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 and an optional battery unit capacity (Ah) to get a battery count.
- Read your bank size, total Ah and number of batteries, they update instantly as you type.
Nothing is submitted or stored: the numbers never leave your device, so you can plan your system privately.
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.
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.