kW, kVA, Amps and HP Converter

Generators are sold in kVA. Loads are specified in kW. Motors are rated in HP. Breakers are rated in amps. This converts between all four for single and three phase supplies.

How this calculation works

Everything routes through kVA, apparent power, as the common currency.

Three phase: kVA = (V × I × √3) ÷ 1000 Single phase: kVA = (V × I) ÷ 1000 Real power: kW = kVA × power factor Motor rating: HP = (kW × efficiency) ÷ 0.7457

Why the √3? In a three phase system the three conductors carry currents 120° apart in time. The total power delivered isn’t three times the single phase figure — it works out to √3, roughly 1.732, times the line voltage and line current.

What power factor actually means. Apparent power (kVA) is what the supply has to deliver. Real power (kW) is what does useful work. Power factor is the ratio between them. Motors and transformers draw current to establish magnetic fields, and that current does no work but still has to be carried by the cable and supplied by the generator. A 0.8 power factor load needs 25% more current than a purely resistive load of the same kW.

Why generators are rated in kVA. A generator’s limits are thermal — the alternator windings and the cables can only carry so much current, regardless of whether that current is doing useful work. So it’s rated on current-carrying capacity, which is kVA. The engine behind it is rated in kW. This is why a standard set is quoted as, say, 500 kVA / 400 kW.

HP is shaft output, not electrical input. A 100 HP motor produces 100 HP at the shaft and draws more than that from the supply, because no motor is 100% efficient. Converting HP to electrical kW means dividing by efficiency, typically 0.85–0.92 for industrial motors. Getting this backwards undersizes the supply.

Worked example

A specification calls for a 150 HP three phase pump motor at 400 V. What generator does it need?

Shaft output: 150 × 0.7457 = 111.9 kW. Electrical input at 88% efficiency: 111.9 ÷ 0.88 = 127.1 kW. At 0.85 power factor: 127.1 ÷ 0.85 = 149.6 kVA running. Running current: (149.6 × 1000) ÷ (400 × 1.732) = 216 A.

So a 150 HP motor draws around 216 A running. Started direct on line at six times, it briefly pulls over 1,290 A — which is why motors that size are almost never started DOL on generator supply.

Common mistakes

Treating kVA and kW as interchangeable. They differ by power factor, typically 20%.

Using √3 on single phase. Or omitting it on three phase. Either gives an answer wrong by 73%.

Converting HP without efficiency. Understates the electrical demand by 10–15%.

Assuming power factor 1.0. Only correct for purely resistive loads — heaters, incandescent lighting. Motor loads are 0.8–0.85.

FAQ

How do I convert kW to kVA? Divide kW by the power factor. At 0.8, 80 kW is 100 kVA.

How many amps is a 100 kVA generator? At 400 V three phase, about 144 A. At 415 V, about 139 A. Single phase 230 V, about 435 A.

Why is my generator rated in kVA but the engine in kW? The alternator is limited by current, which is kVA. The engine is limited by torque, which is kW. A standard 0.8 power factor set pairs a 500 kVA alternator with a 400 kW engine.

What power factor should I assume? 1.0 for resistive loads. 0.85 for general mixed industrial load. 0.8 for motor-dominated load. If you have a measured figure from a power quality survey, use that.