# Conversion from kW to HP?

Many of the European manufacturers of cars and motorcycles publish their ICE power numbers to reflect kW rather than HP.

Is there a reason for this?

What is the conversion for kW to HP?

• There's PS as well...
– Zaid
Jan 19, 2016 at 19:56

Wikipedia has this nice table:

``````               PS          hp          kW
1 PS     =     1           0.98632     0.735499
1 hp     =     1.01387     1           0.74570
1 kW     =     1.35962     1.34102     1
``````

Finally, this are all different measures of power.

PS an hp are both a measure of the power of a horse, though it's obviously not clear how to define it. It was somehow estimated by James Watt who wanted to compare horses with his steam engines, i.e. how many horses a steam engine can replace... He finally defined that a horse has a power of 33,000ft*lbf/min, which he called 1HP. So, a HP is even not based on any imperial units...

The Germans didn't like imperial units and searched for a more SI-unit based re-definition, which they found with this: 1PS (Pferdestärke, literally horse strength) is the power needed to lift 75kg by 1 meter within 1 second.

However, the difference between PS and HP is more or less negligible.

PS isn't an official unit for power since 1978 in Germany any more. It has to be written together with the value in kW, and since 2010, the kW value must be more emphasized than the PS value.

But though officially kW is the only valid unit, everybody is still talking about PS here.

Ah, and by the way: PS and HP are not the only units for power. Sometimes, they don't differ in definition, but in test conditions for the car...

It's like asking why do certain countries drive on the left side of the road and other on the right side?

kW is part of the SI system of units. Horsepower is a legacy from the imperial units system.

kW stands for kilo-watt - a measure of power, or energy per unit time, named after the Scottish inventor, James Watt.

hp is horsepower. Ironically, it was James Watt (whose name is used to define power in the metric system) who played an important role in defining the quantity of power a horse could provide.

It would be wonderful if everyone could use SI/metric units because they do away with all sorts of arbitrary conversion factors. My favorite example is the US gallon, which holds exactly 231 in³. No more, no less, no discussion.

It is difficult to get regions to change the unit norms that they have been used to for so long, which is why both remain both prevalent and relevant.

• My next question is how much is a stone? Followed by how big is an acre? Jan 19, 2016 at 20:53
• So what exactly is the conversion (part of the OP's question), or were you leaving that out on purpose? Jan 19, 2016 at 23:00
– Zaid
Jan 20, 2016 at 3:18
• @DucatiKiller a stone is easy: 14lb. An acre is more fun: 1 furlong (220 yards, 1/8 mile) x 1 chain (22 yards) = 1/640 of a square mile. A hectare is 0.01 square km, and as a mile is almost exactly 1.6km, this means that an acre is very close to 40% of a hectare. Jan 20, 2016 at 9:37

They are just different units of measurement, kW is metric which is widely adopted in Europe so that is probably why they choose to publish their numbers in this unit.

1 kW is about 1.34 hp, the inverse relation gives 1hp as about 0.75 kW.

• They may not be able to choose. Legislation in at least some European countries is that measurements on products or advertisements have to be in metric first (you may add other units in parentheses, for example). You're simply not allowed to do otherwise. (E.g. in Germany it's a regulatory offence to use other units (unless the official one is emphasised).)
– Joey
Jan 19, 2016 at 19:39

Formula:

HP= kw/(0.746xEficiencia)