kW is real power, while kVA is apparent power. A simplified AC relationship is kW = kVA × power factor, or kVA = kW ÷ power factor. This formula alone is not sufficient for generator selection: load type, efficiency, starting current, load steps and the current product datasheet also matter.
Generator ratings are commonly stated in both kilovolt-amperes (kVA) and kilowatts (kW). They describe different electrical quantities. Confusing them, or applying a fixed conversion factor without checking the load, can lead to an undersized or unnecessarily large generator set.
What do kW and kVA mean?
Kilowatts (kW) describe real power converted by the load into useful work, heat, light or motion. Kilovolt-amperes (kVA) describe apparent power, the total electrical loading made up of real and reactive power.
For resistive loads, kW and kVA are close. Motors, transformers and many electronic loads also require reactive power, so apparent power is higher than real power.
What does power factor describe?
Power factor, often shown as cos φ, describes the ratio of real power to apparent power for sinusoidal quantities. A value of 1 means all apparent power is converted into real power. A lower value indicates a larger reactive component.
Power factor is not a freely interchangeable constant. It depends on the connected equipment and operating state. Motor-starting power factor can differ significantly from the running value.
How are kVA and kW converted?
kW = kVA × power factor
kVA = kW ÷ power factor
Example: 100 kVA at a power factor of 0.8 corresponds to 80 kW. Conversely, 80 kW at 0.8 requires 100 kVA. The calculation assumes the same power factor applies to the operating condition being assessed.
When calculating three-phase power from voltage and current, √3 is also involved. Product selection should use the ratings in the current manufacturer datasheet.
Why is conversion alone not enough for sizing?
A generator set must do more than supply steady-state real power. The alternator must carry apparent and reactive power, while the engine must supply real power and respond to transient changes. The assessment should also cover:
- motor and compressor starting kVA and kW
- starting and running power factor
- non-linear loads such as UPS systems and variable-frequency drives
- load sequence and the size of individual load steps
- permitted voltage and frequency dip
- PRP, ESP or other operating duty
- temperature, altitude, ventilation and other site conditions
What information is needed for reliable selection?
For each load, record at least rated power, voltage, phase, power factor, efficiency, starting method and starting current. Add the load profile, switching sequence, operating duration and environmental conditions.
Final selection should be checked against current manufacturer data and, for demanding loads, a project-specific sizing study.
Frequently asked questions
Does 100 kVA always equal 80 kW?
No. The 80 kW result applies only at a power factor of 0.8. A different power factor changes the conversion.
Can power factor always be assumed to be 0.8?
No. Although 0.8 is a common rating reference, the actual load value depends on the connected equipment and its operating state.
Should a generator be selected by kW or kVA?
Both matter. The engine must supply real power, while the alternator must carry the kVA and reactive-power requirement.
Sources and technical basis
The linked manufacturer sources provide technical context. Current datasheets, manuals and agreed specifications govern each specific project.
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