DG Size Calculator
Estimate preliminary generator-set size while accounting for running load and the largest motor starting demand.
How this calculator works
The calculator first checks steady running kW. It then estimates a starting-demand case by replacing the largest motor's running kW with its starting multiplier.
Peak estimated kW = Other running kW + Largest motor kW × Starting Multiplier.
The higher load is converted to kVA using generator PF and then multiplied by the selected safety margin.
Example
If a site has 100 kW total running load including a 20 kW motor, and that motor needs 3 times running power during start, the preliminary peak becomes 80 + 60 = 140 kW before margin.
Source and engineering note
Cummins generator-sizing guidance emphasizes both running load and starting demand. A practical preliminary approach is to consider the largest starting load in addition to the other running loads, then keep a safety margin. Final generator selection should be checked with the generator manufacturer for transient voltage dip, alternator capability, nonlinear loads and site conditions.
Cummins generator sizing guidance
Last reviewed: 27 August 2026.
Frequently asked questions
Why not just add all running loads?
Generators must also handle short-duration starting demand, especially from motors and compressors.
Should every motor starting load be added together?
Usually the worst practical starting sequence is assessed, often the largest motor starting while other loads are already running.
Does a VFD change starting demand?
Yes. Starting characteristics can be very different with VFDs or soft starters. Use actual equipment data.
Is 25% margin mandatory?
No. It is an editable planning margin. Final OEM sizing should determine the appropriate margin.