Generator Sizing Calculator (kVA)

How is a generator rating estimated?

The first step is to find how much of the load runs at the same time. Multiply the connected load by a demand factor, divide by the power factor to get apparent power and add a spare margin:

S = P × k_e / cos φ × (1 + y)

The result is in kVA. Generators are usually named by their kVA at a power factor of 0.8: a 100 kVA set delivers about 80 kW of active power.

Example

Connected load 80 kW, demand factor 80 percent, power factor 0.8, spare margin 20 percent. Demand is 64 kW, which is 80 kVA. With the margin 96 kVA is needed, and the rated current at 400 V three phase is 139 A.

Why motor starting matters

Motors draw several times their rated current when they start. A generator is not as stiff as the grid: that current pulls the voltage down, contactors may drop out and other equipment is disturbed. In the plant above a 15 kW motor started direct on line draws 117.6 kVA, taking the momentary total to 178 kVA. That is almost twice the 96 kVA found for the running load.

Either the generator is made larger or the starting current is reduced (star delta, soft starter, drive). The starting kVA an alternator can supply briefly and the permitted voltage dip come from the manufacturer's curves.

What this tool does not cover

Harmonic loads, rating class, altitude and temperature derating, load steps and the voltage dip calculation are not included. All factors are values you enter. The result is a first estimate: the final selection must be made by a qualified engineer from manufacturer data and the applicable standard.