Apparent power versus real power
Every AC load pulls two kinds of current. One is in step with the voltage and does the actual work — heating, turning a shaft, lighting a filament. The other lags or leads the voltage and shuttles energy back and forth into magnetic fields and capacitors without consuming any of it. Add them together and you get apparent power, measured in kVA. Keep only the working part and you get real power, measured in kW. The ratio between them is the power factor.
This conversion matters because the two units answer different questions. A generator or UPS is rated in kVA because that’s what its alternator and wiring have to carry. The load you want to run is described in kW because that’s the work it does. To know whether a 5 kVA generator can run a 4 kW load, you have to get both onto the same footing.
The kVA to kW formula
Worked example
The calculator’s defaults are 5 kVA at a power factor of 0.8.
The load consumes 4 kW of real power. The remaining 3 kVAR of reactive power — from
How power factor sets the result
| Apparent power | PF 1.0 | PF 0.9 | PF 0.8 | PF 0.6 |
|---|---|---|---|---|
| 5 kVA | 5.0 kW | 4.5 kW | 4.0 kW | 3.0 kW |
| 10 kVA | 10.0 kW | 9.0 kW | 8.0 kW | 6.0 kW |
| 25 kVA | 25.0 kW | 22.5 kW | 20.0 kW | 15.0 kW |
The same generator delivers noticeably less usable power into a poor-power-factor load. This is why a lightly loaded motor — which has a bad power factor — is such an inefficient use of a generator’s capacity.
Reading a dual-rated nameplate
Generators and UPS units often print both figures, for example “5 kVA / 4 kW.” The kW number already assumes a power factor, almost always 0.8. If your actual load runs at a higher power factor you can approach the full 5 kW; at a lower one you’re capped below 4 kW. Always compare your load’s real power factor against that 0.8 assumption before trusting the nameplate kW.
Common mistakes
- Assuming kW equals kVA. Only true at unity power factor — resistive loads.
- Using the generic 0.8 for a resistive load (should be 1.0) or for a lightly loaded motor (often 0.5–0.7).
- Sizing a generator to the kW figure and ignoring kVA. The alternator limit is kVA; a 4 kW load at 0.6 power factor is 6.7 kVA and can overload a “5 kW” generator.
For the reverse, use the kW to kVA calculator. To get the line current from either figure, see kVA to amps.