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Two demands size a generator. The steady running load in kW converts to kVA by the power factor. But motors draw several times their running current at start, so the largest motor’s inrush can dominate — the generator must supply that surge without excessive voltage dip. Take the larger of running and starting requirements.
- 40 kW running ÷ 0.8 = 50 kVA.
- 20 HP motor start ≈ 120 kVA surge → governs.
Reference tables
Standard standby genset ratings (kW)
| Small | Medium | Large |
|---|---|---|
| 20 | 100 | 400 |
| 35 | 150 | 500 |
| 60 | 200 | 750 |
A number the tables do not give: a single 20 HP across-the-line motor start can demand ~120 kVA of surge — often more than the entire running load.
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FAQ
kW or kVA?
Loads are rated in kW (real power); generators and motor starting are limited by kVA. Convert with the power factor (kVA = kW ÷ PF).
Why does motor starting dominate?
Across-the-line motors draw 5–7× running current at start; that inrush can exceed the running load and set the generator size.
Can I reduce the starting size?
Yes — soft starters or VFDs cut inrush dramatically, letting a smaller generator serve the same motors.
Standby or prime rating?
Standby is for occasional outages; prime/continuous ratings are lower for full-time running. Match the rating to the duty.
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