Set your budget, then ask will you be home or not. If you are home, a DC option or manual generator is fine. Like most of us I'm not always home, so UPC or backup system specific to aquariums is a valid option, for me. I have to search, but with the UPC the life varies with equipment and what you connect. So where some would say an echotech backup battery and power heads are too expensive, I planned it ahead of time and worked that into by budget and builds. It works seamlessly.
It's great you are thinking of this because so many of us run tanks without consideration to power failures, and in the long run you can spend more with losses. With that said, for a 5 gal tank, things happen fast in regards to water quality. So it also depends on how heavily your tank is stocked. For what you mention (OP) as a solution, that is about the norm. A UPC will activate automatically, however you may want a small pump or small power-head to not draw too much power. And don't try to connect everything. If you have a power-head just connect that, but look to see what is less draw (powerhead vs' return pump). Also, test your solution. On a day off shut it down and see how long you have. Then you can plan accordingly. It's better for you to test in a controlled failure than the other option.
