What size motor will this supply run?
When you know how much apparent power is available — a generator’s kVA rating, a spare transformer, the headroom on a feeder — the natural question is how large a motor it can drive. This calculation walks forward through the motor: from apparent power, through the power factor to real electrical power, through the efficiency to mechanical output, and finally into horsepower.
The kVA to horsepower formula
Worked example
The calculator’s defaults are 5 kVA, power factor 0.8, efficiency 90%.
Real power:
Mechanical output, then horsepower:
So 5 kVA supports about 4.83 hp of running motor load. You wouldn’t actually put a 4.8 hp motor on a 5 kVA generator, though — its starting inrush would stall the set. A 2 to 3 hp motor is a more realistic match.
Running horsepower a source can carry
Power factor 0.85, efficiency 88%:
| Available | Running hp | Realistic motor (start margin) |
|---|---|---|
| 3 kVA | ~3.0 hp | 1–1.5 hp |
| 5 kVA | ~5.0 hp | 2–3 hp |
| 10 kVA | ~10.0 hp | 5–7 hp |
| 25 kVA | ~25 hp | 15 hp |
The gap between the running figure and the realistic motor is the starting-inrush allowance.
What the estimate assumes
Efficiency and power factor use general defaults. Real values depend on the motor’s frame size, its load, and its manufacturer, and both fall when a motor runs lightly loaded. Use nameplate figures for an actual decision, and always keep starting current in mind.
Common mistakes
- Matching a motor’s running hp to the source with no starting margin.
- Dividing by power factor instead of multiplying in this direction.
- Ignoring that partial load worsens power factor, raising the kVA a given motor actually needs.
For the reverse, use the horsepower to kVA calculator. For line current, see kVA to amps.