I know I am worried about that too and also my first winter with a fish. I just have the one tank with my Betta and I live back in the country so if there is a big snow coming I will go stay with my son who lives in the city because if there is a power outage due to snow the city would be the first most likely to get their power back on first. So I need to get a tank and keep one at his house just in case we have to go stay there for awhile.
Also I will get a smaller tank I could put him in and get some of those heat packs and put the tank inside a box along with the heat packs and cover the box or I have heard those coolers work well too. This is the reason I am sticking with only one Betta right now because I am scared of what the winter might bring, and for you with so many tank to keep warm would need alot of smaller tanks or bowls and could put them in them along with the heat packs and a bigger box or cooler would work I think to keep them warm I think.
I live in the midwest, and our winters aren't usually too horrible, but every five years or so we get a nast one. We have a small generator that can power my reptiles cages, the fridge, and now that I have them, the fish.That thing has been a lifesaver (literally with the reptiles)!
One winter, the gas went out, so we had no heat, then after a week of no heat, the power went out too. This will be my first winter with a fish as well. If just our power goes out, I can crank the gas heater in the bedroom to keep Fishy warm, but if both go out, I don't know what I'll do :/
A backup generator would be the most versatile thing you could choose. Just make sure you keep enough fuel for it to run for a week. I think they're pretty expensive to buy new but you could probably find a deal on a used one.
In an emergency I would move them all to 1 gals in the smallest room of your house (preferably on the top floor; heat rises. Or in the kitchen near the oven, with the oven on if the gas works) and wrap whatever you can around the tanks to insulate them. Then stay in that room so your body heat and respiration heat stay in that space. Tape up the windows with plastic and shove towels under the door so the warm air cannot escape.
i know they sell current converters where you can hook up electrical appliances to a car battery, but i have no idea where you'd buy such things (home depot maybe?) or how long the charge would last, but it's something to consider.