If the electricity goes out, depending on the type of bator you have, you are better off not opening it. Adding hot water bottles after the fact will be difficult to control the temp.Sooo, if the electric goes out and I had a way to put hot water in some bottles, it might help? As long as the bottle would be away from the eggs?
The eggs will survice a few hour outage.
I have an 12V to 110V inverter for a car hooked up to a marine battery (actually a bank of them) for extended outages