if you build the overflow as directed in the stickies, you will not have any problems with overflow if you have a power outage. the pipe leading out the window is unnecessary, and actually, wouldnt even work as designed.
many people here have been using DIY overflows for a long time and can vouch for their reliability.
i shut mine off all the time for feedings and sump maintenance, and have not had any problems with the tank overflowing.
i made a couple of "power outage simulation" videos, and posted them on youtube. ill try to post them when i get a chance.
just follow the design in the sticky. make sure the pipe is sealed and the check valve is properly installed, and you wont have any problems.