To size a generator, convert your total load to kVA (kW ÷ 0.8), size the set so that load sits at roughly 70–80% of its prime rating, then check that your largest motor can start without exceeding what the alternator can absorb. Whichever of those two checks demands the bigger set wins. That is the whole method — the rest of this guide is the detail that stops it going wrong.
TL;DR
- Running kVA = total kW ÷ 0.8 (the ISO 8528 rating convention every Indian genset is nameplated to)
- Target 70–80% loading of the prime rating — ISO 8528-1 caps the 24-hour average at 70%
- Motor starting, not running load, usually decides the rating: a DOL motor draws ~7× full load for a few seconds
- Below ~30% sustained loading you risk wet stacking — an oversized set is also a wrong set
- Use the calculator, then let an engineer confirm it free
Why generators are rated in kVA, not kW
A genset’s nameplate kVA is an apparent-power rating at 0.8 power factor lagging — the ISO 8528 convention. The engine limits the real power (kW); the alternator limits the current, and therefore the kVA. A 500 kVA set is a 400 kW set (500 × 0.8), and both limits are real: exceed the kW and the engine lugs; run at poor power factor and the alternator windings overheat even though the kW looks fine.
Practical rule: sum your loads in kW, divide by 0.8, and you have running kVA. If a motor is rated in HP, multiply by 0.746 to get kW first.
How much headroom should the set have?
Size so your running load lands at roughly 70–80% of the prime (PRP) rating:
| Running load as % of prime rating | What it means |
|---|---|
| Above ~80% | No margin for surges; ISO 8528-1’s 70% average-load-factor cap is likely breached |
| ~70–80% | The target band — margin for starting surges and future loads |
| 30–70% | Fine — comfortable, slightly generous |
| Below ~30% sustained | Wet-stacking territory — unburned fuel glazes the exhaust; Caterpillar recommends loading to at least 30% |
The last row surprises people: too big is also wrong. A set loafing at 15% load for months carbons up, smokes, and eventually fails the very afternoon you need it.
Why does motor starting change everything?
Because an induction motor started direct-on-line (DOL) draws about seven times its full-load current for the seconds it takes to run up (Stamford’s AGN090 application guide; 6–7× is the standard planning range). The genset’s alternator must absorb that surge without the voltage dipping so far that contactors drop out — in practice a Class-H alternator absorbs roughly 1.75× its rating in starting kVA at an acceptable ~25% dip.
The starter changes the arithmetic:
| Starting method | Starting demand (× motor full-load kVA) |
|---|---|
| Direct-on-line (DOL) | ~7× |
| Star-delta | ~3× (and a mistimed transition can spike to 5×) |
| Soft starter | ~3–3.5× at typical current-limit settings (limits below ~3× often cannot accelerate the load) |
| VFD / drive | ~1.1–1.5× — essentially no inrush |
Two identical factories — same machines, same running load — can need gensets two ratings apart purely because one starts its compressor DOL and the other fitted a star-delta starter. Sometimes a ₹40,000 starter beats a ₹4,00,000 bigger genset.
A worked example
A 30,000 sq ft warehouse: 12 kW of lighting and sockets, and one 20 HP (14.9 kW) DOL compressor.
- Running load: (12 + 14.9) ÷ 0.8 = 33.6 kVA
- Headroom check: 33.6 ÷ 0.75 → a ~45 kVA set would cover running load
- Starting check: compressor surge = 14.9 ÷ 0.8 × 7 = 130.5 kVA, on top of 15 kVA already running = 145.5 kVA. Alternator needed: 145.5 ÷ 1.75 = 83 kVA
- Answer: not 45 — the next standard rating above 83: a 100 kVA set.
“Just add up the kilowatts” would have undersized this site by two ratings. The kVA calculator runs exactly this method and snaps the answer to the real fleet ladder.
What a simple method can’t catch
Be honest about the edges — these are the cases where you stop calculating and talk to an engineer:
- UPS- and VFD-heavy sites (server rooms, modern process lines): harmonics heat the alternator; the fix is often an oversized or specially-wound alternator, not more engine.
- One very large motor relative to the total load: the alternator, not the load sum, sizes the set.
- High-inertia loads — crushers, big fans, loaded conveyors — with long run-ups can trip protection even when the numbers fit.
- Sensitive loads (medical imaging, labs) need tighter transient classes (ISO 8528-5 G3/G4) than a standard sizing rule assumes.
- Altitude and heat derate engine output — manufacturer data, not rules of thumb.
- Motor nameplates quote shaft power; electrical input is a little higher. On one motor it’s noise; on twenty it’s a rating step.
The fastest way to get it right
Run your list through the calculator, then send the result to an engineer on WhatsApp — sizing confirmation is free, same-day, and occasionally saves you from renting a set two sizes too big. If the load is real and imminent, every rating from 25 to 1250 kVA in the rental fleet carries its full Cummins specification so you can see exactly what you’d be getting.
Method sources: ISO 8528 rating conventions (Cummins PowerHour reference), Stamford AGN090 “Motor Starting Fundamentals” (starting multiples, alternator transient capability), Caterpillar white papers on underloading and wet stacking. Figures are planning values for 415 V/50 Hz LT installations at 0.8 PF — final sizing is always confirmed against the actual load list.