How big is your tank and sump? What kind of Overflow? Assuming you have a UTube - When you have a power outage, the overflow will continue to siphon until the tank water level gets below the teeth on the overflow. The return line will also starts a reverse siphon and puts water into the sump until the tank water level clears the opening of the return line. Once the power comes back on, the return pump will start to pump water back into the tank, water level increases in the tank, and the UTube siphon should continue to work.
Take out a bunch of water from our sump. Perform a fake power outage. Let the overflow and return pump finish draining the water. Once they are done, look at your level in the sump, now you can add the appropriate amount of water to the sump. This will ensure you avoid a sump overflow.
When you turn the power back on, let the water flow through for about 5 minutes. When the sump and tank levels are normal, take a marker and mark the water level on the outside of the sump. This will be your top off line for future reference. Never top off above that line!