[wtallis]

For an ordinary consumer UPS that's not trying to keep the system up for hours but just a few minutes, the peak power consumption probably matters more than the average: a 750VA UPS might simply trip its overcurrent protection if you're unlucky enough to have a power outage coincide with a spike in system load. With a big enough GPU, even a 1000VA unit might not be safe.

And it might be hard to get your system configured to throttle itself quickly enough when the power goes out: having the UPS connected over USB to a userspace application that monitors it and changes Windows power plans in response to going on battery could be too slow to react. It's a similar problem to what gaming laptops face, where they commonly lose more than half of their GPU performance and sometimes a sizeable chunk of CPU performance when not plugged in, because their batteries cannot deliver enough current to handle peak load.