How to use this battery bank calculator
- Daily energy use, enter how much energy your loads use per day and pick the unit (kWh or Wh).
- Days of autonomy, choose how many days the bank should run with no solar input (2 to 3 is common).
- System voltage, select 12, 24 or 48 V to match your inverter and wiring.
- Battery type, choose lead-acid or lithium so the right usable depth of discharge is applied.
- System efficiency, then your single battery's own voltage and capacity (Ah) to get a full wiring plan.
- Read your bank size, target Ah, series/parallel counts and total batteries, plus the actual voltage and capacity the plan builds, they update instantly as you type.
How to size an off-grid battery bank
The goal is to store enough energy to carry your loads through the night and through any days with little or no sun. The core formula is the watt-hours you use each day, multiplied by your days of autonomy, divided by the usable share of the battery you can actually draw on: required Wh = daily Wh × days of autonomy ÷ (depth of discharge × efficiency). The depth of discharge and efficiency terms make the bank bigger than your raw usage, because you should not flatten a battery to zero and because some energy is lost as heat in wiring and conversion.
Once you have the required watt-hours, convert to amp-hours at your system voltage: Ah = required Wh ÷ system voltage. A higher voltage gives fewer amp-hours for the same energy, which is why larger systems use 48 V to keep currents and cable sizes down. Battery type matters too: lead-acid banks are typically sized to about 50% depth of discharge to protect their lifespan, while lithium (LiFePO4) banks happily run to 80 to 90%, so a lithium bank can be noticeably smaller for the same usable energy.
To get a battery count, divide the total amp-hours by the capacity of one battery (for example 100 Ah) and round up. Wiring several batteries in series and parallel builds the bank up to your target voltage and capacity.
Wiring batteries in series and parallel
A single battery almost never matches both your target system voltage and your target bank capacity on its own, so real battery banks are built by wiring batteries in series and parallel. Batteries wired in series have their voltages added together while their amp-hour rating stays that of one battery, batteries wired in parallel have their amp-hour ratings added together while the voltage stays that of one battery. The calculator works out both counts for you: batteries in series = target system voltage ÷ one battery's voltage, rounded up, and parallel strings = target capacity (Ah) ÷ one battery's Ah rating, rounded up. Multiplying the two gives the total batteries needed.
Because both counts round up to whole batteries, the bank you actually wire can come out slightly above your targets, for example four 10V batteries in series for a 24V target actually deliver 40V, not 24V. The calculator shows this actual voltage and capacity alongside the targets so you are never left guessing whether the numbers on screen are what you asked for or what you will actually end up with.
Every battery in a series string, and every string wired in parallel, must be identical, same voltage, same capacity, same age and same chemistry. A weaker or older battery mixed into the bank drags the whole string or the whole bank down to its level, which is called charge imbalance, and shortens the life of every battery around it. Never mix battery ages, capacities or chemistries (for example lead-acid with lithium) in the same bank.
Estimate only: These figures are a planning starting point. Real capacity depends on temperature, battery age, cable and inverter losses, and charge controller efficiency. Confirm the final design with your installer or the battery manufacturer before buying.
Frequently asked questions
How do I size an off-grid battery bank?
Take your daily energy use in watt-hours, multiply by your days of autonomy, then divide by the usable depth of discharge times the system efficiency. That total in watt-hours, divided by your system voltage, gives the amp-hours your bank must hold.
What is depth of discharge and why does it matter?
Depth of discharge is how much of a battery you can safely use before recharging. Lead-acid lasts longest at about 50%, while lithium handles 80 to 90%. A lower usable share means you need a larger bank for the same usable energy.
How many days of autonomy should I plan for?
Most off-grid systems plan for 2 to 3 days so the bank carries you through cloudy weather. More autonomy means a bigger, costlier bank, so balance reliability against budget.
What system voltage should I choose?
Small setups often use 12V, mid-size 24V, and larger homes 48V. A higher voltage means lower current for the same power, which cuts wiring losses and lets you use thinner cables.
Is this a guarantee of the exact bank size?
No, it is a planning estimate. Temperature, battery age, cable and inverter losses, and charge controller efficiency all affect real capacity, so confirm the final design with your installer.
How do I wire batteries in series and parallel to hit my target voltage and capacity?
Wire batteries in series until their combined voltage reaches your system voltage, target system voltage divided by one battery's voltage, rounded up. Then wire that many series strings in parallel until the combined amp-hours reach your target capacity, target capacity divided by one battery's Ah rating, rounded up. Multiply the two counts for the total batteries needed, and keep every battery identical in voltage, capacity, age and chemistry.
Is this calculator a guarantee of bank size?
No. It gives a planning estimate. Real-world capacity is affected by temperature, battery age, cable and inverter losses, and charge controller efficiency. Always confirm the final design with your installer or the battery manufacturer.