About the Battery Bank Sizing
Battery bank sizing works out the storage capacity (in amp-hours or kWh) needed to run your loads for a required backup duration, accounting for depth of discharge, system voltage, and round-trip efficiency.
Formula
Battery capacity (Ah) = (Load Wh × Autonomy) ÷ (System V × DoD × η)
- Load Wh
- Daily energy the battery must supply (Wh)
- Autonomy
- Days of backup without charging
- System V
- Battery bank voltage (V)
- DoD
- Usable depth of discharge (0.5 lead-acid, ~0.9 Li-ion)
- η
- Round-trip / inverter efficiency
Worked example
For 2,000 Wh/day of backup on a 48 V bank with 0.8 usable DoD and 0.85 efficiency: 2,000 ÷ (48 × 0.8 × 0.85) ≈ 61 Ah. Round up to the next standard battery size.
What you can calculate
- LFP / NMC / Lead-Acid
- DoD & cycle life
- S×P wiring
- Temp derating
Standards & references
Frequently asked questions
How do I size a solar battery bank?
Multiply your daily backup energy by the days of autonomy, then divide by the bank voltage, usable depth of discharge, and efficiency. Lithium batteries allow ~90% depth of discharge versus ~50% for lead-acid, so they need roughly half the rated capacity for the same usable energy.
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