Battery Bank Sizing Calculator — Free Online Calculator
Calculate the battery bank capacity needed for off-grid solar systems. Factor in depth of discharge and autonomy days.
How to Use This Calculator
Enter your daily energy consumption, number of backup days, battery type (affects depth of discharge), and system voltage.
The Formula Explained
Bank kWh = (Daily kWh × Days) / DoD. Amp-hours = (Bank kWh × 1000) / System Voltage. Lead-acid batteries should only be discharged to 50%, while lithium can go to 80-90%.
Battery Banks: Sizing for Real-World Use
Battery sizing is the most critical and most often bungled part of an off-grid system. Undersize the bank and you run out of power on cloudy days, deep-cycling the batteries and killing them within a year or two. Oversize the bank and you tie up thousands of dollars in storage you never use while the batteries slowly die from underutilization and partial-state-of-charge sulfation. Getting it right requires accurate load calculation, honest assessment of solar resource variability, and correct handling of depth-of-discharge limits.
The core calculation: daily energy use in Wh, divided by system voltage to get Ah per day, multiplied by days of autonomy to cover cloudy stretches, divided by the usable depth-of-discharge fraction. Days of autonomy is the judgment call — 2 days for cabins with backup generator, 3-5 days for full off-grid homes, 7+ days for critical medical or telecommunications sites. Depth of discharge is a chemistry decision: 50% for lead-acid, 80% for lithium. Doubling either factor doubles the bank size.
Worked Example: Weekend Cabin
A weekend cabin used Friday through Sunday uses these loads: LED lighting 100W × 4 hours = 400 Wh. Small refrigerator 50W × 24 hours = 1,200 Wh. Water pump 200W × 0.5 hours = 100 Wh. Phone/laptop charging 30W × 4 hours = 120 Wh. Total daily use: 1,820 Wh.
Weekend total (Friday arrival through Sunday departure): 2.5 days × 1,820 Wh = 4,550 Wh. During the week the fridge runs at vacation mode (off or very low, say 600 Wh daily) and minimal other loads. Weekly average: 1,100 Wh daily.
System voltage: 24V (good for 200-400W range). Battery sizing for 3 days autonomy at 50% DoD (flooded lead-acid): (1,820 × 3) / 24 / 0.5 = 455 Ah at 24V. That's four 12V 225 Ah batteries in 2-series 2-parallel, about 500 lb of batteries, around 1,800 USD.
Same bank in lithium at 80% DoD: (1,820 × 3) / 24 / 0.8 = 285 Ah at 24V. Two 12V 150 Ah LiFePO4 in series gives 150 Ah at 24V, so you'd need 300 Ah total — three batteries in a 2-series 1.5-parallel arrangement (or two 24V batteries in parallel at 150 Ah each). Weight: about 120 lb. Cost: about 3,500 USD. Higher upfront but 10 years life vs 4-5 for lead-acid, and 1/4 the weight and space.
Worked Example: Full-Time Off-Grid Home
A small off-grid home uses 8 kWh per day average (well-designed with LED lighting, efficient appliances, propane cooking/water heating). In winter with shorter days and heating loads, use climbs to 12 kWh daily. System voltage: 48V (essential above 4 kWh daily load).
Peak daily Ah at 48V: 12,000 / 48 = 250 Ah. With 4 days autonomy (typical for full-time off-grid in sunny climate) at 80% DoD lithium: 250 × 4 / 0.8 = 1,250 Ah at 48V. That's 60 kWh of usable storage.
Options: Tesla Powerwall has 13.5 kWh, need 5 units at 48V (though Powerwalls are 400V, so a different architecture). LG Chem RESU 16H: 16 kWh per unit, need 4 units. DIY lithium with cylindrical LiFePO4 cells (like EVE or CATL prismatic): build your own 48V 1,000 Ah bank for about 12,000 USD materials. Ready-made turnkey: 20,000-35,000 USD installed.
Cost per kWh of usable storage (lithium, full life): Tesla/LG commercial: 400-600 USD per kWh. DIY lithium: 200-300 USD per kWh. Lead-acid: 100-150 USD per kWh initial but lasts 1/3 as long, making lifecycle cost higher than lithium.
Five Battery Bank Mistakes
1. Mixing old and new batteries. Adding new batteries to an aging bank drags the new ones down to match the old. Always replace the entire bank at once.
2. Mixing batteries of different sizes or chemistries. Two 200 Ah and one 150 Ah in parallel make a bank that behaves like 450 Ah, but current distributes unevenly and the smaller battery stresses faster.
3. Going below 50% state of charge on lead-acid. Each deep cycle shortens life. Flooded lead-acid discharged to 20% might only last 200 cycles; discharged to 50% it lasts 1,500+ cycles. Monitor state of charge and enforce low-voltage cutoffs.
4. Forgetting that cold weather reduces capacity. At 0°C, lead-acid delivers about 80% of rated capacity. At -20°C, about 50%. Lithium is similar without internal heating. Design for worst-case temperature, not room temperature.
