Generators are sold by their kVA rating, but the equipment you plug into them is usually listed in kW. Picking a generator therefore always involves one conversion — and one assumption about power factor. Get either wrong and you end up with a set that trips under load or one that is oversized and runs inefficiently.
Why generators are rated in kVA
An alternator’s limits are thermal: winding temperature and insulation depend on the total current flowing, which includes both real and reactive components. Because the manufacturer cannot know what power factor your loads will have, they rate the machine in apparent power (kVA) and quote kW at a standard power factor — almost always 0.8.
A 200 kVA generator at 0.8 PF can therefore deliver 160 kW of real power.
Step 1 — Add up the real power (kW)
List every load that will run at the same time and total its running kW. Use nameplate data or measured values rather than breaker ratings.
| Load | Running kW |
|---|---|
| Lighting and small power | 18 |
| HVAC compressors | 55 |
| Pumps | 22 |
| IT and controls | 25 |
| Total | 120 |
Step 2 — Convert kW to kVA with the load power factor
Divide the total real power by the lowest expected power factor of the combined load:
For 120 kW at 0.8 PF: 120 ÷ 0.8 = 150 kVA.
Step 3 — Add design margin
Motors draw 3–7× their running current when starting, and future loads are almost always added. A 20–25% margin is common practice:
150 kVA × 1.25 = 187.5 kVA
Step 4 — Select the next standard size
Round up to the next standard rating — here, a 200 kVA set. Check that its kW rating (160 kW at 0.8 PF) comfortably covers the 120 kW running load.
Common mistakes to avoid
- Treating kVA as kW. A 150 kVA generator is not a 150 kW generator unless the load is purely resistive.
- Ignoring motor starting. Large motors may need a soft starter or VFD, or a larger alternator, even if running kW fits.
- Sizing for 100% continuous load. Most generators run most efficiently between 50% and 80% of rated capacity.
- Forgetting non-linear loads. UPS systems and drives create harmonics that may require alternator derating.
Ready to run the numbers? Use the kVA to kW calculator or the kW to kVA converter on the homepage.