In short. A generator is rated in kVA (apparent power) and kW (real power). Size it from the real running load and its power factor, then check the largest motor start, load steps, site derating and future growth. The result is a preliminary size; a final selection needs the load details and the manufacturer’s data.
The basic relationship
Generators are commonly rated at 0.8 power factor, so a 500 kVA set is about 400 kW. If your load runs at 0.9 power factor, 400 kW needs about 444 kVA of load apparent power, which the 500 kVA generator can supply, but its engine still limits real power to about 400 kW.
Step-by-step
- List the loads with their kW, power factor and operating pattern. See kW, kVA and power factor.
- Apply demand and diversity to estimate maximum running demand. See demand and diversity.
- Convert to kVA using the combined power factor.
- Check motor starting and load steps against the generator’s ability. See motor-starting capacity.
- Apply derating for altitude, ambient temperature and enclosure. See derating.
- Add allowance for growth and a loading target so the set is not run at its limit.
- Select the next standard size and verify against the manufacturer’s data.
Typical loading targets
| Factor | Typical thinking |
|---|---|
| Running load | Often kept within about 70 to 80 per cent of rating for standby use |
| Minimum load | Avoid long periods at very light load; many engines prefer about 30 per cent or more |
| Growth | An agreed percentage, often 10 to 25 per cent, depending on the site |
| Starting | The largest motor must start within voltage and frequency limits |
These are common rules of thumb, not requirements. Your application and the manufacturer’s data decide.
Worked example (illustrative)
A workshop has a running load of 280 kW at an average power factor of 0.85. Apparent power = 280 ÷ 0.85 ≈ 329 kVA. The largest motor is 55 kW started star-delta. A 20 per cent growth allowance gives about 395 kVA. A 400 kVA generator at 0.8 power factor provides 320 kW, which is more than the 280 kW running load but leaves little margin for growth, so the next size, 500 kVA (400 kW), is chosen after a starting check. The estimate shows why checking both kVA and kW matters.
Why real power matters
The engine supplies kW. The alternator supplies kVA. A load with a low power factor can be limited by the alternator’s kVA, and one with a high power factor by the engine’s kW. Always check both.
The kVA and kW relationship
kVA = kW ÷ power factor; kW = kVA × power factor; I (three-phase) = kVA × 1000 ÷ (√3 × V)
| Quantity | Meaning | Sets |
|---|---|---|
| kW | Real power the engine must supply | Engine size |
| kVA | Apparent power the alternator must carry | Alternator, cable and breaker size |
| Power factor | kW ÷ kVA | Link between the two |
Worked example (illustrative)
A site has 320 kW of load at 0.8 lagging power factor. kVA = 320 ÷ 0.8 = 400 kVA. Line current at 415 V = 400 × 1000 ÷ (1.732 × 415) ≈ 556 A. A generator rated 400 kVA at 0.8 power factor supplies 320 kW, so it is fully loaded at this duty with no margin. Adding 20 percent spare for growth and 10 percent for derating gives roughly 320 × 1.3 ÷ 0.8 = 520 kVA, so the next standard rating of 550 or 625 kVA would be reviewed.
Common sizing path
- Total demand in kW from the load schedule.
- Convert to kVA with the realistic power factor.
- Add the largest motor start check and any step-load check.
- Apply site derating and growth margin.
- Select the next standard size and check loading at light load.
Why this is only a first screen
The result does not test motor starting, harmonics or transient limits. Use it to shortlist, then confirm with the detailed checks. See generator sizing and capacity requirement guide.
Common mistakes
- Adding nameplate kW of all equipment with no diversity.
- Sizing from kVA of the load with no check on kW.
- Ignoring the largest motor start.
- Skipping altitude and temperature derating.
- Choosing a size that leaves no room to grow.
What to send us
Send your load list, motor list with start methods, site altitude and temperature, and any growth plans. Send your application details. See also generator sizing.
Questions engineers ask
How do I convert kW to kVA?
Divide kW by the power factor. At 0.8, 100 kW is 125 kVA.
Why is a generator rated at 0.8 power factor?
It is a standard rating convention that ties the alternator’s kVA to the engine’s kW. Real loads may differ.
Is it bad to oversize a generator?
Yes if it runs at very light load for long, which can cause engine problems and waste fuel. Balance margin against minimum load.
Does the generator also have to carry the starting current?
Yes. The starting kVA of the largest motor, with the base load, must stay within voltage dip and frequency limits.
Can I use this guide for a final purchase decision?
No. It gives a preliminary size. Final sizing needs a load study and the manufacturer’s performance data.
Figures are illustrative. Final sizing requires an engineering review of your loads and site.