Turning a measured wattage into a sizing number
You’ve measured or looked up a load’s real power in watts, and now you need the apparent power in kVA — because that’s how generators, UPS units, and transformers are rated. Real power is the work the load does; apparent power is what the source has to supply once the reactive current is included. The two diverge whenever the load isn’t purely resistive, and the power factor is the ratio between them.
The watts to kVA formula
Worked example
The calculator’s defaults are 4,000 W at a power factor of 0.8.
Divide by the power factor to get volt-amps:
Rescale to kilovolt-amps:
The load consumes 4,000 W of real power but presents 5 kVA to the source. A generator or UPS for this load needs a rating of at least 5 kVA before any margin is added.
Watts to kVA by power factor
| Watts | PF 1.0 | PF 0.9 | PF 0.8 | PF 0.65 |
|---|---|---|---|---|
| 500 W | 0.50 kVA | 0.56 kVA | 0.63 kVA | 0.77 kVA |
| 1,500 W | 1.50 kVA | 1.67 kVA | 1.88 kVA | 2.31 kVA |
| 4,000 W | 4.00 kVA | 4.44 kVA | 5.00 kVA | 6.15 kVA |
| 8,000 W | 8.00 kVA | 8.89 kVA | 10.00 kVA | 12.31 kVA |
Getting the power factor right
For a single appliance the power factor is often a guess, and the guess drives the whole result. Many plug-in energy meters display power factor or VA directly alongside watts — use that reading when you have it. Failing that: near 1.0 for resistive heat, roughly 0.9 for power-factor-corrected electronics, 0.7 to 0.85 for motors, pumps, and older linear supplies. When several loads run together, their power factors don’t simply average; the reactive parts add separately from the real parts.
Beyond the steady-state number
The kVA figure this returns is the continuous demand. Practical sizing of a generator or UPS adds headroom for motor starting surge, for the harmonic distortion that switch-mode loads inject, and for future load growth — commonly 20 to 25% on top.
Common mistakes
- Assuming unity power factor for anything with a motor or a cheap power supply.
- Skipping the ÷ 1,000 and reporting VA as kVA.
- Sizing a source exactly to the result with no margin for inrush or growth.
For the reverse, use the kVA to watts calculator. To get line current from the apparent power, see kVA to amps.