Quick answer: Size a generator for two things — the steady running load (sum the kW, divide by power factor to get kVA) and the motor-starting surge, which can be 6–7× the motor's running kVA on a direct-on-line start. Add ~25% margin. The starting surge, not the running load, is what usually decides the set size. Do it in seconds with the generator sizing calculator.
The mistake: sizing on running load alone
Add up the kW, convert to kVA, pick the next standard set — done? Then the biggest motor starts, the genset voltage sags, contactors drop out and the set trips on under-frequency. The genset passed the running test and failed the starting test. Every real generator sizing has to survive the worst single motor start with the rest of the load already connected.
Step 1 — running load (kVA)
Running kVA = Σ running kW / power factor apply a diversity factor if not everything runs at once typical PF 0.8 → kVA = kW / 0.8
Step 2 — the motor-starting surge
A motor draws a big inrush at start. How big depends entirely on the starting method:
| Starter | Starting current | Starting kVA (× motor kVA) |
|---|---|---|
| Direct-on-line (DOL) | 6 – 7× FLA | ~6 – 7× |
| Star-Delta | 2 – 3× | ~2 – 3× |
| Soft starter | 2 – 4× | ~2 – 4× |
| VFD | ~1 – 1.5× | ~1 – 1.5× |
So the starter you choose for the largest motor directly sets how big the generator must be.
Step 3 — the set must accept the biggest step
The generator has to absorb the largest starting step while already carrying the running load, without the voltage or frequency dipping beyond the limit for your equipment. In practice that means the genset kVA must comfortably exceed running kVA + the starting contribution of the largest motor, then add margin for the future and for altitude/temperature derating.
Worked example
Running load: 50 kW @ PF 0.8 = 62.5 kVA → +25% = ~78 kVA Largest motor: 15 kW, started DOL motor running kVA ≈ 20 kVA DOL starting kVA ≈ 20 × 6 = 120 kVA (momentary)
On running load alone a ~80 kVA set looks fine. But the 120 kVA DOL surge would collapse it. Two options:
- Keep DOL → you need a much larger set (~125 kVA) just to ride the start.
- Switch the big motor to star-delta or a soft starter → the surge drops to ~40–60 kVA and a ~100 kVA set copes easily.
The cheaper fix is almost always the softer start, not the bigger generator.
kW or kVA?
Generators are rated in kVA (apparent power) and kW (real power); kW = kVA × power factor, usually 0.8. Loads are often quoted in kW, so always convert to kVA before comparing with the genset nameplate.
FAQ
What size generator do I need? Enough kVA for the running load plus the largest motor's starting surge, with ~25% margin. Enter your loads into the generator sizing calculator for the number.
Why does my generator trip when a motor starts? The motor's inrush exceeds the set's transient capacity — either the genset is undersized or the motor needs a softer starter.
How much margin should I add? Around 25% for load growth, plus derating for high altitude and ambient temperature (both cut a genset's output).