building a mini-UPS

I really need help in constructing or purchasing some kind of Battery backup for a STB (5v, 2A). The changeover from the main power should be automatic and change back should also be automatic. The backup power should last minimum 10 seconds. IT should somewat act like a DC UPS. The purpose is that we set up this STB and when power goes off, they all freeze and on power returning, they are reseted to default and all 400 of them has to be reconfigured. I wouldnt say I am a learned Electronics hobbyist but i am an electronics engineer so some technical info will not throw me off. My hunch is that some kind of feedback capacitive circuit should do the trick but i want a real practical idea that can be implemented.