Solar Battery Size Calculator

Sizing a battery bank correctly is one of the most important steps when designing an off-grid solar system or adding backup power to a grid-tied installation. An undersized battery bank will leave you without power during cloudy days or at night; an oversized one wastes capital and adds unnecessary weight. This calculator guides you through the key variables that determine how much battery storage is needed for your specific situation.

The calculation starts with your daily energy consumption — how many kilowatt-hours (kWh) your loads consume in a 24-hour period. From there, three system-specific factors shape the final battery capacity requirement: days of autonomy, depth of discharge, and battery efficiency.

Daily Energy Consumption

Daily energy consumption is the foundation of any battery sizing calculation. To determine it, list all electrical loads in the system — lights, pumps, refrigerators, electronics, motors — and multiply each load's wattage by the number of hours it operates per day. Summing these gives daily watt-hours; dividing by 1,000 converts to kWh.

For an existing system, the most accurate approach is to read your electricity meter over several days and divide by the number of days. For a new installation, an energy audit spreadsheet or a clamp meter on the main panel provides a reliable estimate. When in doubt, add a 10–20% safety margin to account for overlooked loads or higher-than-expected usage.

Days of Autonomy

Days of autonomy (also called days of storage or backup days) refers to how many consecutive days the battery bank can power your loads without any solar input. A value of 1–2 days is common for grid-backup systems where mains power is normally available. Off-grid systems in areas with reliable sunshine often use 2–3 days. Systems in regions with extended cloudy periods — northern latitudes, monsoon climates — may require 4–5 days of autonomy.

Higher autonomy requires a larger and more expensive battery bank. The appropriate value depends on your acceptable risk of running out of power, the availability and cost of alternative power sources (generator, grid connection), and the criticality of the loads being powered.

Depth of Discharge

Depth of discharge (DoD) is the percentage of a battery's nameplate capacity that can be regularly used without damaging the cells or significantly shortening battery lifespan. Discharging a battery beyond its recommended DoD causes accelerated capacity fade and can permanently damage the cells.

For flooded and sealed lead-acid batteries, the recommended DoD is typically 50%. Discharging to 80% is sometimes done in emergencies but shortens service life substantially. Lithium iron phosphate (LFP) batteries are designed for 80–90% DoD, and most battery management systems allow 80% as a standard operating limit. Premium lithium NMC cells may support up to 90% DoD with adequate thermal management.

The DoD setting directly multiplies the required nominal battery capacity. For example, if you need 10 kWh of usable energy and your batteries are rated for 50% DoD, the nominal bank must be 10 ÷ 0.50 = 20 kWh. The same 10 kWh of usable energy with 80% DoD lithium batteries requires only 10 ÷ 0.80 = 12.5 kWh nominal — a significantly smaller and lighter bank.

Battery Efficiency

Batteries are not perfectly efficient storage devices. Some energy is lost to internal resistance and heat during charge and discharge cycles. This round-trip efficiency — the ratio of energy out to energy in — typically ranges from 80–85% for lead-acid batteries and 95–98% for lithium batteries.

In the context of this calculator, battery efficiency adjusts the total nominal capacity required to ensure that the stated usable energy is actually deliverable after losses. A 90% efficiency means that for every 1 kWh you want to draw from the bank, you need to store approximately 1.11 kWh. Real-world efficiency is influenced by temperature (cold batteries are less efficient), charge/discharge rate, and age.

System Voltage

Battery banks are configured at a system voltage of 12V, 24V, or 48V. The system voltage determines how batteries are connected in series and parallel to achieve the desired capacity and voltage. Small systems (under about 1 kWh) often use 12V. Medium systems (1–5 kWh) typically use 24V. Large off-grid homes, commercial systems, and modern hybrid inverters commonly operate at 48V.

Higher system voltages reduce the current flowing through cables and connections for the same power level, allowing the use of smaller-gauge wiring and reducing resistive losses. A 48V system carrying 1,000W draws only about 21 amps, whereas a 12V system carrying the same load draws around 83 amps — requiring much heavier and more expensive cable. Most modern off-grid inverters and MPPT charge controllers work most efficiently at 48V for systems above a few kilowatts.

The system voltage is used to convert the required capacity from kWh to Ah. A 10 kWh bank at 48V corresponds to (10,000 Wh ÷ 48V) = approximately 208 Ah. The same 10 kWh at 12V would require 833 Ah — a much larger and heavier bank.

Number of Batteries and Bank Configuration

Once the total required Ah at the system voltage is known, dividing by the Ah rating of an individual battery gives the number of batteries needed. Because partial batteries are not possible, the result is always rounded up to the nearest whole number.

Batteries are connected in series to increase voltage, and in parallel to increase capacity. For a 48V bank using 12V batteries, four batteries must be connected in series to achieve 48V. If more than four batteries are needed, additional groups of four are added in parallel. For example, a 400 Ah bank at 48V using 200 Ah 12V batteries requires 2 series strings of 4 batteries each, totalling 8 batteries.

When paralleling strings of lead-acid batteries, best practice limits parallel strings to three or four to avoid imbalanced charging and cell sulfation. Lithium battery systems with battery management systems (BMS) are generally more tolerant of parallel configurations.

Battery Types and Practical Considerations

Flooded lead-acid (FLA) batteries are the lowest-cost option per kWh of nominal capacity, but their 50% DoD limit means you need twice the nominal capacity for usable storage compared to lithium. They require regular maintenance (checking electrolyte levels, equalisation charging), must be installed in well-ventilated areas due to hydrogen gas off-gassing, and are heavy. Expected service life is 3–7 years depending on usage and maintenance.

Sealed lead-acid batteries — absorbed glass mat (AGM) and gel — are maintenance-free and can be installed in any orientation. They share the 50% DoD limitation of FLA batteries and are more expensive per kWh of nominal capacity, but cheaper per usable kWh than many lithium options in smaller systems.

Lithium iron phosphate (LFP) batteries have become increasingly popular for residential and off-grid solar applications. They offer 80–90% usable capacity (DoD), round-trip efficiency of 95–98%, a lifespan of 2,000–6,000+ cycles (10–15+ years at daily cycling), and significantly lower weight than lead-acid. The higher upfront cost is often offset by longer service life and greater usable capacity per unit of nominal storage.

Cost Estimation

Battery system costs vary widely by chemistry, brand, market, and installation complexity. As a general guide, flooded lead-acid batteries for solar applications range from approximately $100–$200 per kWh of nominal capacity (meaning the cost per usable kWh at 50% DoD is roughly twice that). Sealed AGM batteries are typically $150–$350 per kWh nominal.

Lithium iron phosphate battery prices have fallen substantially and now range from approximately $200–$600 per kWh of nominal (usable) capacity in the mid-2020s depending on brand, warranty, and whether a BMS is included. All-in-one lithium battery storage systems with integrated BMS and communication protocols sit at the higher end.

The total system cost should also factor in the inverter/charger, MPPT charge controller (for solar input), wiring, fuses, disconnects, and installation labour — typically adding 30–60% to the battery cost alone. Enter the per-battery cost in the optional field to get a rough total battery hardware estimate.