Solar battery calculator

Bank size is daily consumption times days of autonomy, divided by the usable depth of discharge. Five kilowatt-hours a day with two days of autonomy at 80% usable needs a 12.5 kWh bank — 260 Ah at 48 V.

Days of autonomy is the expensive decision. Each extra day is a linear increase in battery cost, and batteries are usually the largest single line in an off-grid system. Going from two days to five nearly triples the spend, and in a northern winter even five days may not bridge a stubborn anticyclone. Most off-grid systems settle on two or three days of storage plus a generator, because the generator covers the rare deep gap far more cheaply than the batteries that would otherwise be needed.