Well I personly would not put my heaters on one. The reason is that a power outage would have to be for several hours before my tanks really would sufer from no heaters. The biggest thing I want to maintain is the water circulation.
Im not talking one of those baby ups devices either.
Im talking about high end battery backup equipment like what is used on servers and what not.
To get an idea of what size UPS one would need visit the following site. Enter in the total watts you want to keep and the desired time you want to keep it up and it will spec out a solution.
Of course a gas powered generator would be cheaper than this BUT gas powered generators do NOT start by themselves in a power outage thus they rely on you being there to get them ready, gas them up, plug them in to what ever you waint to maintan plus they are loud and for us city folk a nosy motor in the back yard at night is not gonna make you overly happy with the neighbors.