Battery Bank Sizing — Method
Storage is sized from the energy the system must run on battery alone, times the number of days you want to ride through poor sun (autonomy), divided by the usable fraction of the battery. Required Ah = (daily Wh × autonomy) ÷ (system voltage × depth of discharge). Lead-acid is limited to about 50% DoD to protect life; lithium allows 80–90%. Batteries are wired in series to make the system voltage, then parallel strings add capacity.
Frequently Asked Questions
Required Ah = (daily Wh x days of autonomy) divided by (system voltage x depth of discharge). For 5000 Wh/day, 1 day, 48 V, 80% DoD that is about 130 Ah at 48 V.
Lead-acid batteries should not go below about 50% DoD for reasonable life; lithium (LiFePO4) tolerates 80-90%. Using a higher DoD reduces the Ah needed but check the warranty cycle life.
The number of days the bank can power the load with no charging, for cloudy spells. Off-grid homes often use 2-3 days; grid-backup systems may use less.
Series connection adds voltage to reach the system voltage; parallel strings add capacity (Ah). Total energy = number of batteries x Ah x cell voltage.
Yes. Cold reduces usable capacity (a lead-acid bank can lose 20-30% near 0 C). Add a temperature derating factor and account for inverter/round-trip losses.
Solar Battery Bank Sizing Explained
The battery bank carries the system through the night and through cloudy days. Sizing balances three things: the daily energy, how many days of autonomy you want, and how deeply you are willing to discharge the chemistry you chose.
The core equation
Required Ah = (daily Wh × autonomy) ÷ (V × DoD). Pick the system voltage (12/24/48 V — higher voltage means lower current and smaller cables), then divide by the usable depth of discharge.
Real-world factors
Add temperature derating and round-trip/inverter losses, and confirm the charge source (array + controller) can recharge the bank between cycles. Size DC cabling with the cable sizing tool.
Related: Panel Sizing, Inverter Sizing, PV String.