As a horse owner, you're accustomed to thinking of the power of horses in terms of their muscle strength and endurance. It probably seems impossible to you that the horse could have been the basis of a system of power measurement, when each horse in your barn different, with different pulling power and strength. But it's been a long time since horses were actually the basis of horsepower. Now it's a system of scientific measurement that can be used to describe anything from cars to chainsaws.
Ironically, just as horses are different from each other, there are several different definitions for what a unit of horsepower entails. In the imperial system, one horsepower is equivalent to 550 foot pounds (745.7 watts) of power while the metric system defines one horsepower as being equivalent to 735.499 watts, or 75 kilogram-force meter per second. In Canada, where we use both systems, it can be difficult to tell whether something is based on metric or imperial definitions of horsepower. What is the difference between the two?
In addition to these standardize measurements, there are also several other definitions of horsepower based on the type of engine or machine they refer to. One horsepower for boilers found in homes (like the gas boilers from Wallhungboilers.com) and factories, for example, is much larger - 34.5 pounds of water evaporated per hour at 212 Fahrenheit, which is equivalent to over 9,800 watts. When talking about North American electric motors, one horsepower refers to each parcel of 746 watts of power outputted. For European electric motors, the conversion rate is .0735.
Horsepower as a unit of measurement came about because before there were gas engines, and steam trains, there were only horses to augment our natural abilities to locomote. Therefore when work on steam engines began, the unit of horsepower was invented to keep track of how many horses they estimated they would need to replace the engine. The differences in unit size came about from differing opinions on the average power of a horse. Even this was inaccurate, for while an engine could run continuously, horses would need to be fed, watered, and rested.
Today, however, the use of horsepower as a unit is purely reflexive and traditional. We are familiar with horsepower so we can envision the power of an engine if its output is measured in horsepower even though it is impossible for the brand new Aston Martin to be over 500 horses. Horsepower is therefore used most often in automotive advertising and journalism, with scientists and engineers preferring to work with the Watt as a unit of power, which has only one definition and is accepted as the standard SI unit for power.