It would really depend on how cold the temperature gets in your area. In the summer, I wouldn't worry about it since snakes don't need UVB light like other reptiles do and most snakes can tolerate 60-70F temperatures for a short time. Even if just fed I think they will be fine in short power outages (under 8 hours). If the power is out for several days, I still wouldn't be worried unless the temperature drops substantially outside.
Power failures in the winter here I am more concern with due to the fact where I am it can get as cold as -35 C (around -31 F) so getting back up heat would be adviseable. Though, unless your home has poor insulation, an hour or two outage wouldn't be much of a problem. Then again I have natural gas heat so no power won't be any worry, well except for my snake's food thawing in the freezer..but with temps like that, don't need the freezer, outside is cold enough and I can just put a cooler with all the frozen food in the backyard.
That said, getting a backup generator or one of those mastercraft power supplies (The Eliminator I think it's called) would be good for longer power outages and in areas were having long outages of power is common. Or a natural gas fireplace would be good to (though you can't move the fireplace like you can other types of heaters). It really would depend on how many snakes you have and how they are situated. If you have a large collection in a single room, insulate it well and a backup generator would be good to keep the power on, or some other form of non-electric heating (maybe solar panels would be helpful if you want to spend alot of money).