Well, yes, a 12 volt, group 27 battery won't give you as much capacity as the golf cart batteries, but then you can get bigger 12 volt batteries if you want. Also, golf cart batteries will be affected by discharging too deeply the same as any other deep cycle battery. In all cases, your battery bank will last longer if you don't routinely discharge it below 50%. So, why two 6 volt batteries instead of one big 12 volt battery? The answer is weight and cost. One big 12 volt battery with 220 a/h capacity is going to weigh about the same as the combined total of those two 6 volt batteries. The total weight is about the same, but you're going to have to be a candidate for the Strong Man Competition to pick up and move around the 12 volt battery. By buying two 6 volt batteries you cut in half the weight you have to pick up at one time. Also, because 6 volt batteries are standard in golf carts they are pretty common. That means that two 6 volt batteries will cost less than one big 12 volt battery with the same capacity. On the other hand, there's nothing magical about 6 volts. You could buy two group 27, 12 volt batteries at Wal-Mart, wire them together, and get about the same capacity at the same cost as from the golf cart batteries.