Maintaining some sort of flow is the first priority for a tank in a power outage. I use a vortech battery backup for my MP40's. It recently had to run 18 hours for a power outage due to a storm in my area. This occurred while I was on vacation! The temperature on my tank went from 78 to as low as 73 during that time period, but I did not suffer any losses.
To deal with an extended power outage that might last say 24 to 36 hours, I also have an inverter available that I can run off my car battery. I would obviously need to be at home to use this. For more extended outages, you really need a generator. Even a small generator can be a life saver for a tank. Return pumps typically don't require that much wattage, so a generator could keep your pumps going very easily. If you want something to maintain your lighting systems and heat, you would need a little larger generator as those systems require a lot more wattage.
I guess a lot of it really depends on where you live and the likely scenario you might encounter as far as length of power outage. I'm in an area that I don't really have to worry about earthquakes, hurricanes, or snow storms, so a power outage of more than a day for me is less likely than other areas of the country.