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Generator Sizing Calculator

Most generator calculators add up your running load and stop there. That is not what decides the size. On a site with motors, the set is almost always chosen by the largest motor starting, because the current surge at start drags the voltage down. This one works both out and tells you which is governing.

Running loadTotal running loadKilowatts. Everything that will be running at once, motors included - not the sum of everything installed.Overall power factor0.8 is the usual assumption for a mixed industrial load, and is what generator sets are rated at.Line voltageVolts, three-phase.

Largest motorRated outputKilowatts, of the largest single motor that starts against the generator. Enter 0 if there are no motors.Starting methodDOL starting multipleTimes full load current.Motor efficiencyPer cent.Acceptable step loadPer cent of the set's rating that may be applied in one block. A standard alternator will usually take somewhere around 60 %; your supplier will give the real figure for the machine.

Duty and site conditionsDuty ratingNamed in ISO 8528-1. A set rated 100 kVA standby is worth roughly 90 kVA prime or 70 kVA continuous, so the harder you work it, the bigger it has to be.Site altitudeMetres above sea level.Maximum ambient temperatureDegrees Celsius.

Results

Running demand -

Motor full load current -

Motor starting kVA -

Peak demand while starting -

Derating applied -

Minimum set size -

Governed by -

What "governed by" means. If it says running load, the set is sized by what the site draws day to day and the motor start is comfortably within it. If it says motor starting, you are buying a bigger machine than the running load needs, purely so the lights do not dim when the big pump kicks in. That is normal, and it is the case worth knowing about before you order.

Worked example

A pumping station: 45 kW of running load at pf 0.8 on 400 V, with a 15 kW pump starting direct on line.

Running demand = 45 / 0.8 = 56.3 kVA

The 15 kW pump draws about 30.1 A at full load, so a 6.5 × direct on line start is roughly 195 A - about135 kVA in one block.

At a 60 % step limit the set has to be at least135 / 0.6 = 226 kVA. That is four times the running demand, and it is the motor start paying for it, not the site.

Start the same pump star-delta and the block drops to about 45 kVA. The set needed falls to roughly 80 kVA - and what now governs is simply the peak while starting, not the step limit. Same site, same pump, a third of the generator.

A first-pass estimate for comparing options, not a specification. Real sizing depends on the alternator's reactance, the order loads are brought on, non-linear loads such as UPS and inverters, and the voltage dip the installation can actually tolerate. Confirm the final selection with the supplier before ordering. Derating figures used here are common rules of thumb and vary between manufacturers.

Standards reference: Duty ratings (ESP, PRP, COP) are defined in ISO 8528-1; transient performance classes in ISO 8528-5. Neither standard is reproduced here - consult the published documents for the definitions and limits.