Size a diesel generator in two steps: convert the running load to kVA at a power factor of 0.8, then add the starting demand of the largest motor. A 320 kW running load at 0.85 power factor is 376 kVA, but if a 55 kW motor on that load starts direct-on-line the set has to carry roughly 735 kVA for a few seconds. The largest motor, not the total load, usually decides the rating.
The three numbers to collect before any calculation
Sizing goes wrong when it starts from a nameplate total. Start from these three facts instead, because everything else follows from them:
- Running load in kW (or in kVA if the load list already gives it that way).
- Power factor of the site load. 0.8 is the usual assumption for a mixed commercial or industrial load; a site full of drives can sit closer to 0.9.
- Starting method of the largest motor: direct-on-line, star-delta, soft starter or variable frequency drive.
Ambient temperature, altitude and the duty rating (standby or prime) come next, and they change the answer: the same electrical load needs a bigger set at 50 C than at 25 C. That is covered in prime vs standby power.
Converting kW to kVA: the only formula you need
The alternator is rated in kVA, the engine is rated in kW, and the bridge between them is the power factor:
kVA = kW ÷ power factor · kW = kVA × power factor
| Running load | At 0.80 PF | At 0.85 PF | At 0.90 PF |
|---|---|---|---|
| 200 kW | 250 kVA | 235 kVA | 222 kVA |
| 320 kW | 400 kVA | 376 kVA | 356 kVA |
| 400 kW | 500 kVA | 471 kVA | 444 kVA |
| 640 kW | 800 kVA | 753 kVA | 711 kVA |
| 1600 kW | 2000 kVA | 1882 kVA | 1778 kVA |
A load list that quotes only amps can be converted the same way: kVA = 1.732 × 400 V × amps ÷ 1000 for a 400 V three-phase supply. At 400 V, 1000 A is approximately 693 kVA.
Why the largest motor can decide the size on its own
A motor draws five to eight times its full-load current while it accelerates, and its power factor during that start is poor – typically 0.3 to 0.4. The result is a short, large kVA demand that the set has to supply without dropping the voltage far enough to trip contactors or stall the motor.
| Starting method | Starting current | Starting kVA, rough order |
|---|---|---|
| Direct-on-line (DOL) | 6 to 8 times full load | 6 to 8 times the motor kW |
| Star-delta | About one third of DOL | 2 to 3 times the motor kW |
| Soft starter | 2 to 3 times full load | 2 to 3 times the motor kW |
| Variable frequency drive | About 1 to 1.5 times full load | The set still sees the drive harmonics, so the load is not purely resistive |
Worked example: 320 kW of load, one 55 kW DOL motor
- Running load: 320 kW at 0.85 PF = 376 kVA.
- Largest motor starts direct-on-line: 55 kW × 6.5 = roughly 358 kVA of starting demand.
- Demand while that motor accelerates: 376 + 358 = about 735 kVA.
- Add 10 % for load growth: about 808 kVA.
The steady-state load alone would have pointed at a 400 kVA set. With that one motor starting direct-on-line, the answer is 800 kVA. If the motor is changed to a soft starter, the starting demand falls to roughly 165 kVA and the same site lands between 500 kVA and 630 kVA. The cheapest way to reduce the generator size is often a starter on one motor, not a bigger set.
How much margin, and when margin becomes a problem
Add 10 to 20 % for load growth and for the difference between a clean load list and a real site. Do not add margin twice – once for the motor and once again on top – and do not size the set for the sum of every nameplate in the building, because no site runs everything at once.
An oversized set has its own cost. Running a diesel engine continuously below about 30 % of its rating keeps the combustion temperature low, which leads to wet stacking (unburnt fuel and soot in the exhaust and around the injectors) and to a fuel consumption per kWh that is 20 to 30 % worse than at three-quarter load. A set that is right-sized and loaded properly is cheaper to run than a larger one that idles.
Conditions that change the answer
- Ambient temperature. A rating is declared at reference conditions. A set quoted for a Gulf site in summer is normally de-rated for a higher ambient, and the datasheet figure for your build is the one that counts.
- Altitude. Above 1000 m the engine loses power because the air is thinner; the exact percentage comes from the engine manufacturer’s altitude curve, not from a rule of thumb.
- Power factor below 0.8. The alternator has to be de-rated or specified with a heavier exciter, otherwise the kVA figure on the set no longer holds.
- Duty rating. A standby set and a prime set of the same kVA are not the same machine. See prime vs standby power.
- Harmonics. A site dominated by drives and rectifiers needs a de-rated alternator even when the kW total looks comfortable.
Questions
How do I convert kW to kVA for a generator?
Divide the kW by the power factor. 400 kW at 0.80 PF is 500 kVA; the same 400 kW at 0.90 PF is 444 kVA. Mixed commercial buildings usually sit near 0.8, which is why 0.8 is the safe assumption for a first sizing pass. A 1000 A load at 400 V three-phase is about 693 kVA.
My total load is 300 kW – do I need a 300 kW generator?
No. Set ratings are quoted in kVA, and the engine rating in kW is always lower than the alternator rating in kVA at 0.8 power factor. A 375 kVA / 300 kW standby set supplies 300 kW continuously, but the same 300 kW of load with a 75 kW direct-on-line motor starting on it can need 750 kVA for the duration of the start. The motor decides, not the total.
How much spare capacity should I leave?
Between 10 % and 20 % of the calculated demand. On the worked example above, 735 kVA becomes about 808 kVA with 10 %, which is why 800 kVA is the practical answer rather than 630 kVA. Anything beyond 25 % spare usually means paying for a larger set, worse fuel consumption per kWh and light-load running.
Can I add a canopy or a bigger tank after the set is built?
Sometimes, but the enclosure changes the cooling. A canopy restricts airflow, so the set has to be re-checked against the higher air temperature inside the enclosure; on some ratings the set must be de-rated or the radiator enlarged. Decide the enclosure and the fuel autonomy before the quotation, not after delivery. The trade-offs are compared on the silent and open frame pages.
What to send for a quotation
Send the load list (running kW or amps per item), the power factor if it is known, the starting method of the largest motor, the supply voltage and frequency, and the site conditions. With those five lines the reply carries the rating, the configuration and the lead time. The form is on the request a quote page, and the full power range is on diesel generator sets.
Send the load list and get a rating back
We answer with the kVA, the engine rating, the enclosure and the fuel autonomy – in writing, usually the same working day.
Written and maintained by the KVA Grid sales engineering desk. Figures are for 400/230 V, 50 Hz, three-phase sets unless stated, and every set is quoted against its own datasheet and factory test report. Reviewed 26 September 2026.