The calculation, without hidden assumptions
Size a generator from two separate constraints: steady running apparent power and the worst entered motor-start load step. The calculator takes the larger case, then applies your explicit spare-capacity and derating assumptions. It does not hide motor surge inside a generic percentage.
How to use this calculator
Enter the real kW expected to run simultaneously, not every connected load if loads are intentionally non-coincident.
Enter the average running-load power factor used to convert kW to kVA.
Enter the largest motor running kVA as part of that total, then its manufacturer/study starting-kVA multiple. Use 0 kVA if there is no material motor start.
Enter deliberate spare capacity and a project/OEM derating factor. Keep derating at 1.0 when no verified derating applies.
Use the result as a first-pass electrical load model, then verify a real generator/alternator in the manufacturer sizing tool for voltage/frequency dip, harmonics, duty rating and site conditions.
Where people use it
- •First-pass diesel or gas genset kVA sizing from a known load schedule.
- •Checking whether motor starting, rather than steady kVA, governs the preliminary size.
- •Comparing DOL versus reduced-start scenarios by changing the entered motor start multiple.
- •Translating a required kVA into approximate single- or three-phase rated current.
Example: 40 kW running, 15 kVA motor, 6× start
At PF 0.8 the running load is 50 kVA. The other load is 35 kVA and the motor start is 90 kVA, so the 125 kVA start scenario governs. With 15% headroom and a 0.95 derating factor, the planning requirement is about 151.32 kVA.
What the result does not assume
- •This is a preliminary load-step model, not a generator-model selector. OEM tools such as Caterpillar SpecSizer model voltage/frequency dip and specific alternator/engine response.
- •Use actual motor starting kVA or manufacturer data where possible. Starting current alone cannot be converted to starting kW with a guessed power factor without introducing false precision.
- •The largest motor running kVA must already be included in total running kW/kVA; the calculator subtracts its running contribution before adding its start demand so it is not double-counted.
- •Headroom and derating are separate. Do not use a generic margin as a substitute for manufacturer altitude, ambient-temperature or fuel-specific derating curves.
- •Nonlinear loads, UPS/VFD harmonics, regenerative loads, cyclic loads, simultaneous motor starts and strict voltage/frequency-dip limits require a detailed OEM sizing study.
- •Do not use this result to size conductors, breakers, transfer switches, grounding, ventilation, fuel systems or to claim compliance with a particular standby/prime/emergency code.
Frequently asked questions
How do I calculate generator size in kVA?+
Convert simultaneous running kW to kVA using kVA = kW/PF, compare it with the worst credible starting-load step, take the larger demand, then apply explicit project headroom and verified derating.
Why can motor starting determine generator size?+
Motors can demand several times their normal current during acceleration. Caterpillar notes motor starting current can be roughly two to eight times normal running current, so the alternator voltage-dip response can govern selection.
Should I simply add the motor starting kVA to all running kVA?+
Not if that motor running load is already included in the total. Subtract its running kVA first, then add its starting kVA; otherwise the motor is counted twice.
Is 0.8 power factor always correct?+
No. Enter the load-study value. Generator nameplates are often discussed in both kW and kVA, but actual load PF depends on the connected equipment.
What derating factor should I use?+
Use the generator manufacturer data for the actual altitude, ambient temperature, fuel and rating. Leave the factor at 1.0 rather than inventing a derating percentage.
Does this guarantee a motor will start?+
No. A specific genset must be checked for acceptable transient voltage/frequency dip, alternator capability, motor torque/acceleration and the real starting method using OEM sizing data or software.
Semantic next steps
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