The obvious answer is to buy a small generator, which you can get for under $100.
We have the following:
A small generator, which we will use to power incubators when needed, and a fridge or freezer.
An indoor kerosene room heater, which will be used to keep the reptile room at a comfy 80F+, and extra kerosene for it.
72 hour heat packs, in case the animals need to be moved.
A few more ideas:
Heated water can be used to provide a source of warmth, if you have a gas stove. Just fill water bottles with it, and wrap them in towels. These would have to be checked pretty often.
While hardly desireable to leave a car idling for an entire day, the collection can be packed into bags and put into a nice warm car.
My house is not well insulated, so I would go for the kerosene heater if the power were out for longer than a half hour to 45 minutes. Ball pythons do not handle temperatures under 70F well.