Solar Battery Bank Calculator
Size an off-grid battery bank from your daily load and days of autonomy — capacity in kWh and Ah, plus a series/parallel layout for the cells you choose.
Results
Required nameplate capacity: 15.28 kWh
Bank capacity at system voltage: 636.6 Ah @ 24 V
How it works
The bank must store enough energy to carry your load through several sunless days, after inverter losses and without discharging the cells too deeply.
required_wh = daily_load_kwh × 1000 × autonomy_days / (dod × inverter_efficiency) × (1 + safety_buffer)
bank_ah = required_wh / system_voltage
series = system_voltage / battery_voltage
parallel = ceil(bank_ah / battery_capacity_ah)
daily_load_kwh- Energy your loads use per day, in kilowatt-hours.
autonomy_days- Consecutive days the bank must run with no meaningful charging.
dod- Fraction of capacity you draw before recharging; the reciprocal oversizes the bank so cells aren't over-discharged.
inverter_efficiency- Round-trip inverter efficiency for AC loads.
safety_buffer- Extra headroom for aging and temperature derate, as a fraction.
system_voltage- Nominal DC bus voltage (12, 24, or 48 V).
required_wh- Storage the bank must hold once losses and DoD are accounted for.
bank_ah- Required capacity expressed as amp-hours at the system voltage.
series- Blocks wired in series to reach the system voltage.
parallel- Series strings wired in parallel to reach the required amp-hours.
Assumptions
- DoD defaults to 0.80 for LiFePO4 and 0.50 for lead-acid — conservative for cycle life. Many LiFePO4 packs tolerate 0.90; raise it only if your datasheet allows.
- Inverter efficiency defaults to 0.90. AC loads pass through the inverter; DC-only systems lose less.
- A 0.10 safety buffer adds headroom for aging and cold-weather derate. Set it to 0 for a bare-minimum bank.
- Sizing is by energy (Wh). It does not model charge acceptance, temperature, or wiring losses beyond the buffer.
- Series count must be a whole number. A block voltage that doesn't divide the bus voltage is reported as a mismatch rather than silently rounded.
FAQ
How many batteries do I need for a 5 kWh/day off-grid system?
At 2 days autonomy, 48 V, 80% DoD, 90% inverter efficiency and no extra buffer, 5 kWh/day needs about 13.9 kWh of storage (~289 Ah at 48 V). With 12 V / 100 Ah LiFePO4 blocks that is 4 in series × 3 in parallel = 12 blocks.
Should I use 80% or 90% DoD for LiFePO4?
80% is a safe default that extends cycle life. Many LiFePO4 cells are rated for 90% or deeper; using 90% shrinks the bank by about 11% but leaves less reserve. Follow your battery's datasheet.
12 V vs 24 V vs 48 V — which should I pick?
Higher voltage means lower current for the same power, so thinner wiring and less loss. 12 V suits small RV/van builds, 24 V mid-size systems, and 48 V is standard for whole-home off-grid inverters.
Sources
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