Running an Online UPS on a Generator: Sizing, Frequency & Compatibility
The UPS bridges the start; the genset carries the long haul. How to size and set it up so they cooperate.
Quick answer
An online UPS bridges the few seconds while a generator starts and stabilises, then runs off the genset. Size the generator comfortably above the UPS's input power (load ÷ efficiency) — typically a 25-50% buffer for a modern low-THD UPS, more for older units — and make sure the genset's frequency is stable, or the UPS may keep rejecting it and stay on battery.
Why they work well together
A UPS is instant but short; a generator is slow to start but runs for hours. Put them together and the UPS carries the load for the seconds the genset needs to crank and stabilise, then hands over to genset power for the long haul. The bonus is that you don't need a huge battery bank if a generator backs the UPS — the bank only has to bridge the start, not carry the whole outage.
The essentials
- The UPS covers the start-up seconds; the generator carries the long outage.
- Size the genset above the UPS INPUT power (load ÷ UPS efficiency), not just the load.
- Modern low-THD / PFC online UPS need less genset headroom than older 6-pulse units.
- Frequency stability matters: a wandering genset can make the UPS sit on battery.
- A wider input window or a 'generator mode' on the UPS helps it accept genset power.
Sizing the generator
Start from the UPS's input power, which is your load divided by the UPS efficiency, then add headroom — for the rectifier's harmonic current, for anything else the genset feeds, and for motor start surges. A modern power-factor-corrected online UPS draws clean current and needs only a modest buffer; an older 6-pulse unit draws harmonic-rich current that stresses the alternator and needs much more. Always confirm against both the UPS and generator spec sheets.
| UPS type | Genset headroom over UPS input | Why |
|---|---|---|
| Modern PFC / low-THD online UPS | ~25–50% | Clean input current, low harmonics |
| Older 6-pulse / high-THD UPS | Up to ~2× or more | Harmonic current stresses the alternator |
| UPS plus other loads on the genset | Add those loads + any motor surge | The genset feeds everything, not just the UPS |
Generator sizing for an online UPS (rule of thumb)
Why frequency stability matters
An online UPS rebuilds a clean, steady frequency for the load, but its input still has an acceptable voltage and frequency window. A small or overloaded generator whose frequency swings as load changes can fall outside that window, so the UPS rejects the input and keeps running on its battery — which then drains and defeats the point of having a generator. A stable, adequately sized genset keeps the UPS happily accepting its power.
Settings that help
Many online UPS let you widen the input voltage and frequency window or switch on a dedicated generator mode, which makes them tolerant of a genset's wander. Make sure the earthing is clean, let the generator stabilise before the load transfers, and don't run the genset near its limit. These small steps are usually the difference between a UPS that accepts genset power smoothly and one that sulks on battery.
A practical setup
Because the generator carries the long outage, the UPS battery bank only needs to bridge the start and stabilise — a few minutes, not hours. That keeps the bank small and the cost down while still giving you seamless, clean power. We size the UPS, the bridging bank and advise on the generator match so the three actually cooperate.
Where to next
Frequently Asked Questions
What size generator do I need for an online UPS?
Size it above the UPS input power (your load divided by the UPS efficiency) with headroom — about 25-50% for a modern low-THD UPS, more for older 6-pulse units — plus any other loads the generator feeds and their start surges.
Why does my UPS stay on battery when the generator is running?
Usually the generator's frequency or voltage is unstable, or it's undersized, so the UPS input window rejects it. A stable, adequately sized genset, plus a wider input window or generator mode on the UPS, fixes it.
How big should the UPS battery bank be if I have a generator?
Only big enough to bridge the generator's start and stabilise — a few minutes — since the generator carries the long outage. That keeps the bank small and the cost down.
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