

If you’re remotely interested in the car industry, at one point in time you’ve asked yourself this question: What’s with all the various power ratings? We’ll see an engine develop X amounts of either kW, HP, BHP, PS or even WHP, depending on where we read the information.

That’s why we decided to answer this question, and explain in simple terms just what is the difference between all these ratings, and how they are measured nowadays with new cars.

Curious to find out? Hit the jump then.

All these measuring units, be them kW, HP, BHP, PS or WHP, tie into the history of engines and how they were rated over the years, so that every one could understand the power they could output at peak operating performance.

Let’s start with HP shall we?

Believe it or not, horses were pretty popular back in the day. But steam engines were quickly catching up … steam (bad pun, we know). So James Watt decided to market his engines more accurately, so that everyone could understand why it was better to use his machines than conventional horses.

He then measured the power output of one horse, and started saying that his engines developed X amounts of this so-called Horse Power. People were really excited, and the rating system became pretty popular, as eventually every new engine in English-speaking lands was marketed with how many HP it could deliver.

But enough with the history lesson. HP became used in every day life, even though the International System of Units insisted on using watts to accurately measure power. As such, 1 HP = 745.7 watts. Seeing as how most engines or motors these days produce adequate amounts of power, the kilowatt is used (1 kW=1,000 watts). This results in 1 HP = 0.745 kW.

The difference between HP and BHP (brake horse power) didn’t even exist until recently. In order to get bigger numbers, engine manufacturers (including carmakers), placed a single unit on a testing rig, and connected the drive shaft to a device called a “brake”. It measured the pure horse power coming from the engine, without any auxiliary units.

But seeing as how that wasn’t accurate with what customers were experiencing, a new SAE certification process appeared. The engine was now measured with all of its auxiliary systems, like transmission, fuel pumps, etc, while an independent observer was present. As you can imagine, a reduction in power ratings was experienced, as many systems take a toll on the eventual output of the engine.

BHP is still used regularly in Great Britain (it is mistakenly explained as British Horse Power, sometimes). The term is slowly being phased out in favor of the regular HP. There isn’t an accurate mathematical connection between HP and BHP, as it all depends on the engine and the systems connected to it.

PS is often overlooked, especially in the English media. It is the abbreviation of a German term, Pferdestärke, which basically means horse power. But it isn’t exactly the same as the ol’ English HP. 1 PS = 735.5 watts (or 0.735 kW). In order to accurately figure out the difference, 1 PS = 0.98632 HP.

Carmakers and tuners often add this measurement as well, particularly because it is bigger when talking about powerful machines. Besides Germany, PS is also used in Japan and other territories.

Last but not least, WHP. It comes from wheel horse power, but you can also find it as RWHP (rear-wheel horse power). Basically this is the rating that comes out when you put the car on a rolling road (aka dyno). It measures the amount of power the wheels actually send to the road. Of course the tires make a difference, and the type of dyno, but the rating is usually pretty close to the SAE-verified HP rating.

So there you have it. The official measuring unit (rating) for a vehicle engine is kilowatts (kW). Most of the times you’ll find it as HP, BHP or WHP (all equaling 0.745 kW) but also as PS (meaning 0.735 kW). Next time a carmaker flaunts various ratings in your face, you’ll know just how powerful the engine really is.

This has been a part of our RPMGO Answers feature, which tackles various automotive debates or questions, and tries to find a simple explanation or to reveal all sides of the discussion.

If you want a specific question to be answered, why not contact us?

2 Responses to What’s the difference between kW, HP, BHP, PS or WHP?

  1. Nosfer says:

    Now this is a definitely useful info. I always thought PS is the same with HP and I was a bit confused when press releases had PS and Bhp with two close values, and didn’t knew which one is exactly HP.

  2. dr kvr says:

    why in nissan car cc is more and ps is less ?

Leave a Reply

Your email address will not be published. Required fields are marked *


You may use these HTML tags and attributes:

More in RPMGO Answers (2 of 4 articles)