Just out of interest... Whats the main issue with a power outage? I guess over extended times and in extreme conditions temperature could be an issue (but water retains its heat well if its cold and fish can survive hotter periods- like the summer we have just had in UK). So is it lack of water disturbance and therefore lack of oxygen? Bacteria dying off due to lack of filtering?
TIA
Depending on the season heat could be an issue. My power just went out for around 20 minutes earlier, but being summer it wouldn't pose a problem for several hours. In winter, it could be a problem, room temperature will decrease, increasing the rate at which your tanks would cool.
Filters not working means no bio filtration. As long as the media is wet, the bacteria should hold out for around 12 hours before dying off. It also means no aeration. The combination of ammonia & low O2 could get out of hand quickly in an overstocked tank.
I have a 5kw generator for any extended outages. It will run all the tanks, a couple of refrigerators, the sump pump, and the furnace blower in the winter. Without a generator, I would wrap tanks in blankets or other insulating material in the winter. Find a plastic cup or other container you don't mind destroying, something big enough to hold your filter media with room to spare. Drill or punch a bunch of holes in the bottom. Put your filter media in this & clip it to the side of the tank. Pour water through the filter media, you are now the pump of your filter.