Guides
The UPS carries the load through the seconds the generator needs to start, then runs from the generator for the rest of the outage. For that to work, the generator must hold voltage and frequency within the UPS's input window, carry its load and recharge, and cope with the UPS's input characteristics.
The UPS carries the load through the generator start: Eaton notes that a generator typically takes about 10 seconds to come online, with no AC until it does, and that it provides no protection from spikes and disturbances by itself. Most often, though, what goes wrong is that the UPS refuses the generator. Eaton explains that every UPS has an input voltage and frequency window, and if the generator's frequency swings outside it the UPS treats the generator as unstable and stays on battery, until the battery runs out and the load drops. An undersized generator causes the same problem: it cannot hold voltage and frequency within tolerance when the UPS comes off battery and onto it.
Bigger than the UPS rating, by a factor that depends on the UPS and the generator. Eaton's UPS basics paper suggests 1.5 to 3.0 times the UPS kVA, depending on the UPS design, alternator size, regulator type and fuel. Its generator page gives guidance for UPS up to 25 kW: 2 times the total load for natural gas and mechanical-governor generators, 1.5 times for propane or diesel generators and those with an electronic governor. Large data-centre systems are sized with both makers; Vertiv notes that UPS with active input power factor correction avoid the need to oversize generators. See also UPS sizing.
A non-linear, constant-power load with its own quirks. Cummins lists UPS rectifiers among the data-centre loads that create harmonics, capacitive filters among those with a leading power factor, and IT power supplies as constant-power loads that draw more current as voltage falls. It warns that low kW with high leading kVAR is a risk for the alternator, and recommends selecting a generator mode on the UPS where one exists and asking for the alternator's capability curve.
Walk-in is a UPS feature that transfers its load onto the generator gradually. Cummins describes it: the UPS senses the voltage and frequency excursion, lets the batteries take the load first, then ramps it onto the generator, so the generator can accept what would otherwise be a full load step. Cummins suggests asking whether a UPS walk-in function is more appropriate than demanding full load acceptance from the generator.
Yes. Eaton notes that standby and line-interactive UPS use the battery to protect the load from frequency variations, so on a generator they may cycle to battery often, while a double-conversion UPS filters frequency variations in normal operation and preserves the battery. That is one reason double conversion is standard for generator-backed sites; see types of UPS.
By testing them together, under load, before an outage does. Kohler's data-centre paper notes that historical confusion over generator and UPS compatibility led to oversized, lightly loaded generators; the fix is testing, not guesswork. A combined test runs the UPS on generator power at a known load and watches the transfer, the UPS input and the recharge. The generator test itself is explained in generator load bank testing; in Canada, emergency generators required by the building code fall under CSA C282, which does not cover UPS. Eaton's last advice applies: check that the generator maker has tested and approved its use with UPS. For UPS service on generator-backed sites, see UPS service on generator-backed data centres.
Usually because the generator's voltage or frequency is outside the UPS's input window. Eaton notes that the UPS then treats the generator as unstable and stays on battery until the battery runs out.
Eaton suggests 1.5 to 3.0 times the UPS kVA, depending on the UPS design, alternator, regulator and fuel; confirm with both makers.
A setting some UPS offer to widen input tolerance or change behaviour on generator power; Cummins recommends selecting it where the UPS supports it.
No. CSA C282 covers emergency generator sets for buildings and explicitly excludes emergency power from batteries and UPS.
Sources, checked 2026-10-04:
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