Formula
S (kVA) = P (kW) ÷ PF
The kW–kVA relationship is the same for single and three phase; the phase count matters only when converting to amps.
Worked example
A home with 3.5 kW of essential load (fans, lights, fridge, TV) at PF 0.8: S = 3.5 ÷ 0.8 = 4.38 kVA — so a 5 kVA single-phase generator is a sensible choice.
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
Residential backup is single-phase territory in most of the US, UK and beyond, and this is its sizing formula. Total the running watts of what must stay on — refrigerator, furnace blower, lights, internet, a window AC — divide by 0.8, and you have the minimum kVA before surge headroom. It's the storm-prep calculation millions of households redo every hurricane season.
Common mistakes to avoid
The killer omission is motor starting surge: a refrigerator compressor or sump pump briefly draws 3–6× its running watts. Size for the surge of the largest motor on top of the steady load, or the generator will trip exactly when the basement is flooding. Portable generator wattage labels are 'starting watts' — read the smaller 'running watts' figure.
Frequently asked questions
Is the formula different for single phase?
No — kW ÷ PF = kVA in both systems. Single vs three phase changes the current calculation, not the power relationship.
What size generator for a typical house?
Add up the wattage of appliances you'll run simultaneously, divide by PF (0.8 is a safe assumption), then choose the next standard kVA size up. Motor appliances like refrigerators and water pumps briefly draw 3–5× their running power when starting.
Why does my 5 kVA generator struggle with a 4 kW load?
At PF 0.8 a 4 kW load is 5 kVA — the generator is at 100%. Add starting surges and it will trip or stall. Leave 20–30% headroom.