Battery Bank Sizing Calculator
A battery bank is built from identical blocks: series strings raise the voltage, parallel strings raise the capacity. Enter your system voltage, the voltage and capacity of one battery, and the capacity you need, and this calculator returns how many batteries in series, how many strings in parallel, and the total count.
Formulas: Series count = system voltage ÷ battery voltage (rounded up) · Parallel strings = needed Ah ÷ battery Ah (rounded up) · Total = series × parallel
Worked example: a 48 V system from 12 V, 100 Ah batteries needing 400 Ah: series = 48 ÷ 12 = 4 batteries per string, parallel = 400 ÷ 100 = 4 strings, total = 16 batteries, giving a 48 V, 400 Ah bank (19.2 kWh). If the division does not come out even, the calculator rounds up — a bank must never mix different voltages or capacities in one string.
For how long that bank will run your load, use the battery runtime calculator; for the solar array to charge it, use the solar PV sizing calculator.
Frequently asked questions
Can I mix different battery sizes in one bank?
No. Mixing ages, capacities or chemistries in series or parallel unbalances charging — the weakest battery gets overcharged or deep-discharged and fails early, dragging the rest with it. One bank, one identical battery type.
Why round up instead of using exact division?
Because you cannot install a fraction of a battery. Rounding up gives a bank at or above your target voltage and capacity; rounding down would leave you short on both.
Does a bigger bank last proportionally longer?
Roughly, yes — double the amp-hours at the same voltage and the runtime roughly doubles, though very large banks also need heavier cabling and fusing to match.