You are installing a closed loop or a sump/return pump? A closed loop never goes into a sump, it goes from the tank, into plumbing, through the pump, into plumbing and then back into the tank. If the power goes out, the water just stays where it is.
In the case of a return pump, from overflow to sump to pump to return line, check valves don't work very well anyway. They are nice to have, but don't count on them -- you still need enough space in your sump to handle all the overflow water.