There are several brands of these backup power sources. Home Depot calls them “Battery Generators.” They are large batteries, often on wheels, that need to be kept charged. Kohler made one of the first ones, intended for construction job sites. Other brands include EcoFlow, Duracell and GoalZero. You would likely have to order one online. Amazon carries some; I think Lowes does as well. The problem with them is twofold - once they draw down they can’t be refueled, and the amount of wattage available is usually smaller than a fueled generator. You need to compute the amount of electricity you need for the mini-fridge, as well as anything else, and figure out how many hours or days the wattage you select will give you what you need. I was thinking about buying a very large battery generator, but I couldn’t power a full size refrigerator for too long... There isn’t enough wattage from most of them to power, say, a microwave, refrigerator and a box fan.