I read a college report that I found interesting (shame I can not find the link).
The report basically shows the link between temperature and heart rate which explains why certain things happen.
At 99.5 the heart rate is normal. As the temp increases so does the heart rate which explains why chicks are born early in incubators that are running hot.
The heart rate does have a upper limit and when that is reached the heart stops which explains why even a short burst of high temperature can kill all of the eggs.
Now the fun part is that when a egg cools down the heart rate drops to conserve energy. This is useful when the hen leaves the nest.
If a incubator runs low then the chick will be late or non viable as a slower heart rate conserves energy but its still using energy and the egg has a limited amount.
Now it has been tested that you can remove a egg from a incubator and stick it in the fridge until the heart rate reaches 60bpm (they did this for over a hour). If the egg is then placed back in the incubator the heart rate will return as the egg warms back up and the chick will hatch with no ill affects. At around 55bpm the heart rate stops and will not restart.
So during a power outage as the temp drops the heart rate in the egg will slow down and as long as the heat is restored before the egg reaches 60bpm then the egg will still be viable. In a 70 degree home that will take hours for the heart rate to drop into the danger zone. As the incubator will take a while to cool down even long power cuts may have very little affect on the eggs.
im still searching for the report. If I find it I will post it, (read it on my phone which is why I didn't copy the link)