About this tool
Off-Grid Solar Battery Bank Sizing Calculator is a free, in-browser tool that works out how big a battery bank you need to run an off-grid load through cloudy days. It returns the required capacity in amp-hours, the nominal bank energy in kWh, and how many batteries to wire in series and parallel.
The capacity is Ah = (dailyWh × daysAutonomy) ÷ (systemVoltage × DoD), where depth of discharge (DoD, entered as a percent) is how far you safely drain the batteries — about 50% for lead-acid, 80–90% for lithium. Nominal bank energy is Ah × systemVoltage ÷ 1000. The layout wires ceil(systemV ÷ batteryV) cells in series to reach the bank voltage and enough parallel strings, ceil(Ah ÷ singleAh), to reach the capacity.
Use it to plan a cabin, RV or backup system: enter your daily watt-hours, how many days of no-sun autonomy you want, and the voltage and Ah rating of the batteries you are pricing. Everything is computed locally in your browser.
Frequently asked questions
Should I enter daily load in watt-hours?
Yes — total watt-hours consumed per day. If you know it in kWh, multiply by 1000 (3 kWh = 3000 Wh). The same figure a load audit produces by summing each device's watts times hours of daily use.
What depth of discharge should I use?
DoD is the fraction of capacity you routinely use before recharging. Flooded and AGM lead-acid last longest at around 50%; lithium (LiFePO4) tolerates 80–90%. A lower DoD needs a bigger bank but extends cycle life.
Why do days of autonomy matter so much?
Autonomy is how many consecutive low-sun days the bank must cover with no charging. Each extra day multiplies the required capacity, so it is the biggest lever on bank size and cost — 2–3 days is common, 5+ for critical loads.
How is the series-parallel layout decided?
Series cells add voltage until they reach your system voltage (48 V ÷ 12 V batteries = 4 in series); parallel strings add capacity until they reach the required Ah. The battery count is series × parallel. All computed in your browser.
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