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Battery Bank Size

Bank Ah + count for autonomy-driven off-grid storage.

Battery

Inputs (SI units)

Average daily AC energy consumption the bank must supply during autonomy days.

Number of consecutive days the bank must carry the load with no solar recharge (cloudy-day reserve).

Maximum fraction of nominal capacity usable before BMS/LVD cut-off — lower DoD extends cycle life.

Charge/discharge coulombic + resistive losses inside the battery/BMS (datasheet round-trip efficiency).

DC-AC (or AC-DC charging) conversion efficiency between the bank and the load.

Operating cell temperature — capacity derates below 25 °C and life derates above it.

Extra oversizing to cover ageing, unmodelled loads and inverter surge draw.

Target DC bus voltage of the assembled battery bank.

Nominal voltage of a single battery module/block from its datasheet.

Rated Ah capacity of one battery module at the manufacturer's reference discharge rate.

Maximum continuous discharge current per unit from the datasheet — checked against actual worst-case draw.

Results

Required nominal storage
30.47 kWh
Required bank capacity
635 Ah @ 48 V
Selected configuration
4S4P = 16 units
Installed capacity
40.96 kWh (800 Ah @ 51.2 V)
Usable capacity
32.77 kWh
✕ FAIL — Series string voltage 51.2 V deviates >5% from target bus 48 V — pick a unit voltage that divides evenly.
✓ PASS — Estimated worst-case DC draw 205.6 A is within bank rating 800 A.
✓ PASS — DoD 80% is reasonable for the chemistry implied by unit voltage.
✓ PASS — Parallel strings (4) within common BMS/paralleling guidance.

Engineering Recommendations

  • →Confirm the BMS's own continuous/peak discharge current rating, not just the cell rating.
  • →Round total units up in whole series strings to preserve balanced paralleling.
  • →Engineering-grade preliminary calculation. Final design must be verified against project-specific site conditions, manufacturer datasheets, applicable standards and utility requirements.

Detailed Calculation Log

Validated inputs: load=10 kWh/day, days=2, DoD=80%, RTE=95%, inv η=95%, T=25°C.
Capacity temperature factor computed from lead-acid style curve: 1.
Bank sized to 4S4P using unit 12.8 V / 200 Ah / 200 A cont.
Achieved bus voltage 51.2 V vs target 48 V.
Worst-case DC current 205.6 A vs bank rating 800 A.

Save & Load Project

Designs are stored privately in this browser — nothing is uploaded.

Engineering Formula

  • E_req = Load × Days / (DoD × η)
  • C_ah = E_req × 1000 / V_bat
  • N_blocks = ⌈C_ah / C_block⌉

Usable storage accounts for DoD ceiling and round-trip efficiency. Bank Ah follows from E=Q·V. Lead-acid ≤ 50% DoD; LFP up to 90%.

Step-by-step Calculation

  1. 1.Raw energyLoad × Autonomy days20 kWh
  2. 2.After DoDE_raw / DoD25 kWh
  3. 3.After round-trip efficiencyE_dod / η_RTE26.32 kWh
  4. 4.After inverter efficiencyE_rte / η_inv27.7 kWh
  5. 5.Temperature capacity factorf(T_cell)1
  6. 6.After temperature correctionE_inv / f(T)27.7 kWh
  7. 7.With design marginE_temp × (1+margin)30.47 kWh
  8. 8.Required AhE_req×1000 / V_bank635 Ah
  9. 9.Series countround(V_bank / V_unit)4
  10. 10.Parallel count⌈Ah_req / Ah_unit⌉4
  11. 11.Total unitsSeries × Parallel16
  12. 12.Installed / usable energyAh_inst×V_bus/1000, ×DoD40.96 / 32.77 kWh

How to use this calculator: Battery Bank Size

Bank Ah + count for autonomy-driven off-grid storage. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Confirm that the Battery Bank Size matches the quantity or design check you need.
  2. 2Enter Daily load, Autonomy, and Depth of discharge using the units printed beside each field.
  3. 3Check every value before calculating, especially decimal points and measurement units.
  4. 4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
  5. 5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Daily load10 kWh/dayAverage daily AC energy consumption the bank must supply during autonomy days.
Autonomy2 daysNumber of consecutive days the bank must carry the load with no solar recharge (cloudy-day reserve).
Depth of discharge80 %Maximum fraction of nominal capacity usable before BMS/LVD cut-off — lower DoD extends cycle life.
Battery round-trip efficiency95 %Charge/discharge coulombic + resistive losses inside the battery/BMS (datasheet round-trip efficiency).
Inverter efficiency95 %DC-AC (or AC-DC charging) conversion efficiency between the bank and the load.
Electrolyte/cell temperature25 °COperating cell temperature — capacity derates below 25 °C and life derates above it.
Design margin10 %Extra oversizing to cover ageing, unmodelled loads and inverter surge draw.
Bank nominal voltage48 VTarget DC bus voltage of the assembled battery bank.
Battery unit nominal voltage12.8 VNominal voltage of a single battery module/block from its datasheet.
Battery unit capacity200 AhRated Ah capacity of one battery module at the manufacturer's reference discharge rate.
Battery unit continuous discharge current200 AMaximum continuous discharge current per unit from the datasheet — checked against actual worst-case draw.

