Where are you going to get the electricity to recharge the batteries if you were to do this? Not to mention that power is lost in the charging and also the inversion from 12V DC power to 120V AC. Batteries are very expensive, and so are inverters.
Just keep your operation small. The electric company isn't going to notice and extra one or two hundred dollars on the bill. It's pretty easy to calculate your costs for each item you're using such as lights, pumps, fans, etc.
(watts used divided by 1000)*(hours per day the thing will run)*(30 days in a month)*(cost per kilowatt hour)
It's a good idea to have a generator around in case the power gets knocked out, but otherwise it'll cost you easily 10 times as much money to power your stuff with a generator, and it'll make noise / heat / exhaust. Just be efficient with your watts.