Genset Sizing Calculator
A 40 kW plant whose biggest motor is a 30 kW DOL compressor needs about a 100 kVA genset — double what the running load suggests — because that motor starts at six times its running current. Put a VFD on it and 75 kVA does the job.
Enter your running load and your largest motor. The calculator sizes the set both ways — steady loading per ISO 8528-1 and motor-starting surge per IEC 60034-12 — takes the worse case, rounds to a standard frame, and shows what the diesel actually costs per kWh.
Your Loads
Sizing Results
| Step | Value | Basis |
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The Method
Steady-state requirement (keep average loading at or below target — ISO 8528-1 PRP convention):
Motor starting requirement (largest motor FL kVA × starting multiplier, plus the rest of the load, within the alternator's brief surge capability):
Recommended set — the worse of the two, rounded up to a standard frame size.
Method & Sources
This is a screening tool with stated assumptions — final selection belongs with the genset supplier against the alternator's transient curve.
- Rating definitions — ISO 8528-1: ESP (standby, no overload), PRP (prime, ≤70% average load, 10% overload 1 h in 12), COP (continuous). Quotes often lead with the bigger ESP number — compare like with like.
- DOL starting current 6–8× full load — IEC 60034-12 (Design N induction motors). We use 6× as the default; star-delta ≈ one-third of DOL, soft starters typically 2.5–4× (settable), VFDs ≈1.2×.
- Motor full-load kVA — kW ÷ (η × PF) with η = 0.92 and PF = 0.85, typical for IE2/IE3 motors in this size range.
- Alternator surge capability — default 2.5× rated kVA briefly at roughly 30% voltage dip; real machines range 2–3×. Editable, and worth checking against the manufacturer's skVA curve.
- Fuel burn ~0.28 L/kWh at 75% load — typical modern diesel fuel curve; your data sheet may differ. Engine wear point: avoid sustained running below ~30% load (wet stacking).
- What we leave out — harmonic loads (UPS, VFD banks) which can force an oversized alternator, altitude/temperature derating, and paralleling. Site-specific, by design.
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Frequently Asked Questions
What is the difference between ESP, PRP and COP ratings?
ISO 8528-1 defines three: ESP (standby — maximum output during outages, no overload, limited hours), PRP (prime — main supply on variable load, ≤70% average, 10% overload 1 h in 12), COP (continuous base load, lower again). A set advertised at ESP is ~10% smaller in PRP terms — check which rating your quote uses.
Why does motor starting decide the generator size?
A DOL induction motor draws 6–8× its full-load current while starting (IEC 60034-12, Design N). A 30 kW compressor running at ~38 kVA can demand over 230 kVA for the first moments. The alternator must ride that without the voltage dipping enough to stall everything else — so the surge, not the running load, often sets the frame size.
What load factor should a diesel genset run at?
Above ~30% — sustained light running glazes the cylinders (wet stacking). ISO 8528 assumes prime sets average no more than 70% of PRP. The sweet spot for fuel rate and engine life is roughly 50–80% load.
How much diesel does a generator burn per kWh?
About 0.28 L/kWh for a modern set near 75% load (check your fuel curve). At ₱85/L that is ~₱24/kWh — roughly double a typical grid tariff. Gensets are for outages, not baseload.
Can I run air conditioning or heat pumps on a genset?
Yes — and inverter-driven units are far kinder to the set: they ramp at ~1.2× running current instead of slamming on at 6–8× like fixed-speed compressors. If your cooling and hot water run on inverter heat pumps, the genset that carries them through an outage can be one or two frames smaller.
Generator or battery for outage cover?
Depends on outage length. Batteries cover short cuts silently and instantly; long outages favour the genset's fuel tank. Many Philippine sites do best with both — the battery rides through the frequent short cuts, and the genset only starts for extended ones, saving fuel, noise and engine hours.
Sizing a Genset Because of Outages?
Tell us what trips when the power goes. Often a battery covers the short cuts, the genset shrinks a frame size, and the diesel bill drops — we will tell you plainly if it does not.
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