If you want power for a full duration of a power outage you buy an electric generator. Here is a small one at 1500 Watts peak, 1000W Continuous: http://www.generatorfactoryoutlet.com/gfo/products/MX1500.asp?rt=2
This and similar may be available as low as $160 or so.
It runs on gasoline, and depending on the load could last 5 or 6 hours on a gallon of gas. They output 120V AC