Some devices actually have a spot for a backup battery internally. Some devices require nothing more than a resistor, a capacitor, a zener, and a couple of blocking diodes to add a minute or two of backup power. Of course, if the time is derived from the line frequency this makes for another problem. And some require a little surgery to disable the clock. I think that one time source per room is more than enough, and see no reason for everything electric or electronic in the house to have the time displayed. I actually prefer the soothing tick tock of a windup clock, with bells on the top for an alarm. And if all else fails, why then add a small ups just for the device.