One generator may show kW, kVA, engine kWm and standby output at the same time. These figures describe active output, apparent capacity, mechanical input and operating duty. Mixing them destroys a consistent basis for quotation comparison, sizing and acceptance.
Size the required kW from real loads first, then verify kVA against power factor, current, harmonics and transient behavior. A 0.8 rating basis does not mean the site load always operates at 0.8 power factor.
1. Understand what this decision changes
kW represents active power that performs useful work
Motors, heating, lighting and production equipment consume active power. The engine must supply it while covering conversion losses.
kVA represents apparent capacity carried by the electrical path
Alternator windings, breakers and cables are limited by voltage and current, so lower power factor raises current and kVA for the same kW.
Power factor belongs to the load
A 0.8 rating is a common nameplate basis, but motors, UPS systems and drives must be evaluated at their real operating power factor.
2. Inputs to confirm before procurement
Separate steady operation, starting events and nonlinear loads before calculating capacity.
- Rated kW, kVA, current and power factor for each load
- Maximum simultaneous combination and staged restoration order
- Motor starting method, starting current and allowable voltage dip
- Share and harmonic behavior of UPS, rectifier and VFD loads
- Distribution and imbalance of single-phase loads
3. How the main options compare
| Option or parameter | Where it fits | What must be verified |
|---|---|---|
| Resistive or high-power-factor loads | Lighting, heating and other loads with a direct current-to-kW relationship. | Confirm diversity, cold inrush and line current. |
| Motor and mixed loads | Pumps, fans, compressors and production equipment. | Check running kW, running kVA, starting kVA and sequence. |
| UPS, VFD and rectifier loads | Data, telecom, medical and automation systems. | Check input power factor, harmonics, peak current and alternator transients. |
4. The most common decision errors
Common mistakes
- Comparing one quotation in kVA with another in kW
- Assuming every site load always has 0.8 power factor
- Using engine nameplate power without conversion and derating
- Adding only steady loads while ignoring starts and load steps
Compare values only after aligning voltage, frequency, duty, ambient conditions and test boundaries.
5. What the technical specification must state
The specification should state at least
- Prime, standby or continuous duty with corresponding kW and kVA
- Voltage, frequency, phase, rating power factor and current
- Engine mechanical output and alternator capacity
- Temperature, altitude and other derating conditions
- Permitted voltage-frequency transient and load acceptance
State both input assumptions and calculated results so capacity margin remains explainable.
6. How to verify and make the final decision
Verification and acceptance
- Cross-check engine, alternator and complete-set nameplates
- Record kW, kVA, power factor and phase currents with a power analyzer
- Perform planned load steps and record voltage-frequency recovery
- Review the highest phase current, not only the average
- Confirm metering wiring and transformer ratios
The correct set must satisfy both useful active power and the current carried by the electrical system; multiplying one number by 0.8 is not enough.
If load data is incomplete, measure and document it before adding margin. Oversizing is not a substitute for engineering evidence.