ko888 :
What's the brand and model of the PSU?
Some PSUs won't run on a non-sine wave UPS. If the PSU's Active PFC circuit is incompatible with the UPS' waveform (e.g. simulated sine wave, stepped approximation to a sine wave, square wave, etc.) when operating on battery power the UPS will fail to properly switch over to battery backup mode during a power interruption.
Its not that it will fail, but that the PSU will lose power and then power up again. Unfortunately it acts the same as if you hit the reset button which is not what you want.
I contacted EVGA regarding this problem as well. They told me the problem is not as prevalent as people think and that they believed my PSU would not have an issue with either one, however I went with a Sine Wave UPS "just in case".