I have personally tested with a multimeter when this noise comes from APC UPS the voltage is definitely regulated. At wall, 260+V and from UPS output I get 225V at the same time. Of course when the voltage at wall becomes normal then UPS does nothing and the fan also shuts off.
Yep, I have the smart UPS series that log the data to my server, and a couple of smart plugs that do the same. The ups disables mains in outside acceptable voltage range, and uses the battery to supply power. While their voltage range is slightly on the conservative side (considering all the switch mode supplies can accept a wide range of voltages), I'd take this behaviour anyday over a capacitor popping in the power supplies due to over voltage or exceeding current limit due to under.
Any ups will generate heat when converting battery power up to mains, power conversion is not free, especially at those high current scenarios that you get with 24v battery packs. Understandable that fan noise isn't ideal, but active cooling will only prolong the life of components.
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Can't find the exact specs on the mosfet on the current generation of UPS, but they've used IRFZ24N in the past. RDS_on is 70mohm, max rated current 17A, at that rated current (which is a normal scenario, because mosfets are on the low voltage DC side, hence the higher currents) it will dissipate 20W, and that's just a single mosfet. 20w will absolutely kill the fets in seconds if there's no cooling, and because of space constraints, they have to use active cooling.
They can use better mosfets (and probably do), we have fets these days that have RDS_on <10mohm, but even with those power dissipation is at ~2W, enough to burn the mosfets without active cooling, when you have to provide a few hundred watts output. Then you consider the transformer heat losses, and I'm happy that they have some sort of cooling, despite the fan noise.