The reason why the power of car engines it is measured in horsepower it is to be found in a fairly remote past. Indeed, the invention of the steam engine in the 18th century generated the need to measure engine power.
Originally, energy was measured in units such as “pounds per foot per second” or “kilograms per meter per second”.
Yet, the Scottish engineer James Watt from 1782 chose to base the measurement in question on the manpower of a horsethus introducing a new unit of measurement called “horsepower” (or horsepower in English). From the various experiments it was deduced that a horse could lift a weight of 550 pounds to a height of one foot per second, which is equivalent a circa 745,7 watt.
Given the validity of the studies, the horsepower subsequently became the common unit of measurement for the power of the engines, and shared by the whole world. However, due to the differences between the engines and their efficiency, the brand new unit soon proved insufficiently accurate. Therefore, in the 1920s, the Society of Automotive Engineers (SAE) released a new definition of horsepower, based on a standardized petrol engine.
Even today the power of car engines is measured in horsepower, the unit of which corresponds to 735.5 watts. However, horsepower may vary depending on the make, model and type of engine.
In summary, the bond between horses and engines has ancient roots in the history of engineering. Horsepower as a unit of measurement was then standardized over the years to become more and more reliable and accurate.
Anyway, horses are still used for some sort of custom, because it is easier to associate them with the real power of the car, thinking about the work of how many horses can replace the specific machinery. To know the real power of a car it is necessary to calculate the kilowatts, not the horses.