The "backup" system that you are talking about is a UPS (Uninterruptible power supply).. http://en.wikipedia.org/wiki/Uninter...e_power_supply
If you have a means of heating some water (eg. Gas stove) then you can fill a zip lock lunch bag with warm/hot water, and place it in the aquarium with the fish..
Likewise, if you live in a very hot area, and/or keep cold water fish, a zip lock lunch bag filled with Ice cubes will lower the waters temperature..
How much of either you need to add will depend on the size of the tank, and also how much you are trying to cool/heat the aquarium.. (i.e. trial and error)