I can dig it…but surely the simple option is to have the PC wake up on power restoration, right?
Set the BIOS to “restore last state after AC loss” and let the UPS do its basic job. If power returns before the battery dies, the PC never shuts down. If the battery does die, the PC loses AC while running, then powers back up when AC returns.
Does that not achieve the same outcome?
I get the idea of Pi + NUT + WoL setup (especially if you want a clean shutdown). It just seems…a lot for a single machine.
(He said, conspicuously hiding the shopping cart of solar panels and marine batteries)