Generator kVA ↔ kW Conversion Table

Generator catalogues give power in kVA, while most loads are listed in kW. A “100 kW generator” and a “100 kVA generator” are therefore two different sets. This page gives the formula, a conversion table for our 10–4,125 kVA range and the current at 400 V.

The formula

Apparent power (kVA) and active power (kW) are linked by the power factor, cos φ. Three-phase diesel generator sets are rated at cos φ = 0.8:

• kW = kVA × 0.8
• kVA = kW ÷ 0.8
• Current at 400 V: I = kVA × 1,000 ÷ (√3 × 400) ≈ 1.44 A per kVA

Example: a 250 kVA set delivers 250 × 0.8 = 200 kW. A 200 kW load needs at least 200 ÷ 0.8 = 250 kVA, and more once motor starting and a reserve are added.

The tables below are calculated with these formulas. The exact standby and prime values of each model are on its own product page, because they depend on the engine and alternator fitted.

Sizing from a known load

If the total running load in kW is known: load ÷ 0.8 = minimum kVA. Then add the starting margin for the largest motor and a reserve of about 20%. A pump or compressor started direct-on-line draws several times its running current while starting, so the start, not the running load, can decide the size. Our power calculator gives a first estimate, and the article on what size generator you need goes through the steps.

Standby or prime: which kVA?

The same set has two ratings. Standby is for supply during mains failures with limited hours per year; prime is for unlimited hours under variable load and is lower. If one offer quotes standby and another quotes prime, you are comparing different sets. See standby, prime and continuous power.

Engine kW is not generator kW

Data sheets often show two kW figures: engine shaft power (kWm) and electrical output of the set (kWe). The engine figure is always higher, because the radiator fan and alternator losses take part of it. Only the electrical output counts when choosing a generator.

Common mistakes

• Buying “100 kVA” for a 100 kW load. That set gives 80 kW, with no reserve.
• Comparing engine kW with generator kW. The offers are then not comparable.
• Ignoring motor starting. The set stalls or the voltage dips when a pump or lift starts.
• Ignoring site conditions. In heat or at altitude the set delivers less; see derating for temperature and altitude.

Send us your list of loads and we will select the model: contact us.