A generator and a UPS do two different jobs. Get the sizing and the settings right and they hand the load back and forth cleanly, get them wrong and the genset trips the moment the UPS picks up.
The UPS rides through the gap and conditions the power. When utility fails, the battery carries the load with effectively zero transfer time, and the inverter keeps putting out a clean sine wave no matter what the input is doing. But the battery is a sprint, not a marathon, minutes, not hours.
The generator gives you the long runtime. It does not start instantly: a diesel genset typically needs several seconds to crank, come up to speed and stabilise its voltage and frequency before its breaker closes. During that window the UPS battery is the only thing holding the load. Once the generator is online and steady, it carries the load, including recharging the UPS battery, for as long as there is fuel. The two are complementary: the UPS covers the start, the generator covers the duration.
A UPS is a non-linear load to the generator, and historically a difficult one. Older 6-pulse rectifier UPS drew heavily distorted current with a poor input power factor and high harmonic content (iTHD). That distortion makes the genset’s alternator and its voltage regulator work hard, and it can interact badly with the engine governor. The traditional fix was brute force: oversize the generator so the UPS is a smaller fraction of its rating. Rules of thumb of roughly 1.5 to 2× the UPS kW were common for older units, and some specs pushed higher.
Modern three-phase UPS shrink that factor. Units with active (IGBT) rectifiers, near-unity input power factor and low input harmonics, often a few percent iTHD, look much more like a clean, linear load. With those, a generator sizing factor in the range of roughly 1.1 to 1.25× the UPS rating is often workable, sometimes tighter. The exact number depends on the UPS, the other loads on the genset and the alternator, so confirm it against the manufacturer’s generator-compatibility guidance rather than guessing. The point is that the old 2× penalty is no longer automatic.
A UPS will only accept generator power if it falls inside the input window the UPS is set to tolerate. Two things matter here. First, frequency: a generator under a sudden load step momentarily slows down before the governor catches it, so the frequency dips and then recovers. If that swing falls outside the UPS’s input frequency limits, the UPS rejects the source and stays on battery, which defeats the purpose of having a generator. Second, voltage: the same load step causes a voltage dip until the AVR responds.
The fix is to widen the UPS input frequency and voltage tolerance to suit a generator (most three-phase UPS have a generator or wide-input setting for exactly this), and to make sure the genset’s governor and AVR are tuned to recover quickly. Get this wrong and you see the classic symptom: the generator is running fine, but the UPS never transfers off battery and eventually runs the battery flat.
When the UPS accepts the generator, it should not slam the full load onto it in one step. A UPS rectifier has a walk-in (soft-start) feature that ramps its draw up over a set period, commonly several seconds, instead of demanding full power instantly. This gives the engine and governor time to take the load gradually, so the frequency and voltage stay inside the window and the genset breaker does not trip.
Walk-in matters most at the moment of transfer to the generator and again when the UPS battery starts recharging on top of the live load. A UPS with the battery charger included in the walk-in, or with an adjustable charge current, lets you keep the total generator demand under control. If a genset trips on load acceptance, the walk-in time and the charge-current limit are the first settings to check.
On a site with a generator, the UPS battery does not have to carry the load for a long time. It only has to bridge the gap from utility failure to a stable generator: the genset start time, plus a margin for it to stabilise and for the transfer to complete. That is usually a short window, so a runtime in the region of roughly 5 to 10 minutes is a common target for generator-backed sites, enough to cover a start, a failed first crank and a retry, with margin.
Sites without a generator are the opposite case: there the battery has to last long enough for an orderly shutdown or for staff to respond, so runtimes are sized much longer. Matching battery autonomy to whether a generator exists is one of the first decisions in sizing a three-phase UPS. Oversizing the battery on a generator site just adds cost and recharge load for runtime you will never use.
The handover runs in a fixed order. Utility fails → the UPS goes to battery instantly, with no interruption to the load. The automatic transfer switch (ATS) senses the outage and signals the generator to start. The generator cranks, comes up to speed and stabilises → the ATS transfers the building from the dead utility feed to the generator. The UPS now sees the generator at its input, accepts it once it is inside the frequency and voltage window, walks its load back on, and begins recharging the battery. When utility returns, the sequence reverses: the ATS transfers back, and the UPS rides through that transfer on battery exactly as it did the first one. Throughout, the load never sees an interruption, that is the whole point of putting the UPS downstream of the ATS.
One caution on a generator: be careful with eco-mode. In eco-mode the UPS feeds the load through the static bypass, so the load is exposed to the generator’s less-stable waveform and frequency, and a load step on the genset can force an unnecessary transfer back to double-conversion. On generator power, running the UPS in full online double-conversion is the safer choice; many sites configure the UPS to drop out of eco-mode automatically while on generator. Confirm all of this when the system is installed, because the transfer behaviour and the walk-in tuning are exactly what gets verified during commissioning. We supply and service these systems for industrial sites across Canada and can size the UPS, the battery and the generator interface together.
It depends on the UPS rectifier. Older 6-pulse UPS often needed a generator sized roughly 1.5 to 2× the UPS kW because of high harmonic current. A modern unity-power-factor, low-harmonic UPS often works with a factor of roughly 1.1 to 1.25×, sometimes tighter. Always confirm against the UPS manufacturer’s generator-compatibility guidance and account for any other loads on the genset.
Almost always because the generator’s frequency or voltage is falling outside the UPS input window. Under a load step the genset frequency dips before the governor recovers; if that swing exceeds the UPS limits, the UPS rejects the source. Widen the UPS input tolerance to its generator setting and make sure the governor and AVR recover quickly.
Only enough to bridge the generator start and stabilise time, plus margin for a retry, commonly in the region of 5 to 10 minutes for generator-backed sites. Sites without a generator need much longer runtime to allow an orderly shutdown. Sizing the battery to the wrong case just adds cost or risk.
Walk-in (soft-start) ramps the UPS rectifier’s power draw up gradually over several seconds instead of demanding full load instantly. On a generator this lets the engine take the load without the frequency or voltage collapsing, so the genset breaker does not trip. If a generator trips on load acceptance, check the walk-in time and the battery charge-current limit first.
Tell us the application and we will come back within one business day, sizing, the right system, install and a price. Three-phase installs usually need a licensed electrician, so let us know if you have one.