Power

In Physics it is defined as the rate of doing work.
The problem comes in determining where in a system you take your measurements and what things you consider to be useful work.
The HorsePower?, The Watt? and the PS are commonly used to quote a power rating.
1 British HP is 745.7 Watts, the US one is 746 Watts, the metric one is 735.5 Watts.
I think the metric horse power is called the Pferdstarke (PS), but I am unsure.
The CV or Cheveaux may be the equivalent of a metric Horsepower in France.