kVA to kW Calculator

The formula#

P = S × PF

Where S is apparent power in kVA, PF is power factor, and P is real power in kW.

That is the whole conversion. No voltage, no √3, no phase — those matter for current, not for this. kVA and kW are both power; power factor is simply what fraction of the apparent power does work.

A worked example#

A 100 kVA diesel generator supplying a load at 0.8 power factor:

P = 100 × 0.8 = 80 kW

Which is why the nameplate on that machine reads 100 kVA / 80 kW. Two ratings, one set, and they describe two different limits.

Why a generator has two ratings#

A DG set is two machines bolted together, and each has its own ceiling.

The alternator is limited by heating in its windings. Heating follows current, and current follows kVA — the alternator neither knows nor cares what the power factor is. Its limit is 100 kVA.

The engine is limited by the fuel it can burn and the torque it can produce. That is real work, measured in kW. Its limit is 80 kW.

Connect a load at 0.8 PF and both limits arrive together: 100 kVA of current in the windings, 80 kW of work at the crankshaft. The set is perfectly matched, which is exactly why 0.8 is the convention manufacturers rate to.

Now change the load:

At PF 1.0 the alternator would happily carry 100 kVA, but the engine caps you at 80 kW. You are paying for alternator capacity you cannot use.

At PF 0.6 the reverse: the engine is loafing at 60 kW while the alternator sits at its thermal limit. You bought a 100 kVA set and you are getting 60 kW of useful work out of it.

This is the practical argument for power factor correction on a site running on generator supply. Improving PF from 0.7 to 0.9 does not make the engine bigger — but it lets you draw 80 kW instead of 70 kW from the same machine, because you stop wasting alternator capacity on reactive current.

kVA to kW chart#

The odd-looking ratings — 62.5, 82.5, 125, 320 — are not arbitrary. They are the kVA sizes that give round kW numbers at 0.8 PF: 50 kW, 66 kW, 100 kW, 256 kW. Manufacturers sell engines in kW and alternators in kVA, and the catalogue reflects both.

Transformers work the same way, with one difference#

A 100 kVA transformer at 0.8 PF also delivers 80 kW. The arithmetic is identical.

The difference is that a transformer has no engine. There is no second limit — the transformer will happily deliver 100 kW into a unity power factor load, because the only constraint is winding heating, and that is set by current alone. A transformer's kVA rating is its whole rating.

So on a transformer, poor power factor does not waste a kW capability you paid for. It wastes current capacity: reactive current occupies the windings and the cables feeding them, heats everything on the way, and on a maximum-demand tariff it shows up on the bill. The kW, kVA and kVAr converter works out the capacitor rating needed.

What this means when you are sizing#

Sizing a generator for a known kW load. Work backwards through the kW to kVA calculator, then add margin for motor starting — which usually governs. A 60 kW load at 0.85 PF needs 70.6 kVA on running load, but if it includes a 22 kW motor started direct-on-line, the set will need to be considerably larger to hold voltage through the start. The DG set sizing calculator handles that properly.

Reading a UPS nameplate. Same conversion, different label. A 1000 VA UPS at 0.6 PF is a 600 W UPS. See VA to watts.

Checking a supplier's claim. If a quotation offers "100 kVA = 100 kW", someone has confused apparent power with real power. The only load where that is true is a purely resistive one.

Common mistakes#

Treating kVA and kW as interchangeable. They are equal only at unity power factor, which no industrial load has.

Applying power factor twice. If a figure is already in kW, it has had the power factor applied. Multiplying again by 0.8 halves the answer over two steps.

Assuming 0.8 because the nameplate says so. The 0.8 on a generator plate is the rating power factor, not a measurement of your site. Your actual load might be 0.75 or 0.95, and the set behaves differently in each case.

Sizing on running kW only. Starting kVA can be three to five times running kVA, and it lasts long enough to dip the voltage across the whole site.