Horsepower is common on motor nameplates, but generators, inverters, and wiring calculations are done in watts. The key is knowing whether you need mechanical output watts or electrical input watts.
For full load planning, use the WattSizing Calculator.
Quick Answer
- 1 HP = 746 watts (mechanical output power).
- Motor electrical input is usually higher because of efficiency losses.
- Use:
Input Watts = (HP x 746) / Efficiency.
Conversion Equations
1) Mechanical output power
Output Watts = HP x 746
2) Estimated electrical input power
Input Watts = (HP x 746) / Motor Efficiency
If efficiency is unknown, a rough assumption for small to medium motors is often 0.80 to 0.92, but always use nameplate data when available.
Worked Examples
Example 1: 1 HP motor at 85% efficiency
- Output:
1 x 746 = 746 W - Input:
746 / 0.85 = 878 W
Example 2: 2 HP motor at 90% efficiency
- Output:
2 x 746 = 1,492 W - Input:
1,492 / 0.90 = 1,658 W
Example 3: 0.5 HP motor at 82% efficiency
- Output:
0.5 x 746 = 373 W - Input:
373 / 0.82 = 455 W
HP to Watts Reference Table
| Motor Size | Mechanical Output (W) | Estimated Input at 85% Efficiency |
|---|---|---|
| 0.5 HP | 373 W | 439 W |
| 0.75 HP | 560 W | 659 W |
| 1 HP | 746 W | 878 W |
| 1.5 HP | 1,119 W | 1,316 W |
| 2 HP | 1,492 W | 1,755 W |
| 3 HP | 2,238 W | 2,633 W |
Why This Matters for Generator Sizing
Motor loads also have startup surge, so generator sizing should include:
- Estimated running input watts.
- Starting watts (often 2x to 4x running for many motors).
- Concurrent loads and margin.
Related reading:
FAQs
Is 1 HP always exactly 746 W in real systems?
746 W is the mechanical equivalence. Actual electrical input is higher due to efficiency and power factor.
Can I size a generator from HP alone?
Only roughly. Best practice is to use nameplate voltage, current, power factor, and measured startup behavior if possible.
Do I need power factor in HP-to-watts conversion?
Not for mechanical conversion itself. But yes for accurate electrical input from volts and amps.
Why are my measured watts lower than the formula?
Your motor may not be fully loaded. Real-time draw depends on the actual mechanical load.
CTA
Need to convert HP and account for startup surge in one place? Use the WattSizing Calculator to estimate realistic running and starting requirements for motors, pumps, and compressors.
