Battery bank size calculator: capacity, Ah and series/parallel wiring

How this battery bank calculator works

Sizing a battery bank is a chain of corrections. You start from the energy your loads need and end with a nominal capacity that is noticeably larger, because a battery must never be emptied completely, delivers less when cold, loses some energy as heat and shrinks as it ages. This calculator applies each correction step by step (the bar chart shows how the requirement grows) and then turns the result into a real wiring plan for the battery or cell you have chosen.

1. Energy at the battery: daily energy divided by inverter efficiency if the loads are AC.
2. Autonomy: multiplied by the days the bank must carry the loads without recharging.
3. Discharge efficiency: a few percent are lost inside the battery (about 3% for LFP, 10% for lead-acid).
4. Depth of discharge: only part of the capacity may be used: 80–90% for LFP, about 50% for lead-acid.
5. Temperature: capacity drops in the cold, roughly 1% per °C below 25 °C for lead-acid and 0.4% for lithium.
6. Ageing: an end-of-life margin (IEEE 485 ageing factor) keeps the bank adequate when worn.

The result is divided by the system voltage to give amp-hours. The calculator then puts enough units in series to reach the system voltage and enough strings in parallel to reach the capacity, and checks the bank’s current rating against your peak load.

The formulas

Worked example: 48 V off-grid home bank

The defaults ask for 5 kWh a day from the battery through a 93%-efficient inverter, with 1.5 days of autonomy, LFP batteries used to 80% DoD, a lowest operating temperature of 10 °C and an 80% end-of-life design point.

The chain runs: 5.38 kWh/day at the terminals → 8.06 kWh for 1.5 days → 8.31 kWh after discharge losses → 10.4 kWh at 80% DoD → 11.1 kWh at 10 °C → 13.8 kWh nominal, or 288 Ah at 48 V. With 12.8 V 100 Ah LFP batteries the plan is 4S3P: four in series for 51.2 V, three strings in parallel for 300 Ah. That is twelve batteries and 15.36 kWh. A 3 kW peak load draws 63 A, well within the 300 A the three strings can supply, and the bank still delivers 1.7 days of autonomy at end of life.

The same requirement at 12 V would need eleven batteries in parallel and 252 A of peak current, a clear argument for 48 V.

How to read the results

• Configuration (e.g. 4S3P) is the wiring plan: series units per string × parallel strings.
• Installed bank is usually a little larger than required because batteries come in whole units.
• Actual autonomy (end of life) shows the real margin after rounding up.
• Peak discharge current and C-rate: lead-acid batteries are happiest below about 0.2C, LFP below 0.5–1C (check the BMS rating).

Typical bank sizes

Tips

• Use identical batteries of the same model, capacity and age in one bank; never mix chemistries.
• Wire parallel strings symmetrically. Take the positive from one end and the negative from the other, or use busbars, so every string sees the same cable resistance.
• Fuse every parallel string close to the battery; a short in one string can otherwise be fed by all the others.
• Go to a higher system voltage before adding strings. Fewer parallel strings means fewer fuses, better balance and thinner cables (see the cable size calculator).
• Check runtime at real loads with the battery backup time calculator, and the charging side with the off-grid solar calculator.

Limitations

The calculator treats the daily energy as an average; if short, very high loads dominate, check the battery’s power rating and Peukert losses separately. Temperature factors are simple linear approximations, and lithium charging limits below 0 °C are not modelled. Battery-management-system settings, cell balancing and manufacturer-specific parallel limits always take precedence over these general rules.

Method & assumptions

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