If it burns 1/2 a gallon an hour it's gonna cost you 6 gallons of diesel every 12 hours. At $2.50 a gallon(optimistic right now) that's gonna be what? About $18.00 every 12 hours.
Fuel consumption is hard to pin down because of varying factors, like load on the generator, the size of the engine, etc...
You need to add up the power consumption of all the items that you want to run at the same time on the generator and then size your generator to that level + a little cushion for exta power consumption. Then you can start shopping for generators that meet your power requirements. Then you can determine what engine will be in it and start guestimating fuel consumption from there. One thing to remember is not all appliances have to be on at the same time, but they could be, so you need to size it by your needs.
For instance, we only have a 4000 watt generator. It will run the furnace and some lights or the freezer and some lights, or the refrigerator and some lights or the well and some lights, but it won't run the furnace, freezer, refrigerator, well and some lights at the same time. So in an emergency, we have to cycle the power on those major appliances manually in order to operate off of the small generator. But we still have a warm house, cold beer, fresh meat and a toilet that flushes! /forums/images/graemlins/grin.gif