SPCR

I need a dead silent UPS.

550W would be enough.

The only sources of noise from my computer are 3 Noctua NF-A14 ULN rotating at 650 RPM and a NF-A15 PWM, and if it weren't for those there would be absolutely no sound in my room.

Of course I mean when there is no power outage, I don't care how ridiculously noisy it gets then.

There is a CyberPower (CP1000PFCLCD) that looks nice and doesn't cost much, and I read it's "quiet", but some other feedback or possibly suggestions for better alternatives wouldn't hurt.

I have that same model UPS, and also a CP850PFCLCD, and can confirm that they are in fact totally silent except during a blackout. I've used many different UPS models over the years from APC, TrippLite, and MinuteMan, and these CyberPower units are the best I've had, both in terms of features and in value for the money. They have those killer LCD displays with all the info I love, and in general look cool enough to want to leave them visible instead of hiding them. The accompanying shutdown and status software is simple and effective, and even includes a Windows 7 sidebar gadget for realtime power usage and status display on the desktop (in case you can't see the LCD from where you sit). For all of that, I'd have been willing to pay more than for the long-time standard units like from APC, but the CyberPowers top it all off by being significantly cheaper than the old standards as well. They're just that awesome, and APC should be very nervous.

That said, noise shouldn't really be an issue with any standby or line-interactive UPS unit, since they just sit there and let the wall power flow passively through them until the power glitches. There's no need for a fan or any transformer noise when idle, and I don't remember ever hearing any of my past units. The only noisy UPS units should be the online "always running from the battery" kind, which are always discharging/charging and generating heat, but you don't really need one of those for a regular PC unless your power is so awful it's glitching multiple times per day.

Some UPS are fanned. I set aside an APC XS 900 for this reason. Replaced it with a fanless ES 750.

I also have a CP1000PFCLCD, and I agree that it is silent. It does have a buzzy fan that runs for a while after even a brief power cut. The fan is not loud, but it is noticeable.

In the beginning, I almost returned it. It said it was fully charged, but it would completely shut down as soon as it transferred to the battery. I fixed it when I found that one of the wires was not connected to the battery. Saying that it was fully charged when the battery was not connected was very misleading. Reading the reviews makes me suspect that it may not be unusual for the battery to be disconnected.

Thanks for the feedback.

My main concern though was if there were any buzzing/whining/electronic noise. Since I would place it in a very quiet environment (<20db) I would certainly hear those, even more when the computer is turned off.

APC's ES series is dead silent, except when mains power goes out:
- piercing beep alarm (as if you couldn't tell your lights went out)
- may cause your PSU to loudly buzz (not a true sinusoid)