18 volt battery and power coverter work just as well, to give you some peace of mind. Essentially, all you will need to run is a powerhead and (if it is wintertime when you lose power) a heater. Your tank will be fine without lights for a couple of days. It will not be okay without oxygen and/or a substantial loss of temperature. Oxygen levels can drop in just a couple of hours - that is what you need to be concerned with most of all when you lose power. The powerhead needs to circulate the water and most importantly, break the water surface tension to allow for oxygenation.
You can also run the power coverter off of your car battery with a really long extension cord in a pinch. (I speak from experience!)
I only have to worry about a power loss during the winter in NC though. If you are going to try to run your chiller (i.e. you have a power loss in the summer time), then you may have too high of a power draw to run this way, and may need a generator.