How long eggs can survive in the incubator without power depends on a variety of factors. Length of time the power is out is an obvious one, and so is air temperature in the room --- shorter outages and warmer rooms cause less of an impact. You can close all the vents, add a hot water bottle if you have one, and wrap the incubator up in a blanket to conserve heat which will mitigate the outage to some extent.
Another factor to consider is age of the eggs. The further along your chicks are in their development, the less likely they will be negatively affected by a power outage. As embryos develop, they begin to produce a bit of heat by themselves, which warms the inside of the incubator slightly. Hope this helps!