I would like to buy UPS, but the review I read:
make me alert of one thing — the noise of UPS. So, I understand when powering computer (from batteries) UPS fans kick in, making some noise. OK.
But in normal mode, when there is no blackout, and UPS is working in passive mode (not from batteries), does UPS make any noise (I think about its fans)? In other words — do they work (spin) all the time?
Just in case, I have CyberPower CP900EPFCLCD or APC Smart-UPS 750VA (SMT750I) in mind particularly.
Update
I bought APC Smart-UPS 750VA (SMT750I) and it is noisy all the time — the noise is similar to grasshopper or older models of HDDs. When I close the windows (so all the street traffic is muted) the noise of the UPS is a bit annoying, but since fans in computer are louder I don't blame UPS for discomfort. Also I find that constant hum of the fan is more disturbing, that the ticking of UPS (as I said, it is similar to grasshopper).
Best Answer
The only sensible answer is: It depends.
All UPSes I've seen so far don't produce any noise when they aren't supplying power to a computer, but then again none of them have fans. Also a thing to consider is that internal electronics (not fans) may produce noise while UPS is powering the computer and that some UPSes have buzzers which can get quite annoying, especially if there's no way to turn them off. Also you may hear noise from relays when UPS switches between its internal power source and external power source. In some cases that can get quite annoying, especially if it so happens that your mains voltage is near UPS's tripping point. This can also lead to reduced relay lifetime.
UPSes usually use lead-acid batteries that are often sealed and they don't produce much heat when being charged and the charging circuitry itself, if it's slow enough, will not produce much heat.
So the bottom line is: I wouldn't expect a UPS to require fans when charging batteries or providing power to the computer from an external power source, but if you want to be safe, ask the manufacturer.