You have set the maximum limit on battery storage capacity by stating that your battery box will hold 2 group 24 batteries, and that you are not going to modify it.
If you stay w/ 2 12v batteries, your maximum usable capacity is 80 amp hrs, or 960 watt hrs.
if you have the height for 2" taller batteries, then you can go w/ 2 6v golf car batteries, which will have a useable capacity of 115 amp hrs, or 1380 watt hrs.
Is that by any chance the information you are after ??
Having said that, that would increase your current capacity 13 times ???
