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.
The formula, every assumption, and the primary sources behind this tool are published on this page.
Results
Required nameplate capacity: 15.28 kWh
Bank capacity at system voltage: 636.6 Ah @ 24 V
Battery bank size for 5, 10, or 20 kWh per day
| Daily use | 1 day | 2 days | 3 days |
|---|---|---|---|
| 5 kWh/day | 7.64 kWh | 15.28 kWh | 22.92 kWh |
| 10 kWh/day | 15.28 kWh | 30.56 kWh | 45.83 kWh |
| 20 kWh/day | 30.56 kWh | 61.11 kWh | 91.67 kWh |
Select a result to load its inputs into the calculator.
Plan the generation to support your daily use: Calculate how many solar panels you need.
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.
load_required_wh = daily_load_kwh × 1000 × autonomy_days / (dod × inverter_efficiency) × (1 + safety_buffer)load_bank_ah = load_required_wh / system_voltageseries = round(nominal_target / battery_voltage)string_voltage = series × battery_voltagetarget_required_wh = target_capacity_ah × string_voltagecapacity_P = ceil(load_required_wh / (string_voltage × battery_capacity_ah))target_P = ceil(target_capacity_ah / battery_capacity_ah)current_P = max(ceil(charge_current / max_charge), ceil(discharge_current / max_discharge))parallel = max(capacity_P or target_P, current_P, 1)actual_Ah = parallel × battery_capacity_ahactual_wh = actual_Ah × string_voltageC_rate = bank_current / actual_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- Battery-to-AC discharge inverter efficiency; not grid round-trip efficiency.
safety_buffer- Extra headroom for aging and temperature derate, as a fraction.
system_voltage- Nominal DC bus voltage (12, 24, or 48 V).
nominal_target- The 12/24/48 V label in load mode and lead-acid target mode; 12.8/25.6/51.2 V in LiFePO4 target mode. Target energy uses the matched string's actual voltage; a mismatch retains the demand at the system label.
required_wh- Required nameplate energy from the daily-load path or target Ah × matched string voltage.
bank_ah- Required capacity expressed as amp-hours at the nominal system voltage — the figure you shop with. Block counts are taken from stored energy instead, so a lithium string's real voltage is not rounded away.
series- Cells or modules in series to reach the nominal target voltage.
string_voltage- What one series string actually runs at (series × block voltage). On lithium this sits above the label: a “48 V” LiFePO4 bus is really 51.2 V.
parallel- Series strings satisfying load energy or target Ah, plus any supplied charge and discharge current limits.
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.
- Load mode sizes by energy; target mode starts with bank Ah. Supplied current ratings can increase parallel count. The model does not predict charge acceptance, temperature response or wiring losses beyond the buffer.
- 12 V / 24 V / 48 V are nominal labels, not the voltage a lithium string actually runs at: 3.2 V cells make 4S = 12.8 V, 8S = 25.6 V and 16S = 51.2 V, all sold against the lower label. A string is accepted when it stacks to within 10% of the label, which covers those standard builds; anything further off (20 V blocks reaching 40 V on a 48 V bus) is reported as a mismatch rather than silently rounded. Parallel strings are counted from stored energy, not from amp-hours read at the label, so a 51.2 V string is not credited as if it were 48 V.
- Target mode uses pack Ah; LiFePO4 12/24/48 V labels map to 12.8/25.6/51.2 V nominal targets. Legacy load mode retains its series matching. Parallel count also satisfies supplied charge/discharge ratings, assuming equal sharing. Enter both cutoff voltages to show the string range; confirm BMS and inverter compatibility separately.
FAQ
How do I configure 3.2 V / 100 Ah cells for 48 V and 200 Ah?
Target mode gives 16S2P: 32 cells, 51.2 V, 200 Ah and 10.24 kWh. An 80 A bank discharge is 0.4C. A tighter per-cell current limit can require more parallel strings.
How many batteries do I need for a 5 kWh/day off-grid system?
At 2 days autonomy, 80% DoD, 90% inverter efficiency and a 10% buffer, 5 kWh/day needs about 15.28 kWh of nameplate storage. For a 48 V system using 12.8 V / 100 Ah LiFePO4 batteries, 4 in series × 3 in parallel gives 12 batteries and 15.36 kWh of installed capacity.
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
- Victron Energy — 12.8 & 25.6 V LiFePO4 Smart battery datasheet (cycle life 2,500 cycles at 80% DoD, 5,000 at 50%) — Backs: DoD default 0.80 for LiFePO4
- Victron Energy — Gel and AGM battery datasheet (AGM deep-cycle: 400 cycles at 80% DoD, 600 at 50%) — Backs: DoD default 0.50 for lead-acid
- Victron Energy — Phoenix VE.Direct 250–1600 VA inverter datasheet (max. efficiency 87–92% across the range) — Backs: inverter efficiency default 0.90
- Engineering convention, not an external source — headroom for battery aging and cold-weather derate — Backs: safety buffer default 0.10
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