Solar Battery Bank Calculator
Size an off-grid battery bank from your daily energy use, days of autonomy, system voltage and depth of discharge.
How it works
required Wh = daily Wh × days ÷ (1 − losses%) ÷ (DoD%) · required Ah = required Wh ÷ battery voltage · batteries = ceil(required Ah ÷ 100)
A battery bank must store more than you actually use, because you can only draw part of it (the depth of discharge) and some energy is lost to the inverter, wiring and heat on the way out. Dividing your daily use by the efficiency and by the usable fraction inflates it to the gross capacity the bank must hold, multiplied by the days you want it to run with no sun. Dividing watt-hours by the system voltage converts to amp-hours, the unit batteries are rated in — and moving from 12 V to 24 V or 48 V cuts those amp-hours proportionally, which is why larger systems run higher voltages.
Worked example
For 3,000 Wh a day, 3 days of autonomy, 15% losses and lithium at 80% DoD: 3,000 × 3 = 9,000 Wh used, ÷ 0.85 = 10,588 Wh, ÷ 0.80 = 13,235 Wh of total capacity. At 12 V that is 13,235 ÷ 12 ≈ 1,103 Ah — about 12 of the common 100 Ah batteries, or 13.2 kWh of storage.
Frequently asked questions
How many days of autonomy should I plan for?
Two to three days is typical for a home backup with regular sun, and four to five for cloudy climates or critical loads. More autonomy means a bigger, pricier bank, so many off-grid systems pair a moderate bank with a generator for the rare long stretch of no sun.
Why does depth of discharge matter so much?
Lead-acid batteries last far longer if you only draw about 50% before recharging, so their usable capacity is roughly half the nameplate. Lithium (LiFePO4) can safely give up 80% or more, so you need far fewer amp-hours of lithium to deliver the same usable energy — a big reason it wins despite the higher upfront cost.
Should I use 12, 24 or 48 volts?
Twelve volts suits small RV and cabin systems; 24 V and 48 V suit larger homes because higher voltage moves the same power at lower current, allowing thinner, cheaper wiring and reducing losses. Most whole-home off-grid systems today are built at 48 V.
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Educational estimate only — energy rates, weather and real-world efficiency vary, so treat the figures as a planning guide, not a quote.