There are many ways to measure the power of an engine block of cars, motorcycles and air conditioners… So is a 100 horsepower car equal to 100 horses? The answer is no.
Horsepower is a unit of measurement to measure power, simply understood as the speed at which a job is done. For example, if you are pushing a heavy object from the top of the slope to the top of the slope, the power here is the speed at which you accomplish it.
The term power was coined by Scottish engineer James Watt to compare the power of a steam engine versus – horse power the power of horses. It was later extended to apply to the output power of piston engines for cars, turbines, electric motors and many other types of machinery.
Many people think that horsepower is equal to the maximum power a horse produces. However, this is a misconception. Through measurements, it is confirmed that a horse can produce a maximum of 14.9 horsepower. For ease of comparison, 1 person is capable of producing a maximum power of 5 horsepower.
According to James Watt’s rule, horsepower is equivalent to an amount of energy that a horse can maintain for a period of time . However, until now there have been several variations of the horsepower concept.
Unlike other metrics, the exact definition of horsepower can vary from region to region and in turn to how energy is measured in a particular type of machine. 
Formula for calculating horsepower.
The definition of horsepower proposed by James Watt is called mechanical horsepower (also known as imperial horsepower) and is approximately equal to 745.7 Watt. However there is another “horsepower” called Pferdestarke (also known as PS or metric horsepower) , which is also based on a similar measurement by James Watt but developed in Germany and is equivalent to 735.5 Watt.
The horsepower of the electric motor is equivalent to 746 Watt. Water boiler horsepower is different and 1 horsepower of water boiler is equivalent to 9810 Watt.
For those of you wondering where the above number 745.7 came from. The power of a machine according to James Watt is defined as the Work required to lift 1 kg of mass 1 meter in 1 second, briefly 1 hp = 75 kgf⋅m/s.
In many European countries, this calculation is still used. In the US, this calculation still applies, but because people use Pound and Foot units, it will be defined as 33,000 ft⋅lbf/min.
Again, both of these calculations are based on choosing the value of the gravitational acceleration g = 9.80665/2. And if in another gravity, for example on the Moon, 1 horsepower is only 1/6 of that on Earth.
Particularly in the car sector, there is a difference when car manufacturers in different countries measure horsepower . In the UK, engine power is usually measured by brake horsepower, abbreviated as bhp. Meanwhile, in the US, it is preferable to calculate the car engine power by hp.
On the other hand, we’ve been talking mainly about the horsepower the engine produces . However, when installing a 100-horsepower engine on a vehicle, through many drive systems, the actual power when going to the wheels is no longer the same.