Two in the morning, a regional hospital’s equipment room. The grid went down clean — no warning flicker, no brownout. Just gone.
The generator outside did everything right. Cranked, caught, ramped up, and the automatic transfer switch threw over in about eleven seconds. Inside the room, the UPS should have done the boring part: take generator power, feed the servers, recharge its batteries. Instead it sat on battery. Then it stayed on battery. The facilities guy watched the battery gauge slide toward empty while a perfectly good generator hummed twenty meters away, locked out by the very box it was bought to protect.
That UPS ran its batteries flat. The servers went dark with fuel still in the tank.
I tell that story because it’s the purest version of a fight I see all the time: a UPS and a diesel generator that hate each other. Customers always ask me the same three questions about it. Always. Here are the honest answers.
Because the UPS doesn’t care that the generator is running. It cares whether the power looks right.
An online double-conversion UPS watches its input the way a bouncer watches IDs: voltage within window, frequency within window, waveform not too ugly. Generator power often fails that inspection, and the UPS does exactly what it was designed to do — drops to battery and waits for the “mains” to come back. Except the mains isn’t coming back. The generator IS the mains now.
In the projects I’ve worked on, the usual suspect is the UPS’s input rectifier, not the generator. Older UPS units use a six-pulse SCR rectifier that draws current in gulps — total harmonic distortion of 25 to 30 percent. A diesel generator’s alternator has a fairly high subtransient reactance, which turns that gulping current into a lumpy, distorted voltage waveform. The UPS looks at the lumps and says no. The generator, for its part, is doing nothing wrong. It’s feeding exactly the load it was given.
Frequency is the second bouncer. When the ATS throws a big UPS load onto the generator, the governor dips — maybe half a hertz, maybe two. Most UPS input windows tolerate something like ±2 to 3 Hz, and many can be widened. If yours is set tight and the generator dips past it, down to battery you go. Then the generator recovers, the UPS tries the input again, the load step dips the frequency again, and you get the dreaded bounce: battery, generator, battery, generator, until somebody gives up.
Fixes that actually work: a modern UPS with an IGBT/PFC front end pulls THDi under 5 percent and the whole harmonic problem mostly disappears. If you’re married to the old UPS, an input filter helps. And widen the UPS input frequency window — but do it with your eyes open, because a wide window means the UPS accepts worse power before it protects the load.
Here’s the second question, usually asked right after the first: how big should the generator actually be?
The UPS nameplate says 100 kVA. Fine. That’s output. The input story is different. Right after an outage, the UPS is feeding the full load AND recharging flat batteries at the same time. Add rectifier losses, add harmonic derating on the generator, and your 100 kVA UPS is asking the generator for a lot more than 100 kVA.
The rule of thumb I give customers: with a modern PFC-front-end UPS, size the generator at 1.5 to 2 times the UPS kVA rating. With an older SCR/6-pulse UPS, 2.5 to 3 times. That sounds like overkill. It isn’t. I’ve watched a “correctly sized” 300 kVA set wheeze against a 160 kVA UPS because nobody counted the recharge current and the harmonics. The generator survived. The UPS spent the whole outage bouncing. The customer spent the whole outage on the phone with me.
Then there’s the sneaky one: leading power factor. At light load, a PFC-corrected UPS can present a capacitive, leading power factor to the generator. Synchronous generators hate leading power factor. The automatic voltage regulator gets confused, voltage climbs, the machine hunts. A generator that runs beautifully at 80 percent load can get twitchy feeding a lightly loaded UPS, which is exactly the condition most standby systems live in.
This is why generator sizing for UPS loads deserves its own math, not the nameplate math. Count the load, count the recharge, count the harmonics, then add margin. The generator doesn’t care about your spreadsheet. It cares about physics.
Third question: won’t the batteries just cover the gap while the generator starts? Yes — that is literally the job. But people misunderstand the job description.
The intended sequence is boring and short: grid dies, UPS carries the load on battery, generator starts in about ten seconds, ATS transfers, UPS sees acceptable input and goes back to feeding from its rectifier while recharging. The batteries only need to survive a few minutes. They are a bridge, not a destination. Any UPS sized for a generator-backed site should be planned that way — minutes of runtime, not hours.
Which is exactly why question one is so dangerous. If the UPS rejects the generator, the bridge becomes the whole journey. Minutes of batteries against an outage that lasts hours. That’s the hospital story from the top of this article. Nothing was undersized. Everything worked. The two boxes just never shook hands.
And then there’s ECO mode. In ECO (or bypass) mode the UPS passes input power straight through to the load to save energy — no double conversion, no conditioning. On utility power that’s a defensible trade. On generator power it’s exactly when you don’t want raw power touching your servers. The generator’s voltage and frequency are wobbling around far more than the grid ever does, and ECO mode hands all of it to the load. My standing advice: force the UPS into double-conversion mode whenever the site is on generator. Most UPS units can do this on a schedule or a dry contact from the ATS. Set it up once and forget it.
A data center backup design lives or dies on this pairing, by the way. Redundant generators mean nothing if the UPS won’t accept either of them.
So that’s the whole fight, really. A UPS that rejects generator power, a generator sized for a load that doesn’t exist on paper, and batteries asked to do a job they were never hired for. Fix the input windows, size the set at two to three times the UPS rating, keep the UPS in double conversion on generator power — and the two boxes stop fighting. They were always supposed to be a team. Sometimes they just need an introduction.
If you’re planning a site and want someone to sanity-check the pairing before you buy, talk to us. We’ve seen every version of this argument.
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