Generator Formulas: kVA, Current and Fuel

Generator formulas for kVA from kW and power factor, rated current, fuel use from specific consumption and tank runtime, with worked examples and a fuel table.

The formulas

SFC is the specific fuel consumption in g/kWh and ρ the density of the fuel in g/L. Take SFC from the engine data sheet at the actual load, because it is higher at light load. A generator is rated in kVA at a stated power factor, usually 0.8.

Worked example: a 120 kW load at power factor 0.8

1. S = 120 / 0.8 = 150 kVA
2. I = 150 × 1000 / (√3 × 415) = 208.7 A

Worked example: fuel use at 100 kW

SFC is 220 g/kWh, diesel is 840 g/L, the tank holds 500 L and no more than 90% of it is usable. These are example values.

1. L/h = 100 × 220 / 840 = 26.19 L/h
2. t = 500 × 0.9 / 26.19 = 17.18 hours

Fuel use by load (SFC 220 g/kWh)

Frequently asked questions

Why is a generator rated in kVA and not kW?

The alternator heats according to the current, not the power factor. The kVA rating sets the current limit, and the kW available depends on the load power factor.

Should I size the generator at exactly the load?

No. Leave margin for growth and for starting currents, and check the largest motor start separately because it causes a voltage dip.

Why does fuel use per kW rise at light load?

The engine is less efficient when lightly loaded, so the SFC is higher. Long periods below about 30% load can also harm the engine.

Does the fuel density matter?

A little. Diesel is commonly taken as about 840 g/L, but use the value for your fuel.

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