HP to kW: Horsepower to Kilowatt Conversion Guide
HP to kW Conversion
1 mechanical horsepower = 0.746 kW (exact, by definition)
1 metric horsepower (PS) = 0.7355 kW
1 electrical horsepower = 746 W
Most US motors use mechanical HP. European and Asian motors use kW ratings directly.
Quick Conversion Table
| HP | kW (shaft) | kW input (90% eff) | kW input (85% eff) | 230V 3φ Amps |
|---|---|---|---|---|
| 1/4 | 0.19 | 0.21 | 0.22 | 0.7 |
| 1/2 | 0.37 | 0.41 | 0.44 | 1.3 |
| 1 | 0.75 | 0.83 | 0.88 | 2.5 |
| 2 | 1.49 | 1.66 | 1.75 | 4.8 |
| 3 | 2.24 | 2.49 | 2.63 | 7.2 |
| 5 | 3.73 | 4.14 | 4.39 | 12.0 |
| 7.5 | 5.60 | 6.22 | 6.58 | 18.0 |
| 10 | 7.46 | 8.29 | 8.78 | 24.0 |
| 15 | 11.19 | 12.43 | 13.16 | 36.0 |
| 20 | 14.92 | 16.58 | 17.55 | 48.0 |
| 25 | 18.65 | 20.72 | 21.94 | 60.0 |
| 30 | 22.38 | 24.87 | 26.33 | 72.0 |
| 50 | 37.30 | 41.44 | 43.88 | 120.0 |
| 100 | 74.60 | 82.89 | 87.76 | 240.0 |
Amps formula: I = HP × 746 / (V × √3 × PF × eff). Values above use PF 0.85, V = 230V, 3-phase.
Motor Efficiency Standards
NEMA Premium efficiency motors (IE3) vs standard (IE1):
| HP | IE1 (Standard) | IE2 (High) | IE3 (Premium) |
|---|---|---|---|
| 1 | 78.0% | 82.5% | 85.5% |
| 5 | 84.0% | 87.5% | 89.5% |
| 10 | 86.5% | 89.5% | 91.7% |
| 25 | 89.5% | 92.4% | 93.6% |
| 50 | 91.0% | 93.6% | 94.5% |
| 100 | 92.4% | 94.5% | 95.4% |
NEC 434: NEMA Premium efficiency motors are required for most new installations per EISA 2007.
Common Motor Types
| Type | Efficiency | PF | Application |
|---|---|---|---|
| Induction (NEMA Design B) | 85–95% | 0.75–0.85 | Most HVAC applications |
| Permanent Magnet | 92–97% | 0.90–0.98 | VFD-driven, high efficiency |
| Wound Rotor | 85–93% | 0.70–0.85 | High starting torque |
| Single-phase | 75–85% | 0.60–0.80 | Residential, small loads |
Common Mistakes
- Ignoring efficiency: A 10 HP motor outputs 7.46 kW but draws 8.3–8.8 kW input (depending on efficiency). Size wire and breaker for input, not output.
- Using mechanical HP for electrical calcs: Electrical input = HP × 746 / eff. Don't use 0.746 directly for wire sizing.
- Mixing HP and PS: 1 HP (US) = 0.746 kW. 1 PS (metric) = 0.736 kW. A 10 PS motor is smaller than a 10 HP motor.
Standards Reference
- NEMA MG-1 — Motors and Generators
- NEMA Premium Efficiency Standard
- IEC 60034-30-1 — Motor Efficiency Classes
- EISA 2007 — Energy Independence and Security Act
Frequently Asked Questions
How many kW is a 5 HP motor?
5 HP shaft output = 3.73 kW. Electrical input = 3.73 / 0.895 (IE3 efficiency) = 4.17 kW. A 5 HP motor draws about 4.2 kW from the electrical supply. The difference between shaft output and electrical input is heat loss in the motor.
What's the difference between HP and kW?
HP and kW both measure power. 1 HP = 0.746 kW. The US uses HP for motors; Europe and Asia use kW. They're interchangeable with the conversion factor. 10 HP = 7.46 kW. Motor nameplates in the US show HP; international motors show kW.
How do I calculate motor amps from HP?
For 3-phase: I = HP × 746 / (V × √3 × PF × efficiency). Example: 10 HP, 230V, PF 0.85, eff 0.917: I = 10 × 746 / (230 × 1.732 × 0.85 × 0.917) = 7,460 / 308.8 = 24.2A. Use NEC Table 430.250 for standard FLC values.
Is a 5 HP motor single-phase or 3-phase?
Most motors above 3 HP are 3-phase. Single-phase motors are available up to about 10 HP but are uncommon above 5 HP. Residential homes have single-phase power; commercial buildings typically have 3-phase. Check your supply before selecting a motor.
How efficient are electric motors?
Modern NEMA Premium (IE3) motors range from 85% efficient (1 HP) to 96% efficient (100+ HP). Older motors (IE1) may be 5–10% less efficient. Motor efficiency drops significantly when loaded below 50% of rated capacity — oversized motors waste energy.