What is Horsepower?
Horsepower (HP) is a unit of power invented by James Watt in the late 18th century to compare the output of steam engines with the power of draft horses. He determined that a horse could do 33,000 foot-pounds of work per minute, and this became the definition of one mechanical horsepower.
Today, several different definitions of horsepower exist. The most common are mechanical (or imperial) horsepower (550 ft-lbs/s), metric horsepower (75 kgf-m/s), and electrical horsepower (746 watts). While the kilowatt has largely replaced horsepower in scientific contexts, HP remains widely used in automotive, industrial, and consumer applications.
Horsepower Conversion Factors
Types of Horsepower
| Type | Symbol | Watts | Used In |
|---|---|---|---|
| Mechanical (Imperial) | HP | 745.7 | US automotive, industrial |
| Metric | PS, CV, pk | 735.5 | European/Asian automotive |
| Electrical | HP(E) | 746.0 | Electric motors |
| Boiler | HP(S) | 9,809.5 | Steam boilers |
| Hydraulic | HP | 745.7 | Hydraulic systems |
Frequently Asked Questions
Why are there different types of horsepower?
Different industries and regions developed their own definitions. Mechanical HP was defined by James Watt using imperial units. Metric HP was defined using the metric system (75 kgf-m/s). Electrical HP was standardized to exactly 746 watts. Boiler HP measures steam boiler capacity and equals about 9,810 watts, much larger than the others.
Is a metric HP (PS) the same as a regular HP?
No, but they are close. One metric HP (PS) equals 0.9863 mechanical HP, a difference of about 1.4%. When European manufacturers quote power in PS, the HP equivalent is slightly less. For example, 300 PS = 296 HP. Many modern specs now use kilowatts as the standard unit to avoid confusion.
How much horsepower does a real horse produce?
Ironically, a real horse can produce more than one horsepower in short bursts. Peak output for a horse is about 14.9 HP. However, the sustained output over a full working day averages about 0.7 HP. Watt's definition was based on continuous work capacity, and he intentionally overestimated to make his engines look good by comparison.