5. Skipping battery ventilation (flooded only). Flooded lead-acid batteries vent hydrogen gas during charging. Enclosed battery compartments need active ventilation per ABYC and NEC. AGM and lithium do not produce hydrogen under normal operation.
Battery Chemistry Quick Comparison
Flooded Lead-Acid (FLA): Cheapest upfront (about 100 USD per kWh), 50% usable DoD, 500-1,500 cycles, requires ventilation, weekly maintenance (water topping), best for budget-conscious off-grid.
AGM (Absorbed Glass Mat): Sealed lead-acid, 200 USD per kWh, 50-70% DoD, 700-1,500 cycles, no maintenance, tolerates cold better than flooded, good for boats and RVs.
Gel: Similar to AGM but gel electrolyte, 250 USD per kWh, best deep-cycle performance among lead-acid types, sensitive to overcharging (need specific controller profile).
LiFePO4 (Lithium Iron Phosphate): 400-600 USD per kWh, 80-90% DoD, 3,000-7,000 cycles, lightweight, temperature-sensitive (derate below freezing), safest lithium chemistry (will not thermal runaway). Dominant chemistry for new off-grid builds.
Other lithium (NMC, LCO): Used in pre-built products like Tesla Powerwall, Enphase IQ Battery. Higher energy density but less safe than LiFePO4.
For new off-grid or storage systems in 2026, LiFePO4 is the default choice for most applications. Only very budget-constrained or short-term-use systems should still consider lead-acid.
NEC and Standards for Battery Banks
NEC Article 480 covers stationary battery installations including ventilation, spacing, and overcurrent protection. NEC Article 706 covers Energy Storage Systems (ESS) including residential lithium batteries — added in 2017 specifically for the Tesla Powerwall era of home batteries.
UL 1973 certifies batteries for stationary use. UL 9540 certifies complete ESS including enclosure, BMS, and safety controls. Tesla Powerwall, LG RESU, and Enphase IQ Battery all carry UL 9540 listing. IEC 62619 is the international equivalent for lithium-ion stationary batteries. Always check that your battery has the appropriate listing for its intended installation — a battery rated only for automotive use should not be installed as residential storage.
Battery bank sizing: series, parallel, and how to size for real loads
Sizing a battery bank starts with the load profile: what watts you draw, for how many hours, and how often. Then capacity, expressed in amp-hours at the system voltage, has to cover that load with margin for depth of discharge limits, temperature, and aging. The calculator above takes daily kWh and target days of autonomy, gives back the required bank capacity in Ah at your chosen system voltage.
The formula and what it does
Daily kWh times days of autonomy gives total energy stored. Divide by system voltage to convert to Ah. Divide by depth of discharge (typical 80% for LiFePO4, 50% for lead-acid) and round-trip efficiency (about 95 percent for lithium, 85 percent for lead-acid). Result is required nameplate Ah.
Worked example
Scenario: Off-grid cabin, daily load 5 kWh, 3 days autonomy desired, LiFePO4 batteries, 48 V system.
Required Ah = (5 x 1000 x 3) / (48 x 0.80 x 0.95) = 411 Ah at 48 V. Practical config: 4 x 100 Ah 48 V LiFePO4 modules in parallel = 400 Ah (just under, use 5 x 100 Ah for margin). Or 16 x 100 Ah 12 V LiFePO4 batteries (4 in series x 4 in parallel = 400 Ah at 48 V).
Code references and standards
NEC 706 entire article on energy storage systems. Requires disconnecting means, working space, ventilation for vented batteries, and arc-flash labeling above 100 V DC.
Common mistakes to avoid
undefinedFrequently asked questions
How much can I discharge a lithium battery?
LiFePO4 (Battle Born, Victron, EG4): 80-100 percent depth of discharge regularly without damage. Lead-acid: 50 percent for long life, 80 percent occasionally. AGM: 50 percent for normal use, deeper cycles dramatically shorten life.
Why 48 V vs 12 V vs 24 V?
Higher voltage = lower current for the same kW = smaller conductors, smaller breakers, lower losses. 48 V is standard for residential off-grid and serious solar storage. 12 V is fine for small RV/marine systems. 24 V is a middle ground rarely used in new installs.
Can I mix battery sizes or ages?
No. Different capacities, ages, or chemistries lead to current imbalance, with the weakest battery taking the brunt and failing fast. Always match.
How long do lithium batteries last?
LiFePO4: 3000-5000 cycles to 80 percent capacity, equivalent to 10-15 years of daily cycling. AGM lead-acid: 500-1000 cycles. Flooded lead-acid: similar to AGM but more maintenance.
Do I need a BMS?
For lithium, yes. Built into modern drop-in batteries. Protects from over-charge, over-discharge, over-current, over-temperature. Without BMS, lithium cells can fail spectacularly.
Can I add to my battery bank later?
For lead-acid, no, you must replace all at once. For LiFePO4 with separate BMS per battery, you can add in parallel within the same chemistry and similar voltage. Practical limit on parallel strings is 4-6 before current balancing becomes problematic.