Battery Bank Sizing

Q = (E_daily × days) / (DOD × V)

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Result
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Formula

Q = (E_daily × days_autonomy) / (DOD × V_battery)

Description

Battery bank sizing for off-grid solar systems ensures enough stored energy to power the load through cloudy days without solar input. The required capacity depends on daily energy consumption, the number of days of autonomy (typically 2-5 for residential systems), the maximum allowable depth of discharge, and the battery bank voltage. Deeper discharge shortens battery life exponentially: lead-acid batteries at 50% DOD last roughly twice as long as at 80% DOD, while lithium-ion batteries tolerate deeper cycling.

Variables

  • Q — Required battery capacity (Ah)
  • E_daily — Daily energy consumption (Wh)
  • days — Number of days of backup autonomy
  • DOD — Maximum depth of discharge (0-1)
  • V — Nominal battery bank voltage (V)

Practical Notes

Common battery bank voltages are 12V, 24V, and 48V; higher voltage reduces current and wiring losses. Lead-acid batteries should use DOD of 0.5 or less for longevity, while LiFePO4 batteries can use 0.8-0.9 DOD. Temperature significantly affects capacity: lead-acid batteries lose about 1% capacity per degree below 25°C.

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