Even tanks up to 150 gal would be fine with a simple powerhead mounted at the surface so enough oxygen gets dissolved for the creatures. A deep cycle battery designed to be charged all the way down and up mounted to a controller circuit jacked off a cheap UPS would be ideal. If you really wanted to get the most life out of your battery you could use a DC power head(Tunze) to avoid the costly conversion from DC to AC from your batt via a inverter. A large deep cycle could probably power your power head for days. During the summer this would be more than enough as heat would not really be an issue as you could avoid running your lights and heavy pumps for days and the coral will be just fine. The tank would stay plenty cool just by the top being open to the air. Now in the winter you would need to heat, so for larger tanks you might need a generator, If I ever did a 200+ gal tank I would throw in a generator to the total cost.