Solar Battery Bank Size Calculator
A solar battery bank calculator sizes the battery storage an off-grid or backup solar system needs to carry a load through periods without sun. The size depends on three things: how much energy you use each day, how many days of backup (autonomy) you want, and how deeply you can safely discharge the battery chemistry you choose. This tool multiplies the daily energy load by the days of autonomy, divides by the allowable depth of discharge so you do not over-drain the battery, then divides by the bank voltage to convert watt hours into amp hours, the unit batteries are usually rated in. Every input is editable so you can model lead-acid banks (often limited to 50 percent depth of discharge) or lithium banks (which tolerate deeper discharge), at 12, 24 or 48 volts. The US Department of Energy publishes guidance on solar electric systems and battery storage for homes. Use the result to specify a battery bank, compare chemistries, or check whether an existing bank covers your needs. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
Battery capacity scales the load by autonomy and depth of discharge, then converts to amp hours: Ah = (daily Wh x days / DoD) / voltage. A 5,000 Wh/day load over 2 days at 50% DoD on a 48 V bank needs 416.67 Ah.
Solar battery bank formula
Capacity (Ah) = (E x D / DoD) / V
E = daily energy use (watt hours)
D = days of autonomy
DoD = depth of discharge (fraction)
V = bank voltage (volts)
Dividing by the depth of discharge sizes the bank so the daily load only uses the safe portion of its capacity, which protects battery life.
Worked example
A cabin uses 5,000 watt hours per day, wants 2 days of backup, uses batteries rated for 50 percent depth of discharge, on a 48 volt bank.
- Energy over autonomy = 5,000 x 2 = 10,000 Wh
- Adjust for depth of discharge = 10,000 / 0.5 = 20,000 Wh
- Capacity = 20,000 / 48 = 416.67 Ah
The bank needs about 416.67 amp hours at 48 volts. These are the calculator's default inputs, so the result above matches the widget exactly.
Solar Battery Bank Size Calculator: frequently asked questions
What is depth of discharge?
Depth of discharge (DoD) is the fraction of a battery's capacity you safely use before recharging. Lead-acid batteries are commonly limited to about 50 percent (0.5) to protect their life, while many lithium batteries tolerate 80 to 90 percent. Dividing your energy need by the DoD sizes the bank so daily use stays within the safe range.
How many days of autonomy should I plan for?
Days of autonomy is how long the bank can power your load with no charging. Off-grid systems often plan for 2 to 5 days to ride through cloudy weather, while a grid-tied backup may need only 1 day. More autonomy means a larger and more expensive bank, so balance reliability against cost.
Why divide by bank voltage?
Energy is measured in watt hours, but batteries are rated in amp hours at a given voltage. Watt hours divided by volts gives amp hours. A 48 volt bank needs fewer amp hours than a 12 volt bank for the same energy, which is why higher voltage banks use smaller, lighter cabling.
Does this include charging losses?
No. This calculator sizes the storage capacity from your load, autonomy and depth of discharge. Real systems also lose energy in inverters, wiring and charging, often 10 to 20 percent, so add a margin or increase the daily load figure to account for those losses.
What voltage should my bank be?
Small systems often use 12 volts, mid-size systems 24 volts, and larger off-grid or whole-home systems 48 volts. Higher voltage reduces current for the same power, which allows thinner wiring and lower losses. Match the bank voltage to your inverter and charge controller.
Official sources
- Home solar electric systems and battery storage guidance: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.