Brinsea has a good article about power outages and temperature drops:
https://www.brinsea.com/Articles/Advice/PowerOff.aspx
But if you're anticipating an extended power loss and need to continue incubation under those circumstances(or perhaps it happens when your chicks are already hatching or close to it), things are looking different. What unpowered heat sources do you have? At my house, if the power's out a long time we use the wood stove, even in the summer, so I'd be looking to figure out a setup that can sit near, under or over the stove at an appropriate temperature and humidity- some combination of a big baking pan, maybe full of rice, then a towel, then the eggs. Couple of thermometers, cup of water, cover that all with an upside down Tupperware with one end propped up for ventilation, and see if I can get that setup in the right ballpark. Would take constant tending. Otherwise you're looking at solar maybe? Rotating hand warmers? Depends on your unpowered heat source, it'd be very tricky.
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Thankfully it looks like the storm should lean away from us, but for the next one I think I’ll rig up some candles in the oven and humidity gets so high in a hurricane it shouldn’t be a problem for a few days worst case. The hand warmers are a really great idea too, I’ll have to get some.