[Carl n Susan]

The first question you should answer is: "how many amps do I need?". Whether it is just overnight or for a day or two, that is the amount of power you need to have on hand.

Once you know the "magic" number, then you can decide on the storage medium. FLA and AGMs can only be depleted to 50% so they require more capacity than a Lithium which can be safely drawn down to 10%. There is still a significant difference in cost between FLA/AGM and Lithium batteries although it is shrinking.