Active Power Factor Correction (Power Supply) & Stepped Sinewave Approximation (UPS)
We had a situation where our webcast PC was shutting down suddenly during a power loss even while connected to a brand new APC BR1500LCD 1500VA 865 Watts 8 Outlets BACK-UPS. Unfortunately we first saw the issue while webcasting a live concert during a thunderstorm.
I first contacted APC support and got a replacement unit shipped out but it had the same problem. I then tried a couple more UPS units we had around and to make a long story shorter I learned that the cheaper UPS units have something called Stepped Sinewave Approximation which essentially results in a more square looking current graph instead of the true sinewave that is produced by normal AC current. In most cases this is fine, but with some newer power supplies there is a power saving feature called Active Power Factor Correction which apparently does not like the sinewave approximation. We actually had an APC SmartUPS in our equipment room so we were able to move it to the webcast PC and it stays online as expected now. The stepped sinewave approximation unit is now in the equipment room where everything seems to be happy so far.
Here is a thread with some of the above info and a nice little graph showing the difference.