Generator Sizing Calculator
Combine steady-state load with reserve margin and a first-pass motor-starting case to screen a generator rating before manufacturer selection.
Calculation method
Steady-state generator demand is estimated as running kVA = running kW / PF. A reserve margin is then applied. For the largest motor, the tool estimates full-load current from output power, voltage, efficiency and PF, multiplies it by the selected starting-current factor and converts that starting current to kVA. The governing screening value is the larger of the reserved running load and the motor-starting case.
How to use the result
The next standard kVA is intentionally only a screening suggestion. Use it to establish the order of magnitude for a genset enquiry and to identify when motor starting controls the selection. If several motors can start together, build the actual starting sequence rather than relying on a single largest-motor case.
Method guide: Generator Sizing: Running kVA vs Motor Starting explains the continuous and transient screening cases step by step.
Engineering limits
A real genset selection requires manufacturer transient-performance data. Voltage dip, frequency dip, alternator subtransient reactance, engine step-load capability, temperature, altitude, fuel, harmonic loads, rectifiers, UPS/VFD front ends and allowable starting performance are outside this simplified calculation.
Before design release
Record the source and revision of every project input used in this calculation, then compare the result with the next practical equipment or conductor size rather than treating the numerical minimum as an automatic selection. Check tolerances, environmental derating, duty cycle, protection settings, installation constraints and manufacturer limits that are outside the simplified model. Where the result feeds another study, carry the same voltage, current, power-factor, fault-level and timing assumptions forward so the design chain remains traceable. Save the governing case and the reason for the final engineering choice. ElectroDesigner is intended to make the calculation path visible; it does not replace the project engineer’s verification, the adopted standard or tested manufacturer data.