SUNBLOC
Battery Bank and PV Array Sizing
How many Batteries and PV Panels Do I Need?
Battery count depends on three things most calculators lump together: your load, your chemistry's usable depth of discharge, and your target system voltage. Get the voltage topology wrong and you'll either need a step-down converter or thicker, more expensive wire.
The formula
Size total storage from your load and autonomy target (days without solar charge), adjusted for depth of discharge, then work out how many physical batteries that takes at your chosen chemistry and system voltage.
Storage target (Wh) = (daily Wh × days of autonomy) ÷ usable DOD
Battery count = storage target ÷ (single battery Wh × series count) → rounded up to full parallel strings
Example: A 6,750 Wh storage target on a 48V system using 51.2V/100Ah LiFePO4 batteries (≈5.12 kWh each) needs about 2 batteries in parallel.
Frequently asked questions
How many batteries do I need for an off-grid solar system?
Multiply daily watt-hour use by days of autonomy, divide by usable depth of discharge, then divide by a single battery's usable capacity.
Should I build a 12V, 24V, or 48V battery bank?
Higher voltage moves the same power at lower current, meaning thinner wire and fewer losses. Small setups under ~1,000W often stay 12V, mid-size cabins commonly run 24V, larger off-grid homes typically move to 48V.
How does depth of discharge affect how many batteries I need?
DOD is the usable slice of rated capacity. Lead-acid discharged past 50% wears out fast; LiFePO4 can safely use 80-90%, so the same load can need roughly twice as many lead-acid batteries as lithium ones.
What's the difference between LiFePO4 and lead-acid bank sizing?
LiFePO4 batteries are wired in series to reach system voltage since they usually ship at a fixed nominal voltage. Lead-acid batteries are more often combined in series-parallel strings from individual 6V or 12V units.