Battery Bank Sizing Calculator
Calculate the total battery bank capacity in amp-hours (Ah) needed for off-grid solar systems or backup power, factoring in autonomy, depth of discharge, and system efficiency losses. Use this when designing or expanding a battery-based energy storage system.
About this calculator
A battery bank must store enough energy to power your loads through the intended backup period while accounting for two key constraints: you cannot fully discharge most batteries without damaging them (depth of discharge limit), and the system loses some energy to wiring, inverter, and charge-controller inefficiencies. The required battery capacity is: Required Ah = ((dailyLoad × backupDays) / (depthOfDischarge / 100) / (efficiency / 100)) / systemVoltage. DailyLoad (Wh) multiplied by backupDays gives total energy needed. Dividing by the depth of discharge fraction scales up capacity so that the usable portion equals your requirement. Dividing again by efficiency accounts for system losses. Finally, dividing by systemVoltage converts watt-hours to amp-hours, the standard unit for battery capacity. Lead-acid batteries typically allow 50% DoD; lithium batteries commonly allow 80–90%.
How to use
Suppose you consume 2,400 Wh/day, need 3 days of backup, have a 48 V system, depth of discharge of 80%, and system efficiency of 90%. Step 1 — total energy needed: 2,400 × 3 = 7,200 Wh. Step 2 — correct for DoD: 7,200 / 0.80 = 9,000 Wh. Step 3 — correct for efficiency: 9,000 / 0.90 = 10,000 Wh. Step 4 — convert to amp-hours: 10,000 / 48 = 208.3 Ah. You would need a battery bank with at least 210 Ah at 48 V—for example, four 12 V, 210 Ah batteries wired in series.
Frequently asked questions
What depth of discharge should I use when sizing a battery bank for solar storage?
The recommended depth of discharge depends entirely on battery chemistry. Flooded lead-acid and AGM batteries should not regularly be discharged below 50% (DoD = 50%) because deep cycling drastically shortens their cycle life. Gel batteries can tolerate up to about 60% DoD. Lithium iron phosphate (LFP) batteries are designed for 80–90% DoD and offer far more usable capacity per unit of weight and volume. Always use the battery manufacturer's recommended DoD for cycle-life projections, as exceeding it can void warranties and reduce the number of charge-discharge cycles you get from the bank.
How does system efficiency affect the size of the battery bank I need?
System efficiency captures all the energy losses between stored battery energy and delivered AC power at your loads. An inverter running at 90% efficiency wastes 10% of battery energy as heat. Wiring losses, charge controller losses, and battery internal resistance further reduce usable energy. If you ignore these losses, your battery bank will run out of power before your backup period ends. By dividing required energy by the efficiency factor (e.g., 0.90 for a 90%-efficient system), the calculator ensures you size the bank for the gross energy needed, not just the net energy your loads consume.
How do I convert amp-hour battery capacity to a specific number of batteries to buy?
First, confirm your system voltage (12 V, 24 V, or 48 V). The calculator gives you the required Ah at that system voltage. Individual batteries are rated at their own voltage (commonly 2 V, 6 V, or 12 V cells), so you may need to wire batteries in series to reach system voltage and in parallel to reach the required Ah. For example, a 48 V, 200 Ah bank can be built from four 12 V, 200 Ah batteries in series. If you need more Ah, add parallel strings. Always use identical batteries in each string to prevent imbalance charging problems.