Solar Battery Bank Sizing Calculator
Enter the load, autonomy target, battery limits, and one battery size to estimate the minimum bank capacity and the whole-battery layout that can actually be built.
Total batteries required12
- Minimum nominal bank capacity (kWh)
- 13.59
- Minimum capacity at system voltage (Ah)
- 283.06
- Batteries in each series string
- 4
- Parallel strings required
- 3
- Built bank nominal voltage (V)
- 51.2
- Built bank nominal capacity (kWh)
- 15.36
- Usable AC energy (kWh)
- 11.3
- Achieved autonomy
- 2.26 days
- Voltage compatibility
- Compatible: 51.2 V is within 10% of the 48 V class.
Do not mix battery models, capacities, ages, or states of charge in one bank.
Battery datasheets and battery-management-system limits control allowed series and parallel configurations.
How to use this calculator
- Enter the average daily AC energy delivered to the loads.
- Choose the number of days the bank should carry the load without meaningful solar charging.
- Enter inverter efficiency, allowable depth of discharge, and the battery capacity factor at the design temperature.
- Enter the nominal voltage and amp-hour capacity of one battery or module.
- Check the series count, parallel strings, usable energy, and voltage warning against the equipment manuals.
How the battery bank size is calculated
This calculator uses the stand-alone photovoltaic autonomy method: start with the AC energy the loads need, multiply by the desired days of autonomy, then divide by the usable fractions that stand between stored DC energy and delivered AC energy. Those fractions are inverter efficiency, maximum depth of discharge, and the available capacity at the design battery temperature.
The required nominal bank energy is:
Er = 1000 x L x A / (eta x D x T)
- L is daily AC load in kWh per day.
- A is desired autonomy in days.
- eta is inverter efficiency as a decimal.
- D is maximum depth of discharge as a decimal.
- T is the temperature capacity factor as a decimal.
The amp-hour requirement at the selected DC voltage is Cr = Er / Vs. The calculator then turns that abstract capacity into a whole-battery bank. Batteries per series string are Ns = round(Vs / Vb), with a minimum of one battery. The resulting string voltage is Vstr = Ns x Vb. Energy in one string is Estr = Vstr x Cb, and parallel strings are rounded up with Np = ceil(Er / Estr). Total batteries are Ns x Np.
What moves the result most
Daily load and autonomy move the answer directly: doubling either one roughly doubles the bank. Depth of discharge can be just as important. A bank designed around 50% lead-acid discharge needs much more nominal capacity than a LiFePO4 bank allowed to use 80% or 90%. Cold temperature also matters because many batteries deliver less rated capacity when cold. The temperature field accepts values above 100% for datasheets that rate capacity higher at mild temperatures, but entries are capped at 110%.
The final battery count can jump in steps because the bank must use whole batteries. A calculated need of 2.1 parallel strings becomes three strings. That is why achieved autonomy is often higher than the requested autonomy.
What this calculator leaves out
This is an energy-capacity estimate, not a complete battery design. It does not check conductor size, short-circuit current, overcurrent protection, enclosure ventilation, battery-management-system current limits, charge rates, surge loads, standby consumption, aging, generator charging, or days with partial solar production. Lead-acid Peukert effects are not modeled; use the amp-hour rating that matches the expected discharge rate.
Worked example
Suppose the loads use 5 kWh per day, the autonomy target is 2 days, inverter efficiency is 92%, maximum depth of discharge is 80%, temperature capacity is 100%, the DC system is 48 V, and each battery is a 12.8 V, 100 Ah module.
The required nominal energy is 1000 x 5 x 2 / (0.92 x 0.80 x 1.00) = 13,586.96 Wh, or 13.59 kWh. At 48 V, that is 283.06 Ah. A 48 V class bank uses four 12.8 V batteries in series for a 51.2 V string. One string stores 5.12 kWh, so the bank needs three parallel strings, or 12 batteries total. The built bank stores 15.36 kWh nominal and delivers about 11.30 kWh AC after the entered limits, which is 2.26 days at the entered load.
Common questions
How many days of battery autonomy should an off-grid solar system have?
Many off-grid homes use two to three days as a starting point, then adjust for climate, generator backup, and how critical the loads are. More autonomy improves resilience but quickly increases battery cost and space. Seasonal sites or systems with dependable generator charging may choose less.
What depth of discharge should I use for LiFePO4, AGM, or flooded lead-acid batteries?
Use the value allowed by the battery manufacturer for the cycle life you want. LiFePO4 systems are often designed around 80% to 90% usable capacity. AGM and flooded lead-acid banks are commonly limited closer to 50% for cycle life, though the exact value depends on the datasheet and operating plan.
How does cold temperature reduce usable battery capacity?
Battery chemistry slows at low temperatures, so the same battery can deliver fewer amp-hours before reaching its cutoff voltage. Lead-acid datasheets often publish capacity correction factors for different temperatures. Lithium batteries may also have charge and discharge limits enforced by the battery-management system.
How do series and parallel connections change battery voltage and amp-hours?
Batteries in series add voltage while the amp-hour rating stays the same as one battery. Parallel strings keep the same string voltage and add amp-hour capacity. The calculator uses series batteries to approach the selected system voltage, then adds enough parallel strings to meet the energy requirement.
Why is achieved autonomy higher than the requested value?
The calculated energy requirement is continuous, but batteries are discrete units. If the math says 2.3 parallel strings are needed, the actual bank has to use three strings. That extra fraction of a string increases the usable AC energy and therefore the achieved autonomy.
Should daily energy use include inverter losses?
Enter the AC energy delivered to the loads, not the DC energy drawn from the battery. The inverter efficiency field accounts for conversion losses. If your measurement already came from the DC battery side, set inverter efficiency to 100% or convert the load estimate back to AC first.