The five things that drive genset size
UPS-on-genset is one of those installations where the components are individually correct but the combination misbehaves. The cure is to size the genset based on the worst-case dynamic load the UPS presents, not the UPS's steady-state output rating. Five factors set the answer.
1. Rectifier topology
The dominant variable for an online double-conversion UPS. A 6-pulse SCR rectifier with no input filter generates roughly 30% total harmonic distortion on the input current. That distortion forces a 2.5–3× upsizing of the genset just to avoid voltage collapse during the rectifier's soft-start ramp. A passive input filter (typically a 5th/7th harmonic trap) or a 12-pulse rectifier cuts the distortion to about 10%, dropping the multiplier toward 1.4×. IGBT (active-front-end) rectifiers stay under 5% distortion and need only ~1.25× the UPS kW. The difference between “6-pulse no filter” and “IGBT” is roughly half the generator capital cost.
2. Battery recharge
The UPS starts recharging the battery the instant grid is restored, then again whenever the genset picks up. The recharge load is typically about 10% of UPS kVA, in addition to the steady-state load. Worst case: the genset starts under full UPS load and the battery is depleted — both arrive at the bus simultaneously.
3. Other loads on the same bus
Almost every site has more than just the UPS on the genset — air conditioning, lighting, fire pumps, elevators, motorised dampers. These don't step-load the genset (they're running before the UPS picks up), but they do consume real kW. Add them in.
4. Governor step-load capability
Diesel governors are rated for the largest single instantaneous load they can absorb without dropping frequency. Electronic isochronous governors handle 50–80% step loads in one bite. Mechanical (older or smaller gensets) handle 25–35%. If the UPS rectifier inrush + recharge exceeds the governor's step-load rating, the genset will trip on under-frequency before the UPS finishes its soft-start. The fix: a larger genset, an electronic governor, or a UPS soft-start setting that ramps the rectifier over several seconds.
5. Altitude and ambient temperature
Naturally-aspirated diesels lose roughly 1% of rated output per 100 m above 1000 m, and 2% per 5 °C above 25 °C ambient. A 200 kVA genset at 2000 m elevation and 40 °C is really a 168 kVA genset. Turbocharged engines derate more gently — confirm with the vendor. For sites in hot, high regions (Andean plateaus, parts of East Africa, central Iran) the derating can swallow an entire size step.
Standards behind the math
Topology multipliers from Eaton TD00405018E and Generac Power Systems sizing guidance. Step-load ranges from Cummins / Kohler governor datasheets. Altitude/temperature derating from ISO 8528-1 Annex C. Generator kVA-to-kW conversion at 0.8 PF lagging per ISO 8528.