My answer would be based on what sized sytem you are running and what you have for equipment.
I run my return and the skimmer on my setup with batt. backup. What is required to run these pieces of equipment on any given size tank would obviously vary, I use two large deep cycle batteries for my system, this will run that equipment for a LONG time. A big system will lose heat more slowly as well, if running a small tank a heater might not be a bad idea in case the power goes out when you are not around to provide insulation, when you arrived home to take care of things you could insulate the tank, and unplug the heater for prolonged battery life.
-Dave