Hi Mark - good on you for getting into solar! I'm using two, Group 24 Marine/Deep Cycle batteries from Costco. I can't fit the larger, Group 27 size in my battery box and golf cart batteries are too tall. But, one member modified his battery box by lowering it so that golf cart batteries would fit. The change involved welding on supporting straps that supported it maybe two inches lower than it is now. Looks a pretty easy mod that I may do sometime, since the width and length of the batteries would be good.