Size your battery bank with precision. Accounts for depth of discharge, efficiency losses, temperature derating, and safety margins – built for solar professionals.
This free battery sizing calculator helps solar professionals determine the correct battery bank capacity for off-grid, hybrid, and backup systems. It accounts for all real-world derating factors that affect usable capacity, so you size the bank right the first time.
Calculates for LiFePO4, Li-NMC, AGM, Flooded Lead-Acid, and Gel batteries with correct DoD, efficiency, and temperature factors for each.
Factors in depth of discharge, round-trip efficiency, inverter losses, wiring losses, and temperature derating for accurate real-world sizing.
Converts kWh requirements to actual battery count based on your battery specs and provides installed cost estimates by chemistry type.
Battery sizing is critical for reliable solar storage. Under-sized banks leave you without power; over-sized banks waste money. Use this calculator when:
Homeowners want to know how long their battery will last during a power outage. Enter critical loads and desired backup hours to size the bank correctly.
Off-grid systems need 3–7 days of autonomy. This calculator accounts for the higher battery capacity required including all derating factors for real-world conditions.
Get accurate cost estimates by chemistry type to include in customer proposals. Compare LFP vs. lead-acid to show clients the true cost-per-cycle difference.
Input your daily energy consumption in kWh and select days of autonomy (backup duration).
Select LiFePO4, NMC, AGM, Flooded Lead-Acid, or Gel. Each chemistry has different DoD and efficiency characteristics.
Choose system voltage (12V, 24V, or 48V) and enter individual battery voltage and amp-hour rating.
Fine-tune inverter efficiency, minimum operating temperature, and safety margin if needed.
Review total bank capacity, usable capacity, battery count, wiring configuration, and cost estimate.
Total nominal battery capacity needed after all derating factors are applied.
The actual energy available after applying the depth of discharge limit for your chosen chemistry.
Number of individual batteries needed, arranged in series strings and parallel banks.
How to wire the batteries — series strings for voltage, parallel banks for capacity.
Price range based on current installed costs per kWh for the selected chemistry.
Expected years of service based on rated cycle life at daily cycling.
The calculator uses industry-standard formulas that account for every factor that reduces usable battery capacity in real-world solar installations.
Required Bank (kWh) = (Daily Load × Days of Autonomy) / (DoD × RTE × System Efficiency × Temp Factor)DoD defaults: LFP 80%, NMC 85%, Lead-Acid 50%. RTE: LFP 95%, NMC 92%, Lead-Acid 80%. System efficiency: Inverter (95%) × Wiring (98%) = 93%. Temperature correction applied per chemistry based on manufacturer data and NEC guidelines.
Total Ah = Bank kWh × 1000 / System VoltageParallel strings = ceil(Total Ah / Battery Ah)Series per string = System V / Battery V
Worked example: Daily load: 8 kWh. Autonomy: 1.5 days. LFP battery (DoD 80%, RTE 95%, system efficiency 93%). Required bank: (8 × 1.5) / (0.80 × 0.95 × 0.93) = 12.0 / 0.706 = 17.0 kWh. Using 10 kWh LFP batteries: 2 batteries needed (20 kWh bank). At 48V system: Total Ah = 17,000 / 48 = 354 Ah. Two 200Ah batteries in parallel = 400 Ah — adequate with 13% margin.
Calculations sourced from SurgePV’s Battery Sizing Calculator — surgepv.com/tools/battery-sizing-calculator/
Side-by-side comparison of common battery chemistries used in solar energy storage systems.
| Parameter | LiFePO4 (LFP) | Li-NMC | AGM Lead-Acid | Flooded Lead-Acid |
|---|---|---|---|---|
| Recommended DoD | 80–90% | 80–85% | 50% | 50% |
| Round-Trip Efficiency | 95–98% | 90–95% | 80–85% | 75–85% |
| Cycle Life | 3,000–10,000 | 1,000–2,500 | 500–1,000 | 400–800 |
| Calendar Life | 10–15 years | 8–12 years | 3–7 years | 3–7 years |
| Self-Discharge/month | 1–3% | 2–3% | 5–15% | 5–15% |
| Maintenance | None | None | None | Monthly (water) |
| Cost (installed) | $700–$1,300/kWh | $800–$1,400/kWh | $200–$400/kWh | $150–$300/kWh |
| Cost per Cycle | ~$0.10–$0.25 | ~$0.40–$0.60 | ~$0.40–$0.80 | ~$0.40–$0.75 |
A 10 kWh battery doesn't give you 10 kWh. After DoD, a 10 kWh LFP battery provides 8 kWh usable. Lead-acid? Only 5 kWh. Always calculate usable capacity.
Batteries in an unconditioned garage lose 20%+ capacity at 0°C. If your installation site gets cold, add capacity to compensate. This calculator handles temperature correction automatically.
Never add new batteries to an aging bank. Older batteries drag down the new ones with uneven charging, reducing the lifespan of the entire system. Replace the whole bank at once.
A 12V system at 5 kW draws 417A — requiring massive, expensive cabling. Use 48V instead (104A for the same power). 48V is the industry standard for any system above 2 kWh.
It depends on your daily energy usage, days of backup needed, battery chemistry, and system voltage. For example, a home using 30 kWh/day wanting 1 day of LFP backup at 48V needs approximately 40 kWh nominal capacity, or about 8 units of 48V/100Ah batteries.
LiFePO4 (LFP) is the industry standard for solar storage. It offers 80–90% depth of discharge, 95%+ round-trip efficiency, 3,000–10,000 cycle life, and zero maintenance. The higher upfront cost is offset by the longest lifespan and lowest cost per cycle.
DoD is the percentage of battery capacity you can safely use before recharging. LFP batteries allow 80–90% DoD, while lead-acid should only be discharged to 50%. Using more than the recommended DoD dramatically shortens battery life.
Use 48V for any system over 2 kWh. Higher voltage means lower current for the same power, which reduces wire size requirements and I²R losses. 48V is the industry standard for residential and commercial solar storage.
Cold temperatures reduce battery capacity significantly. At 0°C, lead-acid loses 20% capacity and LFP loses 13%. If your batteries will be in an unconditioned space, the calculator adds extra capacity to compensate.
LFP batteries typically last 10–15 years (3,000–10,000 cycles). NMC lasts 5–8 years (1,000–2,500 cycles). Lead-acid lasts 2–5 years (400–1,000 cycles). Daily cycling at recommended DoD gives these numbers.
For lithium batteries, some manufacturers allow parallel expansion within a certain timeframe. For lead-acid, never mix old and new batteries — older cells drag down new ones. Best practice is to size correctly upfront.
Determine the right solar system size for your energy needs.
Size your solar inverter based on total system load.
Calculate the right solar system size based on energy usage.
Find the correct AWG gauge for solar AC and DC circuits.
Size UPS battery backup systems for critical solar loads.
Calculate power supply requirements for solar systems.
.svg)
