Battery Bank Calculator

Your battery bank decides how long your home keeps running when the sun doesn’t shine. Enter your daily energy use and how many days of backup you want, and the calculator works out the capacity you need, how many batteries that means, and how to wire them in series and parallel.

Estimates for planning only. Always check equipment specifications, local codes and, for electrical work, a licensed professional.

How to use the results

Total capacity is the size of the bank on paper. It is larger than the energy you actually use, because no battery should be emptied completely. Lithium iron phosphate (LiFePO4) batteries are commonly cycled to about 80% depth of discharge; lead-acid batteries (AGM, gel or flooded) last much longer when they are only taken down to about 50%.

Series and parallel: batteries wired in series add their voltages together (two 12 V batteries make 24 V); strings wired in parallel add their amp-hours. A 24 V bank built from 12 V batteries therefore always needs batteries in pairs.

Choosing a system voltage

• 12 V suits vans, sheds and small cabins using roughly 1–1.5 kWh per day.
• 24 V suits most cabins and part-time homes using up to about 4 kWh per day.
• 48 V is the usual choice for full-time homes: lower current means thinner cables, smaller fuses and less loss.

Practical tips

• Use identical batteries of the same brand, capacity and age in one bank. Mixing old and new batteries shortens the life of the whole bank.
• Most LiFePO4 batteries must not be charged below freezing (0 °C / 32 °F) unless they have built-in heating. Keep the bank in an insulated space.
• Fuse every battery string and follow the manufacturer’s cable-size and torque specifications.
• Not sure how much energy you use per day? Start with the solar system calculator, which adds up your appliances.

For a deeper comparison of chemistries, read LiFePO4 vs AGM batteries for off-grid systems.

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