🔌 Solar Battery Bank Calculator
Enter your daily load, how many days of backup you want, your battery type and system voltage to size the bank in Amp-hours.
Battery bank size
156 Ah
at 24 V
Nominal capacity
3.75 kWh
Usable energy
3 kWh
LiFePO4 (lithium): Typically 80% usable (many rated to 100%, but 80% extends life).
Bank size = daily load × days of autonomy ÷ depth of discharge, then ÷ system voltage for Amp-hours. Depth of discharge protects battery life — ~50% for lead-acid, ~80% for lithium (LiFePO4). "Days of autonomy" is how long the bank runs with no charging (cloudy days); 1–3 is common. Note: lead-acid loses usable capacity in the cold (only ~80% near 0°C), and lithium must not be charged below freezing — size up in cold climates. 🔒 In your browser.
How the solar battery bank calculator works
A battery bank has to store enough usable energy to run your loads through the hours (and days) without sun. The tool takes your daily load in watt-hours, multiplies by the days of autonomy you want, then divides by the battery's usable depth of discharge to get the nominal capacity — and divides that by the system voltage to give the size in Amp-hours.
Depth of discharge is the key: lead-acid batteries should only be drawn to about 50% to preserve their life, while lithium (LiFePO4) can use around 80% — so the same usable energy needs roughly a 60% bigger lead-acid bank. Two cold-weather cautions: lead-acid loses usable capacity in the cold (only ~80% near 0°C), and lithium must not be charged below freezing. Size up for cold climates.
Frequently asked questions
How do I size a battery bank for solar?
Bank capacity = daily load (Wh) × days of autonomy ÷ depth of discharge, then ÷ system voltage for Amp-hours. For 3,000 Wh/day, 1 day of backup, 50% lead-acid DoD at 24 V, that's 6,000 Wh nominal ≈ 250 Ah.
What is depth of discharge?
How much of a battery's capacity you regularly use. Draining lead-acid past ~50% shortens its life sharply; lithium (LiFePO4) tolerates ~80% or more. Sizing on usable DoD (not full capacity) is why the bank comes out larger than the raw energy figure.
How many days of autonomy do I need?
How long the bank should run with no charging, through cloudy spells. One to two days is common for a system with a generator backup; three or more for full off-grid reliability in cloudy climates. More autonomy means a bigger, costlier bank.
What system voltage should I use — 12, 24 or 48 V?
Higher voltages carry the same power at lower current, meaning thinner wire and less loss, so larger systems use 24 or 48 V. 12 V suits small setups (small RVs, single panels); 24–48 V suits cabins and homes. It doesn't change the energy, just the Amp-hours.
Does cold weather affect battery size?
Yes. Lead-acid batteries deliver less capacity when cold — around 80% at 0°C — so you need a bigger bank in cold climates. Lithium holds capacity better but its charging must be blocked below freezing. Either way, size up and insulate the bank.