Horsepower
Definition
Horsepower (hp) is a unit of power originally defined by James Watt to compare steam engine output to draft horses. In electrical applications, 1 mechanical horsepower equals 746 watts or 0.746 kW. Electric motor nameplate ratings are commonly given in horsepower: fractional hp motors (1/4 to 3/4 hp) for fans, pumps, and appliances; 1โ5 hp for HVAC compressors, sump pumps, and power tools; 5โ50+ hp for industrial motors. To convert: Amps = (HP ร 746) รท (Volts ร Efficiency ร Power Factor). A 1 hp motor on a 120V circuit draws approximately 8โ10 amps under load.
Why is Horsepower Important?
In electrical engineering and everyday applications, Horsepower is a fundamental concept for understanding how electrical systems work. Whether you are an engineer designing circuits, an electrician sizing wires, or a homeowner estimating energy costs, this metric is essential for safety, efficiency, and accurate calculations.
Our electrical conversion calculators help you quickly convert between units and verify calculations, reducing errors and saving time in both professional and DIY electrical work.
What is Horsepower?
Horsepower (HP) is a unit of power โ the rate at which work is done. It was originally defined by James Watt to compare steam engine output to draft horses. Today, horsepower is used to rate motors, engines, and mechanical equipment across automotive, industrial, and HVAC applications.
Types of Horsepower
| Type | Equivalent | Use |
|---|---|---|
| Mechanical HP (Imperial) | 745.7 watts | Most common โ US motors, engines |
| Metric HP (PS) | 735.5 watts | European/Asian automotive |
| Electrical HP | 746 watts exactly | US electric motor ratings |
| Boiler HP | 9,809.5 watts | Steam boiler capacity |
Horsepower to Electrical Conversions
| HP | Watts | Amps at 120V | Amps at 240V | Common Application |
|---|---|---|---|---|
| ยผ HP | 186 W | ~2.5 A | ~1.3 A | Small fans, pumps |
| ยฝ HP | 373 W | ~5 A | ~2.5 A | Garbage disposal, sump pump |
| ยพ HP | 560 W | ~7 A | ~3.5 A | Pool pump, garage door opener |
| 1 HP | 746 W | ~8โ10 A | ~4โ5 A | Air compressor, table saw |
| 2 HP | 1,492 W | ~16โ20 A | ~8โ10 A | Large compressor, HVAC blower |
| 5 HP | 3,730 W | N/A (needs 240V) | ~20โ25 A | Industrial motors |
Motor HP vs Actual Power Draw
Electric motors draw MORE power from the electrical supply than their HP rating suggests, due to efficiency losses and power factor:
Input Watts = (HP ร 746) รท (Efficiency ร Power Factor)
A 1 HP motor at 85% efficiency and 0.85 PF: Input = 746 รท (0.85 ร 0.85) = 1,033 watts (38% more than the 746W output)