We had our power out for two hours in West Chester last Saturday. Good times, temperature dropped very quickly with lofted ceilings as well the -5 deg blasts of wind.
The basement was the warmest part of the house
. Over 1000 gallons of water that are between 75 and 80 tend to radiate some heat.
After that short stint I was considering getting a generator. I have several colonies of fish that are worth between $500 and $1000, a $300 generator would be a small investment for such a large amount of money of fish.
I'd probably end up running the generator in sections of the fishroom. Starting with clusters of smaller tanks. My 300 gallon takes a long time to cool down. This is another argument for larger tanks...
As far as the CO2 is concerned with planted tanks, I don't believe it'd be a big deal. It's the same thing as running a Black Out where you're keeping the tank dark to kill of unwanted algae. If it was a week without power, I'm pretty sure losing your plants would be the last thing you need to worry about (read: a pretty bad disaster to take out power for a week, probably have to help neighbors or communities clean up).
As far as the rest of the house is concerned, we'd light the fireplace, and get the sleeping bags out. I don't mind living in the cold for a while... I'm an Eagle Scout and have camped in weather that people refuse to go to work in around here.