I'm using the 6 volts from Duracell. Sam's has the best price. Consider you'll be adding considerable weight as they are 60 lbs each. However you get 230 amp/ hrs of 12 volt power. Here's how I installed mine. The advantage of the 6 volt is they have a smaller foot print if hat makes a difference. The are taller.
https://youtu.be/foLGtyt9Rsg