What buyers should know first
A diesel generator set is usually rated in kVA, but the usable real power is measured in kW. The relationship between them is the power factor. If you buy a 500 kVA industrial generator sets expecting 500 kW of motor load but your site power factor is 0.7, you will only get 350 kW of real work before the set is fully loaded. This is one of the most common sizing mistakes buyers make.
This article explains power factor in practical terms, shows when generator sets from 200 kW to 400 kW sets need kVAr compensation, and lists the documentation to request before purchase. It is written for procurement and project engineers who must confirm electrical compatibility without becoming a power-system specialist.
The kW, kVA, and kVAr relationship
Think of kVA as the total electrical ‘size’ the alternator must carry. kW is the useful power that does work. kVAr is the reactive power that builds magnetic fields in motors and transformers. Power factor is simply kW divided by kVA.
| Load type | Typical power factor | Implication for the genset |
|---|---|---|
| Resistive heaters, incandescent lighting | ≈ 1.0 | kW ≈ kVA; almost no reactive power. |
| Standard induction motors | 0.75 – 0.85 | Most common industrial load; needs reactive magnetising current. |
| Compressors, pumps, crushers at partial load | 0.60 – 0.75 | Low power factor at light load; often the hidden limit. |
| Servers / IT without PFC | 0.65 – 0.80 | Can be leading or lagging depending on PSU design. |
| Large capacitor banks | Near 0 leading | Can over-excite the alternator if not coordinated. |
Most industrial diesel generator sets are rated at 0.8 power factor. That means a 1,000 kVA set is designed for 800 kW of real power plus the reactive power that goes with it. If your load power factor is lower than 0.8, the alternator reaches its current limit before the engine reaches its kilowatt limit.
When does a site need kVAr compensation?
Power-factor correction capacitors add leading reactive power that cancels the lagging reactive power of induction motors. The benefits are lower generator current, reduced cable losses, and the ability to use a smaller set. The risks are over-correction, harmonic resonance, and alternator over-excitation.
- Add capacitors when the running power factor is consistently below 0.8 and the generator is current-limited rather than engine-limited.
- Size carefully: target a power factor of 0.90 to 0.95 lagging. Do not aim for unity, especially with variable loads.
- Avoid over-correction because a leading power factor can cause alternator overheating and unstable voltage regulation.
- Check harmonics: if the site has VFDs or UPS systems, standard capacitors can resonate; detuned banks may be required.
Common configuration mistakes
Buyers often assume the generator can simply be derated by multiplying kVA by the load power factor. That is only true if the alternator can carry the resulting current. The real checks are current rating, alternator temperature rise, and AVR stability across the expected power-factor range.
Procurement checklist
- Obtain a load list that states kW, kVA, and power factor for each major load.
- Confirm the generator set power-factor rating (typically 0.8 lagging).
- Ask the supplier for an alternator capability curve showing kW versus kVAr limits.
- If power-factor correction is used, request a single-line diagram showing capacitor steps.
- Verify that capacitors are switched in steps so the generator never sees a leading power factor.
- Request thermal class and temperature-rise data from the alternator datasheet.
RFQ wording that gets useful answers
Ask: ‘Quote a generator set capable of delivering [X] kW at [Y] power factor, and provide the alternator capability curve and voltage-regulation band across 0.8 lagging to 0.95 leading.’ This wording forces the supplier to check the alternator insulation and temperature-rise guide beyond the headline kVA number.
For sites with mixed loads, request a quote with the motor list and any existing capacitor banks. Suppliers can then confirm whether Cummins diesel generator sets, Volvo diesel generator sets, or Perkins diesel generator sets ratings already cover the requirement, or whether external compensation is the safer route.
Questions buyers often ask
What is the default power factor of a diesel generator?
Most sets are rated at 0.8 lagging. This means a 1,000 kVA set delivers 800 kW of real power when fully loaded at 0.8 power factor.
Can I run a generator at unity power factor?
Yes, but only within the alternator capability curve. Unity power factor reduces the alternator’s kVAr capacity and can affect voltage stability under sudden load changes.
Will capacitors damage the alternator?
Over-sized or permanently connected capacitors can cause a leading power factor, which may over-excite the alternator and overheat the windings. Switched, correctly sized banks do not.
Do I need power-factor correction for every motor?
Not necessarily. Correction is worthwhile when the running power factor is low enough that the generator current limit becomes the bottleneck.
How do harmonics affect power-factor correction?
VFDs and some UPS units create harmonic currents. Standard capacitor banks can resonate at harmonic frequencies; detuned or active filters are safer in those cases.
Should I request a power-factor study?
Yes, if the site has more than a few large motors or any variable-speed drives. A study also predicts whether the generator can supply inrush current during motor starting.
Specifying generator sets from 400 kW to 600 kW units without checking power factor is a common source of under-sizing. Treat kVA as the alternator’s current ceiling and kW as the engine’s useful output, then compare both against the site load.