LiFePO4 cells make large capacity banks possible and as we have seen they do not need to be fully charged. Both facts are at odds with LA cells.
The basic equation that Transmitterdan spoke of holds true. That for some given time frame the energy used must be replaced. With the replacement being less than 100% efficient thus if you consumed 100 watt hours you would need to replace it with (oh say) 105 watt hours.
Due to weight and size constraints I suspect that the typical LA house bank was sized so that a days consumption was equal to about 25% to 40% of the bank capacity.
Watt hours do not equal amp hours but for this discussion let's pretend there is no voltage sag....
So a boat that uses 150 AH daily would need a bank that was 600 AH (150/0.25) to 375 AH (150/0.40) in size. (Roughly 6 or 4 T105)
And of course due to LA physics charging
that LA bank to 100% daily was highly desirable.
Enter LiFePO4 - with its desired SOC range of 80% to 20% (LA SOC range is 100% to 50%) yo end up with more usable capacity. Take our 150 AH usage and we find that a LiFePO4 bank of 250 AH would be able to stay within the SOC range while using 150 AH daily.
A 400 AH LiFePO4 bank that goes 2 days without any charging would range between 87.5% SOC and 12.5% SOC which LiFePO4 could handle (with decreased cycle counts). And with some charging during that 2 day period could stay within a 80%-20% SOC range which is desirable.
Some will say that 400 AH is optimal but that presupposes that the conditions and goals are the same for all....
Imagine a wall where the height of the wall represents your total house bank capacity. The bottom of the wall is 0% SOC and the top is 100% SOC. Now place a window on that wall centered right in the middle. The window represents your power usage. The top of the window represents your SOC when you are done charging for the day and the bottom of the window represents the SOC when you are done consuming for the day. Of course it is more complicated than this but go with it.
If you use lots of power and put it all back each day you will have a very tall window. If you only use a little power and put it all back you will have a smaller window.
(now the fun begins)
Some days you use more power than you can replace. lower edge of the window goes down and the upper edge of the window goes down also. thus the window slides downward. Or other days your usage may be smaller or your charging may be greater (or both) so the window slides upward. Thus Sliding Windows.
In any given time period the window changes size and slided up and down on the wall. The key is to keep the window fully within the desired region of the wall (less than 80% SOC and greater than 20% SOC).
With a small(ish) bank (er, wall) of 400 AH the range for the window to slide is small(ish) but with a larger bank your window has a much greater range of freedom.
A week of cloudy weather
as your main charge source) where your run your espresso machine off the inverter
will trend the window to the bottom part of the wall (lower SOC). On Sunny days where you are out doing things all day the window will slide up higher and higher.
With a 1600 AH bank that window can be quite large and/or never get to the upper or lower portions of the SOC wall. Lot's of freedom (as in not a slave to charging).
Of course this all needs a charging source that keeps your SOC above 20% (you pick your number) and shuts off before exceeding 80% SOC.