The nice thing about an air pump backup system is it will cost you less than $20 and it comes on automatically when the power goes out. The air lift is more effective than a powerhead because it moves just as much water but does so from the bottom to the top rather than the less efficient side-side pattern of a powerhead. You can hide a few air lines within your rock work. When the power goes off, the ac plug switches the battery operated air pump on. You will get many hours out of the battery. The B10 model is just an air pump, while the B11 model has the auto-activation when the power goes out.
http://www.mops.ca/pdf/2007cat_p01.pdf