Generator Sizing Calculator (kVA) – Free

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Loads

Everything running when the largest motor starts

Load 1

Active power

kW

Power factor

Largest Motor

Include

Rated output

kW

Started last, with the loads above running

Efficiency

%

Power factor

Starting method

Starting current (optional)

× FLC

Blank for the typical 6 × FLC

Generator

Loading limit

%

Highest continuous load on the set

Allowed voltage dip

%

While the motor starts; 15 to 20% is common

Transient reactance X′d

pu

From the alternator data sheet; often 0.2 to 0.3

Generator Size

Enter the loads, and the largest motor if there is one, to size the generator.

How It's Calculated

Two checks set the size, and the larger one governs.

Running load

The largest motor is included at its input kVA, its output divided by efficiency and power factor. Adding the loads as vectors rather than adding their kVA keeps loads at different power factors from being overstated.

Motor starting

The largest motor starts last, with everything else running. The generator's transient reactance and the motor's starting kVA act as a voltage divider, which gives the initial dip. This is a first-pass estimate: it ignores the voltage regulator's recovery, the low power factor while starting and the engine's load step capability (ISO 8528-5 performance classes), all of which the generator manufacturer's sizing software covers. Confirm the set with the manufacturer before ordering.

Worked Example

Loads of 20 kW at a power factor of 0.95 and 40 kW at 0.85, plus a 22 kW motor (efficiency 92%, power factor 0.85) started direct on line; 80% loading limit, 15% dip, :

Running: ;
Motor: 28.1 kVA running × 6 = 168.8 kVA starting
Starting: 168.8 × 0.25 × 0.85 / 0.15 = 239.1 kVA
Motor starting governs, so the next common size is 250 kVA, with a 14.4% dip

Starting the same motor star-delta brings the starting check down to about a third, and the running load (125 kVA set) decides the size instead.

Typical Starting Currents

The calculator's defaults, as multiples of the motor's full load current. Use the motor's locked-rotor current or the starter's current limit setting when you have them.

Starting method	Starting current (× FLC)
Direct on line	6
Star-delta	2
Soft starter	3
Variable speed drive	1.5

Frequently Asked Questions

Generator Sizing Questions Answered

How do I size a generator in kVA?

Add up the loads that run together as kW and kVAr, take the kVA as √(kW² + kVAr²), and divide by the highest loading you want on the set, typically 80%. Then check the largest motor can start without dipping the voltage too far; with a direct-on-line motor that check usually decides the size.

Why does motor starting need a bigger generator?

A direct-on-line motor draws about 6 to 8 times its full load current while it accelerates. A generator's internal reactance is much higher than a utility supply's, so that inrush pulls the voltage down; a dip beyond 15 to 20% can drop out contactors, stall the motor or upset other equipment.

How much does a soft starter or VSD reduce the generator size?

Star-delta starting cuts the starting current to about a third of direct-on-line, a soft starter typically limits it to about 3 to 4 times full load current, and a variable speed drive starts at around full load current. Where motor starting governs, the generator can shrink in proportion, until the running load decides the size instead.

What is the transient reactance X′d?

It is the alternator's effective reactance during the first few cycles of a load step, in per unit of its own rating, and is on the alternator data sheet. A lower X′d means a smaller dip for the same motor; values of 0.2 to 0.3 per unit are common.

Should a generator run at 100% of its rating?

Standby ratings allow full load only for the duration of an outage and prime ratings typically average 70% over 24 hours, so sizing the running load to about 80% of the set leaves margin for load growth and for steps. Running a diesel set at very light load for long periods is also bad for it, so do not oversize grossly.

Reviewed by Suheib Sherif, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

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