🔌 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.