A UPS is a harder load than its rating suggests, and it gets a vote. Two things decide the generator size: how the UPS draws its current, and whether it will accept your generator as a source at all. Get the second one wrong and the batteries drain while a perfectly healthy generator runs beside them.
If you are still deciding whether you need a UPS at all, what an ATS does and does not do answers that first — the short version is that the 10–30 second transfer gap is a real outage at the socket.
Why a UPS is a difficult load
A generator prefers a smooth, roughly sinusoidal current draw. A UPS input is a rectifier: it charges its DC bus in short pulses near the peak of each cycle, not evenly across it.
That has two consequences the nameplate does not show:
- The alternator sees more current than the kW figure implies, because the current is concentrated rather than spread.
- Those pulses distort the voltage waveform the generator produces, and the distortion feeds back into everything else on the same set.
How bad this is depends entirely on the UPS’s input stage. Older six-pulse rectifier designs are the hardest on a generator. Units with an active power-factor-corrected (PFC) input draw something much closer to a clean sine wave and are far easier to supply.
The failure nobody plans for: the UPS never transfers
Every UPS watches its input and will only accept a source inside a defined voltage and frequency window. When the mains fails and the generator starts, the UPS compares what arrives against that window.
If the generator’s frequency wanders outside the window — which is most likely during the first load steps — the UPS rejects it and stays on battery. The generator runs, burns fuel, and powers nothing on the protected side. The batteries run down on their normal timer, and you get an outage anyway.
This is the classic way a correctly specified generator and a correctly specified UPS still fail together. The usual fixes are widening the UPS’s acceptance window (most are adjustable) and choosing a set whose speed holds steady under load steps — not buying a bigger anything.
Three numbers to get before you size anything
Ask the UPS supplier — or read the UPS datasheet — for these. A serious UPS datasheet states all three:
| # | What to ask for | Why it decides the size |
|---|---|---|
| 1 | Input rectifier type — six-pulse, twelve-pulse, or active PFC | Sets how much oversizing the generator needs; the gap between the worst and best case is large |
| 2 | Input voltage and frequency acceptance window, and whether it is adjustable | Decides whether the UPS will transfer at all |
| 3 | Battery recharge current on top of the load | After transfer the UPS both feeds the load and refills its batteries — that is an extra load step, not a footnote |
Most UPS datasheets state a recommended minimum generator rating directly. If yours does, use that number rather than any general rule of thumb — the manufacturer knows its own rectifier.
Why oversizing is the normal answer
For any load other than a UPS, you size the generator from running load plus starting surge — the method in the kVA sizing chart. A UPS adds a third term: the penalty for a distorted, pulsed current draw.
The practical effect is that the generator serving a UPS is usually rated well above the UPS’s own kVA — modestly so for an active-PFC unit, substantially for an older rectifier. This is not waste. An undersized set here does not simply run hot; it produces the unstable output that causes the rejection described above.
Where variable speed changes the picture
A conventional set holds a fixed speed and corrects voltage with an AVR — every generator in our range has one. An inverter set works differently: the engine speed varies with demand and the output is rebuilt electronically, so the waveform and frequency are held to a tighter tolerance than mechanical governing alone can manage.
For small sensitive loads behind a UPS, that tighter output is exactly what keeps the UPS from rejecting the source. Our inverter models run at a variable 2,200–3,600 r/min depending on the model, rather than a fixed speed, and they are the ones to look at first when the protected load is electronics rather than motors.
A worked shape, not a worked number
Take an office keeping about twenty computers, network gear and lighting alive. The computers and network sit behind a UPS; the lighting does not need to.
Size it in that order: the UPS covers the seconds, the generator covers the hours, and the generator is sized for the UPS plus everything outside it plus battery recharge — not for the computer load on its own. Buyers who size from the computer load alone end up with a set that cannot absorb the recharge step.
What to tell your supplier
- The UPS make, model and rectifier type
- Its acceptance window, and whether you can widen it
- The non-UPS loads on the same set — lighting, air conditioning, pumps
- Whether anything on that set is a motor, since starting surge is a separate calculation
Where HTX fits
Our side of this is the generator, and it is the half that has to be sized around the UPS — which is the part buyers get wrong. Our published range spans 2.8 to 55 kVA, every set with AVR voltage regulation, including three variable-speed inverter models for loads that need a tighter waveform. Send us the UPS datasheet page and we will tell you which of our sets it will accept and which it will not — including when the honest answer is that you need a larger set than the UPS kVA suggests.
Protecting electronics, not just keeping the lights on? Send us the UPS model, the loads outside the UPS, and whether any of them are motors — or message us on WhatsApp — and our engineers will come back within 24 business hours with a sizing that accounts for the rectifier, not just the kilowatts.