Quick answer: A genset must cover both the steady running load and the transient motor-starting surge. Size it as the greater of (running kVA ÷ PF) and (running load + largest motor starting kVA), then round up to a standard IS 10000 rating with a margin.
How to size a generator
Running kVA = Total running load (kW) / PF
Starting kVA = running + (motor kW × starting factor)
DOL start factor ~ 6-7 x FLC; Star-Delta ~ 2-3 x; VFD ~ 1x
Selected kVA = next standard rating ≥ max(running, starting)Steps
- Total the running load in kW and convert to kVA.
- Identify the largest motor and its starting method.
- Add the motor starting kVA to the base running load.
- Select the next standard genset rating above the worst case, plus margin.
Worked example
Running load 200 kW at PF 0.8 = 250 kVA. Largest motor 30 kW DOL adds ~180 kVA transient → starting case ≈ 430 kVA. Select a 500 kVA genset, or use star-delta/VFD to reduce the requirement.
Running load + motor starting kVA to a standard IS 10000 rating. Free tool.
Open the Generator Sizing Calculator →Frequently Asked Questions
How do you calculate generator size in kVA?
Convert running load in kW to kVA by dividing by power factor, then check the motor starting case (running load plus the largest motor starting kVA). Size the genset for the greater of the two, plus a margin.
Why does motor starting affect generator sizing?
Motors draw 6-7 times their full-load current on direct-on-line start, causing a large transient kVA demand and voltage dip. The genset must handle this surge, which often governs the rating.