Formula inputs & variables for Battery Bank Size

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / inputUnitMeaning in this calculation
Daily loadkWh/dayAverage daily AC energy consumption the bank must supply during autonomy days.
AutonomydaysNumber of consecutive days the bank must carry the load with no solar recharge (cloudy-day reserve).
Depth of discharge%Maximum fraction of nominal capacity usable before BMS/LVD cut-off — lower DoD extends cycle life.
Battery round-trip efficiency%Charge/discharge coulombic + resistive losses inside the battery/BMS (datasheet round-trip efficiency).
Inverter efficiency%DC-AC (or AC-DC charging) conversion efficiency between the bank and the load.
Electrolyte/cell temperature°COperating cell temperature — capacity derates below 25 °C and life derates above it.
Design margin%Extra oversizing to cover ageing, unmodelled loads and inverter surge draw.
Bank nominal voltageVTarget DC bus voltage of the assembled battery bank.

How the Battery Bank Size works

The Battery Bank Size uses Daily load, Autonomy, Depth of discharge, Battery round-trip efficiency, Inverter efficiency, Electrolyte/cell temperature, Design margin, Bank nominal voltage, Battery unit nominal voltage, Battery unit capacity, and Battery unit continuous discharge current to calculate Required nominal storage, Required bank capacity, Selected configuration, Installed capacity, and Usable capacity. Its engine applies E_req = Load × Days / (DoD × η); the worked values below come from that same live calculation rather than a separately typed example.

With Daily load 10 kWh/day, Autonomy 2 days, Depth of discharge 80 %, Battery round-trip efficiency 95 %, Inverter efficiency 95 %, Electrolyte/cell temperature 25 °C, Design margin 10 %, Bank nominal voltage 48 V, Battery unit nominal voltage 12.8 V, Battery unit capacity 200 Ah, and Battery unit continuous discharge current 200 A, the main worked-example result is Required nominal storage = 30.47 kWh.

How each Battery Bank Size input is used

Daily load

The Battery Bank Size worked example uses Daily load = 10 kWh/day. This value is passed directly into the calculation, with an allowed minimum 0.1. Average daily AC energy consumption the bank must supply during autonomy days.

Autonomy

The Battery Bank Size worked example uses Autonomy = 2 days. This value is passed directly into the calculation, with an allowed minimum 0.5 and maximum 10. Number of consecutive days the bank must carry the load with no solar recharge (cloudy-day reserve).

Depth of discharge

The Battery Bank Size worked example uses Depth of discharge = 80 %. This value is passed directly into the calculation, with an allowed minimum 10 and maximum 100. Maximum fraction of nominal capacity usable before BMS/LVD cut-off — lower DoD extends cycle life.

Battery round-trip efficiency

The Battery Bank Size worked example uses Battery round-trip efficiency = 95 %. This value is passed directly into the calculation, with an allowed minimum 60 and maximum 99. Charge/discharge coulombic + resistive losses inside the battery/BMS (datasheet round-trip efficiency).

Inverter efficiency

The Battery Bank Size worked example uses Inverter efficiency = 95 %. This value is passed directly into the calculation, with an allowed minimum 70 and maximum 99. DC-AC (or AC-DC charging) conversion efficiency between the bank and the load.

Electrolyte/cell temperature

The Battery Bank Size worked example uses Electrolyte/cell temperature = 25 °C. This value is passed directly into the calculation, with an allowed minimum -20 and maximum 55. Operating cell temperature — capacity derates below 25 °C and life derates above it.

Design margin

The Battery Bank Size worked example uses Design margin = 10 %. This value is passed directly into the calculation, with an allowed minimum 0 and maximum 50. Extra oversizing to cover ageing, unmodelled loads and inverter surge draw.

Bank nominal voltage

The Battery Bank Size worked example uses Bank nominal voltage = 48 V. This value is passed directly into the calculation, with an allowed minimum 12 and maximum 800. Target DC bus voltage of the assembled battery bank.

