Battery Bank Storage Calculator
Enter daily load, autonomy days, system voltage and chemistry to size the bank in kWh and Ah and the number of batteries in series and parallel.
Step-by-step
- Enter valid values to begin.
Capacity = load / inverter efficiency × days / (DoD × temperature factor). Round-trip efficiency does not enlarge the bank; it sets the recharge energy your array must supply. Preset DoD and efficiency values are typical for cycling use and are not manufacturer ratings. The low-temperature capacity factors for lead-acid (about 0.97 at 20 °C, 0.88 at 10 °C, 0.78 at 0 °C, 0.68 at −10 °C, 0.55 at −20 °C, linearly interpolated) are typical values; use the datasheet curve for the exact cell. LiFePO4 is not derated here, but most cells should not be charged below about 0 °C without heating. Cycle-life figures are typical ranges. Check the NEC edition, battery listing (UL 9540/UL 1973) and local AHJ before installing.
Battery Bank Sizing: Autonomy, Depth of Discharge and Real Capacity
Core Engineering Principles
A battery label shows nominal capacity, but you can only use part of it. Depth of discharge (DoD) is the fraction you are willing to drain on a regular basis, and it sets the bank size. Take the energy for your days of autonomy, divide by the DoD, and you have the nominal energy to buy. Lead-acid is normally limited to about 50% DoD, because deeper cycling sulfates the plates and wears the bank out in a few hundred cycles. LiFePO4 is comfortable at 80–90% and lasts several thousand cycles. Cold matters too: lead-acid at 0 °C delivers only about 78% of rated capacity.
Convert energy to amp-hours at the system voltage, then build it from the units you can actually buy. Series strings raise voltage, parallel strings raise capacity. With lead-acid we limit parallel strings to three, because small differences in internal resistance cause strings to share current unequally and the weakest one ages fastest. Remember too that round-trip efficiency (about 80–85% for lead-acid, 95% for lithium) does not change the bank size. It changes how much solar you need to refill it each day.
Ah = kWhnom × 1000 / Vsys • Nseries = Vsys / Vunit • Npar = ⌈ Wh / (Ns Vunit Ahunit) ⌉
NEC & Standard References
NEC Article 706 covers energy storage systems and NEC Article 480 covers storage batteries, including disconnecting means, overcurrent protection and ventilation. UL 9540 is the safety standard for storage systems and UL 1973 for the batteries. IEC 62619 sets safety requirements for lithium cells in industrial use, and IEEE 485 gives the recommended practice for sizing lead-acid batteries in stationary applications. The temperature factors here are typical values only; use the manufacturer’s curve. Verify the adopted code edition and the requirements of your AHJ.1. DC energy = 10 / 0.92 = 10.87 kWh/day.
2. Usable = 10.87 × 2 = 21.74 kWh.
3. Nominal = 21.74 / 0.90 = 24.15 kWh, which is 24,155 / 48 = 503 Ah at 48 V.
4. Series = round(48 / 12.8) = 4 (51.2 V). Parallel = 24,155 / (51.2 × 200) = 2.36, rounded up to 3. That is 12 units and 30.72 kWh installed.
5. Lead-acid at 50% DoD needs 43.5 kWh, or 20 units in 5 strings, a clear warning. Recharge energy is 10.87 / 0.95 = 11.44 kWh/day.
- Fuse every string. A shorted battery can deliver thousands of amps; each parallel string needs its own overcurrent device near the battery.
- Don’t charge lithium below freezing. Lithium plating on the anode causes permanent damage, so add heating or low-temperature protection.
- Ventilate flooded lead-acid. Charging releases hydrogen, which is explosive at about 4% in air.
- Balance parallel cables. Equal cable lengths to each string keep the current split even.