Formula
S (kVA) = P (kW) ÷ PF
Apparent power is always greater than or equal to real power, because PF ≤ 1.
Worked example
A 10 kW load with power factor 0.8 needs S = 10 ÷ 0.8 = 12.5 kVA of generator capacity — before adding any safety margin.
Reference table
Effect of power factor
| Power factor | kVA needed for a 10 kW load |
|---|---|
| 0.7 | 14.29 |
| 0.8 | 12.5 |
| 0.85 | 11.76 |
| 0.9 | 11.11 |
| 0.95 | 10.53 |
| 1.0 | 10 |
Where this shows up in the real world
This is the generator-shopping conversion. Loads are added up in kW, generators are sold in kVA, and power factor is the exchange rate. A US home with 8 kW of essential storm load needs 10 kVA of generator at the standard 0.8 PF — skip this step and the 8 kVA unit on sale will stall the first time the well pump and AC start together.
Common mistakes to avoid
Never assume PF = 1 to make the math friendlier — real mixed loads run 0.7–0.9, and the optimistic assumption under-buys the generator by 10–30%. The reverse error: applying the 0.8 conversion *and* a separate 25% surge margin twice. Convert once, add headroom once.
Frequently asked questions
Why is the kVA always bigger than the kW?
Power factor is at most 1, so dividing kW by PF can only increase the number. The gap is the reactive power the supply must still carry.
What PF do generator suppliers assume?
The industry standard rating point is PF 0.8 — a '100 kVA' generator is typically rated to deliver 80 kW.
Does improving power factor reduce my kVA need?
Yes. Correcting PF from 0.7 to 0.95 cuts the kVA demand of the same kW load by about 26%, freeing transformer and generator capacity.