is an electrical budget pdf sheet I found online at WM a bit ago. "It May" help one decide how big of a batter bank, charging system etc one needs. It does help on figure out how may amps per day/hr etc one uses, then you can figure out how big of a bank, charging system etc you will need.
Something like this will allow one to figure out if you can get away with a single size 24-31 battery as I use for my house bank. OR< if you need a bigger setup as mainsail has, showed in his last post why that client was not able to get away with a size 27, where as I am able to. BUT, if we discussed the power needs of my travel trailer I had.....I used two 6V batteries with the same foot print of a 27 12V, to get me as much or more juice as the 6V MS has, those would last me anywhere from 24 to 48 hrs depending upon how cold it was outside, with the furnace fan running. If my kids turned on the tv, they were drained in 4-6 hrs!
This link or equal links to battery usage charts, will help one decide on how big, many, style of batteries one needs. As one size does not fit all!