I don't know what kind of overflow system you have, but if you have an overflow box, once the water level drops below the weir, the water will cease flowing to the sump. With some of the DIY overflows, strategically placed holes serve as siphon breaks. Have enough space in the sump to accommodate additional water in the sump tank until no more water is entering the overflow system. I have a wet/dry filter, and the biggest problem was to get the bioballs submerged to prevent die off of the BB. I had to add water to the main tank so it could flow into the wet/dry filter and cover the bioballs. This was OK if I was at home, but the nagging question was "what if I'm not home when the power goes off?" Peace of mind for me was getting a whole house generator that kicks on within 10 seconds of the power going out.