Are you talking about a battery with a 110VAC power inverter?
If so, they won’t last long – especially with a fan running. You’re much better off with a deep cycle boat battery and 12V appliances. RV stores carry 12V fans, and you can power a car XM radio setup from the battery. As far as taking credit cards goes, it sucks the profit out of your business. I was quoted a 50 cent transaction fee PLUS 2.5% of the sale. That’s why many merchants have a 20 dollar minimum credit/debit purchase. Screw the credit cards.