This was originally posted on APC forums on 11/18/2011
Hi again.
I finally bought the Back-UPS Pro 900 and sadly had to sent it back for noise reasons. I've tried two units, both with the same result. This unit has no fan but produces a lot of noise. The units I tested had three different noise levels. The louder one was when the system was running on battery power, which is expected and not a problem as it will only happen when there is a power cut and for a short period of time. There is a different noise when the battery is being charged. It's not as loud as when the UPS runs on battery but it's annoying. Notice it takes a lot of time to charge the battery. It shouldn't be a problem with a new battery, but it could become an issue when the battery gets old and requieres periodic recharging or if your power supply has frequent cuts. It's not critical, but there are other UPS that don't make a notable noise when recharging the battery. The real problem was a third noise, the one that the UPS produced all the time. It's much quiet than the one that's produced when the battery is being charged, but it's noticeable if you have a quiet computer, as I do. This noise was produced even with nothing connected to the UPS.
My power supply is stable, so I assume the AVR function was not working (and producing the noise). The LCD panel on the UPS showed stable in voltage and frequency.
So, in my opinion, this UPS is not good for a quiet computer (regulated fans, semi fanless PSU, SSD, 5400rpm drives soft mounted, quiet case). If you have a noisy computer it may hide the UPS running noise.