Motors — especially in the US and in older or imported machinery — are frequently rated in horsepower (HP) rather than kilowatts. If you're trying to size a generator, calculate a motor's energy consumption, or just compare specs across equipment from different regions, converting horsepower to kilowatts is usually the first step.
The Conversion Factor
There are actually two slightly different horsepower standards in common use, which is a genuine source of small discrepancies people run into:
- Mechanical (imperial) horsepower: 1 hp = 0.7457 kW
- Metric horsepower (PS/CV, common in European specs): 1 hp (metric) = 0.7355 kW
For most US and general engineering contexts, use the mechanical horsepower conversion unless the equipment explicitly specifies metric horsepower (common on European car and small-engine specs).
Worked Example
A 5 hp water pump motor:
Real-World Sizing: Why Motor Efficiency Changes the Calculation
Motor horsepower ratings describe mechanical output power at the shaft, not electrical input power drawn from the wall. Because electric motors lose energy through heat, friction, and magnetic hysteresis, a motor is never 100% efficient. Typical commercial and residential motor efficiencies range from 75% to 95% depending on frame size, pole count, and NEMA design class (Premium Efficiency motors achieve 90%+).
Example: A 5 hp (3.73 kW mechanical output) well pump motor operating at 85% electrical efficiency:
Starting Surge vs. Running Load: When sizing a backup generator, solar inverter, or circuit breaker, you must also account for Locked Rotor Amperage (LRA). Inductive motors pull an initial inrush surge 3 to 7 times their steady-state operating current during the first few seconds of startup before settling to their rated Full Load Amperage (FLA). Sizing against the 3.73 kW mechanical rating alone will reliably trip breakers and stall generators.
Mechanical Horsepower to Electrical Output Reference Table
| Horsepower | kW (mechanical output) |
|---|---|
| 1 hp | 0.75 kW |
| 2 hp | 1.49 kW |
| 5 hp | 3.73 kW |
| 10 hp | 7.46 kW |
| 20 hp | 14.91 kW |
| 50 hp | 37.29 kW |
| 100 hp | 74.57 kW |
Frequently Asked
Do I need to worry about motor starting current (inrush)?
Yes, for sizing circuits or generators — many motors draw 3-7x their running current for a brief moment at startup. This calculator handles the steady-state running power conversion; for sizing electrical infrastructure, add a safety margin for inrush, or check the motor's specific starting current rating.
Is horsepower still used, or is it outdated?
It's still the standard rating unit for many motors, pumps, and engines in the US and several other markets, even as most engineering documentation elsewhere defaults to kW. Both remain common enough that conversion between them comes up regularly.
For three-phase motor load calculations from voltage and amperage, see the main calculator's three-phase power section.
