In my area I've never lost power for more than about 6 hours. Famous last words, I know.
For an emergency the might span a few days, I have several battery operated air pumps. They need a new battery every 12 to 24 hours. That would keep me going a few days. Beyond that, I'm stuck.
In my opinion those small UPS systems, like you might put on a computer to give you some time to shut down will not provide anywhere near the required power to run even basic pumps on a reef system. A backup generator is ideal, but that's going to need fuel, and it can go through quite a bit. Also, my condo association would never let me install one outside, so that just about settles that issue for me.