[Dearth]

If you have a generator never be afraid to use it regardless of how long the power is out for and depending on how powerful it is you will have to choose what to run the smaller the generator. I have a small generator that can run one powerhead and a small heater or it can run just the return pump.

I also have a battery powered airstone as a backup for moving oxygen in the tank.

UPS(uninterrupted power supply) for computers will work in a pinch but you have to choose what to run on them as they are designed for computers not SW equipment and get the largest you can afford if it says 4 hours of life expect at best for it to last 2.5hours running one powerhead as they draw a lot of power.

As to retaining heat just cover the tank in a thick blanket I had to do that several years ago when it hit -28c and we lost power for 18plus hours my tank cooled by 8 degrees but that was livable considering the options. You will lose a lot of heat through the top of your tank so wrapping it will conserve heat.

The biggest thing is water movement either by powerhead or via airstone both of which will supply oxygen nothing in your tank will be happy but they will at least stand a better chance of survival.