boatjohn, it is very hard to answer how much you would use because we don't know anything about your pot-bellied stove.
Typically a pot-bellied stove will burn coal, but not all that efficiently. If you had a name brand well known stove that someone here was using, then we could estimate what you might burn.
How much are you spending for wood per night?? I can't imagine it is all that much money for wood.
If you can give us a name for the stove or at least some photos of the firebox and grate, ashpan, and describe the way it burns wood. Maybe we can estimate coal burn for a night.
Maybe the best bet would be to buy say four or five bags of nut coal and give it a try, and see what happens.