Battery operated incubator
Are you in an area with a lot of sun? Perhaps you can get a solar panel that can provide you with power when the electricity goes out. Perhaps you can find ways to insulate the incubator, so that the heat is retained longer. The other suggestion I would have is to surround it with gallon jugs of warm water. Water cools off more slowly than air.
Do you use an alternative power source for other items when the power is out - such as a generator? In a situation where you do not have a reliable source of power your next best option may be to select breeds that are naturally inclined to broodiness (or even cast around your local area to find someone with a perpetually broody bird or two that they don't want due to lack of production) and go the more natural route which is not electrically dependent.