If you work with generators, transformers, or UPS systems, you've run into this exact confusion: nameplate ratings are often given in kVA (kilovolt-amperes), but the actual usable power you can draw is in kW (kilowatts) — and the two are not the same number unless the power factor is exactly 1.0, which almost never happens in real electrical systems.
The Difference Between kVA and kW
kVA measures apparent power — the straightforward product of voltage and current, without accounting for the phase relationship between them in an AC circuit. kW measures real power — the power actually doing useful work (running a motor, lighting a bulb, heating an element). The gap between the two is the power factor (PF), a value between 0 and 1 that reflects how efficiently the current and voltage are aligned in time.
Resistive loads (heaters, incandescent bulbs) have a power factor close to 1.0, so kVA and kW are nearly identical for them. Inductive loads (motors, transformers, fluorescent ballasts) typically have a power factor of 0.8-0.95, meaning a device's real usable power is noticeably less than its apparent kVA rating.
Worked Example
A generator rated at 20 kVA, with a load that has a power factor of 0.85:
This is the number that actually matters for sizing — if you assumed the generator delivered its full 20 kW, you'd be overestimating capacity by 3 kW, which is exactly the kind of mistake that causes a generator to trip or stall under a load it "should" have handled on paper.
Why This Trips People Up When Buying a Generator
Generator and UPS spec sheets almost always lead with the kVA number because it's the larger, more impressive-looking figure. The real, usable kW capacity — what actually determines whether your equipment will run — is smaller and often buried in the fine print or a separate spec line. Always check both numbers, and always size against kW, not kVA, unless you're certain your load's power factor is at or near 1.0.
Typical Power Factor by Load Type
| Load Type | Typical Power Factor |
|---|---|
| Incandescent lighting, resistive heaters | 0.95–1.0 |
| LED lighting | 0.9–0.95 |
| Standard induction motors | 0.80–0.90 |
| Fluorescent lighting (older ballasts) | 0.5–0.9 |
| Welding equipment | 0.5–0.7 |
| Mixed household/office load | ~0.85–0.9 (typical assumption) |
Frequently Asked
If I don't know my exact power factor, what should I assume?
For general household or office equipment, 0.8-0.9 is a reasonably safe planning assumption. For a critical sizing decision (a backup generator for essential equipment), get the actual power factor from the equipment's nameplate or manufacturer spec rather than guessing.
How do I convert kW back to kVA?
Rearrange the formula: kVA = kW ÷ PF.
Does this apply to single-phase and three-phase the same way?
The kW = kVA × PF relationship holds for both — the difference between single- and three-phase only affects how kVA itself is calculated from voltage and current (three-phase includes the √3 factor).
For three-phase power calculations from voltage and amperage, see the main calculator's three-phase section.
