So, something to note is that a lot of UPSs have a configuration for sensitivity. Your power actually fluctuates quite a bit, but you don’t notice. I have my UPS on the default sensitivity, and there have been a few instances of it going onto battery power when none of my other devices even flickered.

So, with that in mind, I use NUT. My server has it setup and it’s set to gracefully shutdown after my UPS hits 25% battery remaining. That way false positives don’t shut it down, nor will small flickers, nor will an outage less than an hour or so. My UPS says I can run for about 90mins on current load.