Mechanical vs Electrical vs Metric Horsepower: Why 3 Different Numbers?
Historical definitions of horsepower. Why HP scales differently depending on standards.
Understanding the Three Main Types of Horsepower
When you look at the specifications of an engine, motor, or electrical device, you will often come across the term "horsepower" (HP). However, not all horsepower ratings are calculated the same way. The three most common standards you will encounter are Mechanical Horsepower, Electrical Horsepower, and Metric Horsepower. In this comprehensive guide, we'll explain the origins of these measurements, how they differ, and why they matter for sizing your equipment.
1. Mechanical Horsepower (Imperial)
Mechanical horsepower, often referred to as imperial horsepower, is the original measure created by James Watt in the late 18th century. Watt needed a way to market his improved steam engines to coal miners, so he compared the power of his engines to the work done by draft horses.
- Definition: 1 Mechanical HP is the power needed to lift 33,000 pounds by one foot in one minute.
- Equivalence in Watts: 1 Mechanical HP = 745.7 Watts.
- Use Cases: Automotive engines, heavy machinery, and traditional mechanical engineering in the United States and UK.
2. Electrical Horsepower
Electrical horsepower is used primarily to define the output power of electric motors. While it is derived from the mechanical horsepower definition, it has been rounded for standardized electrical engineering calculations.
- Definition: Standardized specifically for electric motors and drives in the US.
- Equivalence in Watts: 1 Electrical HP = exactly 746 Watts.
- Use Cases: AC/DC motors, pumps, HVAC compressors, and industrial electric drives.
3. Metric Horsepower (PS, cv, ch)
Metric horsepower was developed in Europe in the 19th century as a localized standard using the metric system instead of imperial units. It is often denoted as PS (Pferdestärke in German), cv (cheval-vapeur in French), or ch.
- Definition: The power needed to lift 75 kilograms by one meter in one second.
- Equivalence in Watts: 1 Metric HP = 735.498 Watts.
- Use Cases: European and Asian automotive engine specifications, European electric motors.
Comparison Table: Types of Horsepower
| Type of Horsepower | Symbol/Abbreviation | Equivalent in Watts (W) | Primary Application |
|---|---|---|---|
| Mechanical (Imperial) | HP, hp(I) | 745.7 W | Combustion engines, mechanical drives (US/UK) |
| Electrical | hp(E) | 746 W | Electric motors, pumps (US) |
| Metric | PS, cv, hp(M) | 735.5 W | Automotive & industrial (Europe/Asia) |
How to Calculate Between HP and Watts
Example (Electrical): 5 HP × 746 W/HP = 3,730 Watts
Worked Example: Sizing a Generator for a 3 HP Motor
Suppose you have a 3 HP air compressor (rated in electrical HP) and you need to size a generator to run it. First, convert the HP to Watts:
Running Watts: 3 HP × 746 W = 2,238 W.
However, electric motors have an inrush current when starting up, typically requiring 2 to 3 times the running watts. Therefore, you should size your generator for at least 6,700 surge watts.