Off-Grid Battery Bank Capacity Calculator
Work out how big an off-grid battery bank needs to be. Enter your daily load in watt-hours, the days of backup you want, the system voltage, the usable depth of discharge and the round-trip efficiency.
How to size a battery bank
- Add up your daily load in watt-hours (watts times hours for each device).
- Choose how many days of autonomy you want with no charging.
- Pick your system voltage, usually 12, 24 or 48 volts.
- Set the usable depth of discharge and round-trip efficiency.
- Read the required bank size in watt-hours and amp-hours.
Examples
4000 Wh per day, 1 day, 12 V bank
4000 Wh/day, 1 day, 12 V, 0.5 depth of discharge, 0.85 efficiency
About 9412 Wh, or 784 Ah at 12 V
Frequently asked questions
Why divide by depth of discharge and efficiency?
You cannot use every watt-hour stored. Dividing by the usable depth of discharge and efficiency grows the bank so the part you can actually draw still covers the load.
What depth of discharge should I use?
Lead-acid banks last longer at about 0.5, so half the rated capacity. Lithium can run to 0.8 or higher. Use the figure your battery maker recommends.
How do days of autonomy change the size?
Each extra day multiplies the required capacity. Two days of backup needs twice the bank of one day for the same daily load.
Should I size at 12, 24 or 48 volts?
Higher voltage means fewer amp-hours and thinner wiring for the same energy. The watt-hours stay the same, only the amp-hour figure changes.
Does this include the solar panels needed to recharge?
No. This sizes the storage only. Use a solar panel calculator to size the array that recharges the bank each day.
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