Running Load · Motor Starting · Standby / Prime
Size a standby or prime power generator by comparing continuous running load requirements against the temporary starting demand of the largest motor on the system. Generators must be sized for whichever condition is more demanding — steady-state running load, or the voltage-dip-limited starting requirement of large motor loads. A future capacity margin can be applied for planned load growth.
Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.
Two independent kVA requirements are calculated: the steady-state running requirement (total kW ÷ generator PF), and the starting requirement (background load kVA plus the motor's starting kVA demand at the selected starting method's multiplier). The generator must be sized for whichever is larger — for installations with a large motor relative to background load, the starting condition frequently governs sizing, not the running load alone.
Standby and Prime are ISO 8528-1 duty ratings, not interchangeable labels for the same set at a different size — a Standby set is limited to roughly 200-500 hours a year at variable load with no continuous-load guarantee, while a Prime set is rated for unlimited hours provided its AVERAGE load stays within 70% of the rated capacity over the duty cycle. That 70% figure is the one numeric rule this calculator checks against for Prime duty. Manufacturers typically publish a Standby rating 10-25% higher than the Prime rating for the same physical set, but published sources are explicit that this is not a fixed percentage — it varies by engine and manufacturer, so this tool does not apply a derating factor between the two and instead sizes to the load and checks the load-factor rule directly.