Battery unit nominal voltage

The Battery Bank Size worked example uses Battery unit nominal voltage = 12.8 V. This value is passed directly into the calculation, with an allowed minimum 1.2 and maximum 800. Nominal voltage of a single battery module/block from its datasheet.

Battery unit capacity

The Battery Bank Size worked example uses Battery unit capacity = 200 Ah. This value is passed directly into the calculation, with an allowed minimum 1. Rated Ah capacity of one battery module at the manufacturer's reference discharge rate.

Battery unit continuous discharge current

The Battery Bank Size worked example uses Battery unit continuous discharge current = 200 A. This value is passed directly into the calculation, with an allowed minimum 1. Maximum continuous discharge current per unit from the datasheet — checked against actual worst-case draw.

Battery Bank Size formulas and result interpretation

Formula 1: relationship used

In the Battery Bank Size, E_req = Load × Days / (DoD × η). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Battery Bank Size, C_ah = E_req × 1000 / V_bat. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 3: relationship used

In the Battery Bank Size, N_blocks = ⌈C_ah / C_block⌉. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Required nominal storage

For the displayed Battery Bank Size worked example, Required nominal storage is 30.47 kWh. Verify Daily load, Autonomy, and Depth of discharge and their units before relying on this output.

Required bank capacity

For the displayed Battery Bank Size worked example, Required bank capacity is 635 Ah @ 48 V. Verify Daily load, Autonomy, and Depth of discharge and their units before relying on this output.

Selected configuration

For the displayed Battery Bank Size worked example, Selected configuration is 4S4P = 16 units. Verify Daily load, Autonomy, and Depth of discharge and their units before relying on this output.

Installed capacity

For the displayed Battery Bank Size worked example, Installed capacity is 40.96 kWh (800 Ah @ 51.2 V). Verify Daily load, Autonomy, and Depth of discharge and their units before relying on this output.

Usable capacity

For the displayed Battery Bank Size worked example, Usable capacity is 32.77 kWh. Verify Daily load, Autonomy, and Depth of discharge and their units before relying on this output.

Battery Bank Size accuracy, checks and limitations

  • Battery Bank Size units check: confirm Daily load (kWh/day), Autonomy (days), Depth of discharge (%), Battery round-trip efficiency (%), Inverter efficiency (%), Electrolyte/cell temperature (°C), Design margin (%), Bank nominal voltage (V), Battery unit nominal voltage (V), Battery unit capacity (Ah), and Battery unit continuous discharge current (A) before calculating.
  • Battery Bank Size result check: compare Required nominal storage, Required bank capacity, Selected configuration, Installed capacity, and Usable capacity with the substituted formula steps and the displayed rounding precision.
  • Battery Bank Size: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Battery Bank Size

  • Do not mix units for Daily load (kWh/day), Autonomy (days), Depth of discharge (%). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not replace the displayed E_req = Load × Days / (DoD × η) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Required nominal storage = 30.47 kWh from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Battery Bank Size is useful

Battery Bank Size is designed for cases where Daily load, Autonomy, Depth of discharge, Battery round-trip efficiency are known and you need Required nominal storage, Required bank capacity, Selected configuration. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is E_req = Load × Days / (DoD × η). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Daily load and Autonomy: what changes the answer

The worked example uses Daily load = 10 kWh/day, Autonomy = 2 days, Depth of discharge = 80 %, Battery round-trip efficiency = 95 %. With those values, Required nominal storage is 30.47 kWh. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Daily load (kWh/day): Average daily AC energy consumption the bank must supply during autonomy days. Autonomy (days): Number of consecutive days the bank must carry the load with no solar recharge (cloudy-day reserve). Depth of discharge (%): Maximum fraction of nominal capacity usable before BMS/LVD cut-off — lower DoD extends cycle life. Battery round-trip efficiency (%): Charge/discharge coulombic + resistive losses inside the battery/BMS (datasheet round-trip efficiency).

How to sanity-check a Battery Bank Size result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Required nominal storage, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with AC Cable Sizing Calculator

AC Cable Sizing Calculator covers the same practical workflow from a related calculation angle, making it a useful cross-check after Battery Bank Size.

Open AC Cable Sizing Calculator

Design Assumptions

  • • Constant discharge to reach autonomy over full days.
  • • Temperature 25 °C (derate 1%/°C below 20 °C for lead-acid).

Engineering Tips

  • ◆Prefer 48 V DC bus over 12/24 V for > 3 kW loads.
  • ◆Use LFP for daily-cycle systems (> 3,000 cycles).

Warnings

  • ▲Never parallel more than 4 lead-acid strings.
  • ▲Follow BMS SoC limits — over-discharge voids warranty.

Standards & References

IEC 62619 (Li-ion)IEC 61427 (Lead-acid for RE)IS 16270

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