This thread is a good resource! Thanks to everyone who contributed!

Reporting a new one, in case someone still cares:

APC Back-UPS Pro 900 (BR900GI):
Virtually silent on mains at any reasonable distance.
Quite noisy when on battery power (with a 150W load), but not unbearably so given the unusual occurrence and that I'd ratter prefer to notice it has switched to battery in any case.
Has a much noisier alarm which can be turned on and off with the touch of a button, and a computer interface which may be used to receive status notifications and automate shut down procedures.

I also have the APC Back-UPS 750 (BE750G) as a replacement for the discontinued Back-UPS ES 725 Broadband (BE725BB). When first plugged in after taking it out of the box, or when on battery power there is a buzz audible at 3 feet. Otherwise it is totally silent - I sleep with one near the foot of the bed.

However the green LED on the power button (which is on unless there is a problem) is insanely bright. When the room lights are on it's not very noticable (green LEDs mix well with room light) but in the dark it looks like I have a nightlight plugged into it - I'm not exaggerating how much it lights up the room. A small piece of electrical tape placed carefully over the button fixes that.

I would like to thank you all for posting information here .

I was planning to buy an APC Back-UPS of somewhere between 400VA and 900VA*, and had not really thought about the noise yet.

- do all your APC units have the possibility to connect to your computer with USB or UTP or something, to tell it to shut off in case of power failure?
My computer is on 24/7, so there is a large chance I will not be there to turn it off. My main reason to buy a UPS is simply to prevent broken hardware or lost data, I don't need to have my computer continuously during a (long) power failure, I just need my computer to shut off safely .

- Has anyone tried if putting some foam around your UPS helps against the sound it makes when it is not running the fan? (Without blocking any significant ventilation openings of course.)
There is of course a small chance the unit depends on it's outer casing for some cooling, and will run hot enough to start the fan because of the foam, which would be horrible, but we will never know without trying it first .

*Something like 700VA should be enough to run this C2D+4870+10xHDD system for 5 minutes, right?

Almost all of the APC models seem to have USB connectivity (it uses a cable provided by APC- one side is USB to plug into the computer, the other side is some kind of proprietary RJ45 connection). With the units I've had you can either install APC's PowerChute software (more bloatware, no thank you) or just let Windows recognize it as a battery - it shows up in your power profile just like the battery on your laptop and you can set power profiles for 'on battery', 'battery low' and 'battery critical' just like a laptop.

Free operating systems have a non-bloated piece of software called apcupsd. I guess it works with all models but I've only used it with somewhat pricier models. If you check the documentation, you'll know for sure which models are supported.

For years, we have been using APC Back-Ups 650; heavy metal, no discernible sound. When a battery of one of these went bad, I decided to trade in the entire UPS for the newer model Back-UPS 650 CS; unfortunately this new model produces a high-pitched whine; while the whine is so high that I can't hear it anymore, my 21-yo son found it sufficiently annoying that he put in it a corner closed off with damping material. Two other units have the same problem.

We also acquired an APC Back-Ups ES 400 (for the cable modem), and this is silent.

Bumping an old thread, but I figure this information is good to know.

I exchanged my APC SmartUPS 1500VA fan-always-on model with my friend's older model. I haven't seen/heard the fan spin unless it was testing itself. Have not yet experienced power-loss situation, but this thing is essentially "silent".

With my head next to it, I can hear some faint electrical noises from it (similar to what you hear from a laptop's power management circuitry), but when it is under my desk and I'm in normal seating position I cannot hear it at all.

That bump serves as a reminder to me . This thread was useful before buying my UPS, but now that I have had one for a while, I can confirm something for others:

Back-UPS ES 700G: Silent.

Hi

first I'm new on this forum and I've always found good informations here .
But now, I'm stuck so I need some help

I'm looking for the most silent Inline Pur sinus wav UPS for my home studio. it's mostly for my audio converters and my samplers.
it seems that the Square wav UPS are not really goo for my audio setup .(actually I don't know)
I just want to be safe during a blackout and get them properly shut down.

So I wanted to buy a APC Smart-UPS 750VA LCD 230V...I just wanted to know how is it noisy ? is it noisy ? hum ? ventilation ? vibration ?

Or if you know something good in Pur Sine wav silent for 350w...just let me know please...
Or..If I don't need an UPS Pure sinewav and just take an Elipse MAX 850 ?

thanks a lot for your help and sorry for my poor english..hope you'll understand what I wanted to say

I have posted comments on the LCD-less version somewhere.
edit: I can't remember where but the short version is that it's noisy when drawing down the battery but normally has only a slight whine you only hear when you're next to it in a relatively quiet environment.

has anyone tried using a DIY UPS?

http://www.mini-box.com/micro-UPS-load-sharing

Id love to have a self contained PC/UPS, along the lines of how google builds it servers.

http://upsonnet.com/Articles/Learning-F ... esign.html

I've seen models that are designed to be put in a 5.25'' bay. They were crappy but I suppose you could do them well.
It's not what Google is doing obviously.