Autonomy and longevity are the words.
Where autonomy is defined in terms of a political reference it still lends itself to the action of a true deep cycle flooded cell lead acid battery when referring to the entire system of a renewable energy source maintaining an adequate state of charge of that battery over several days, and in conjunction with, longevity over the course of years.
Personally, I have found that sizing a battery bank for this purpose is best done by determining the expected number of amphours to be removed in a 24 hour period and multiplying that by no less than 5. In the case of photovoltaics to also size the array to replace the same amount of power used in a 24 hour period in one clear sunny day. This at times will create some surplus PV electricity but at other times permit you to go indefinitely without polluting the air. Of course this varies from region to region.
This keeps the bank in the top 20% state of charge effectively shallow cycling them. Which in return, during inclimate weather, gives one several days of Independence from polluting charge sources such, as the grid or a fuel fired generator. At the same time it will increase the overall life expectancy of the battery bank with proper care and maintenance.
