how long do your power outages last?
water is a great "thermal mass", and as long as evaporation is minimized (keeping a hood on the tank and reducing surface turbulance), it will retain it's heat for a 'long' time
if your power goes out for less than a day, and the aquarium is located inside an insulated house (even if the house heat goes out, it doesn't instantly become cold inside right?) ... I don't think you'll have any problems
if you lived here though, when it's 8°F outside with 25 mph winds, even a well insulated house (and everything in it) gets cold easily, even with the house being heated ... I lost a bunch of fry once due to heater failure - they were without a heater for about 2 days before I noticed, their water temp was around 62
if your power-outages are frequent and last days at a time, you might want to look into cold water fish
... or insulating the back and sides of your tank with some fiberglass or sytrofoam insulation ... building your aquarium stand out of concrete or ceramic bricks would also provide a thermal mass ... as long as the bricks were heated by the houses heater, they would retain that heat a lot longer than a wood or metal stand