If I ask a Frenchman, or any other European for that matter, how much power he’s got under the hood, am I going to get the answer in horses or watts ?
You’ll often see the abbreviation PS - Pferde Starke - German for “horse power”. It’s roughly similar to the imperial horsepower.
In Germany Watt are used for official purposes but when talking about cars almost everybody still uses HP (“PS”) informally. However our HP is defined in a slightly different, metric-derived way. It’s the power required to lift a mass of 75 kg with a speed of 1 m/s at normal gravity. That’s about 735.5 W.
In Germany, as kellner said, for commercial/official purposes kW but informally mostly horsepower (in ads etc. the kW rating must come first and the HP number in brackets only).
I suspect that HP (PS in German) are hanging on much longer in common usage than other non-SI units because the power expressed in HP happens to be a higher number and both manufacturers and car owners prefer the higher number as being more bragworthy.