Battery Bank Calculator
Size your off-grid battery bank by daily usage and autonomy days. Supports lithium, AGM, flooded, and gel batteries with DOD calculations and cost estimates.
Energy Requirements
Total Capacity
22.2 kWh
Recommended Battery Setup
Battery Type Comparison
| Type | DOD | Cycles | Cost/kWh | Lifespan |
|---|---|---|---|---|
| Lithium (LiFePO4) | 90% | 5,000 | $400 | 10-15 yrs |
| AGM Lead-Acid | 50% | 800 | $200 | 3-5 yrs |
| Flooded Lead-Acid | 50% | 1,000 | $150 | 3-5 yrs |
| Gel Lead-Acid | 50% | 1,000 | $250 | 3-5 yrs |
- Never discharge lead-acid batteries below 50% - it dramatically shortens lifespan
- LiFePO4 batteries cost more upfront but have 5-10x the lifespan of lead-acid
- Higher system voltage (48V) means smaller wire sizes and less energy loss
- Keep batteries at room temperature - extreme temps reduce capacity and life
- Always use a Battery Management System (BMS) with lithium batteries
Related Calculators
About This Calculator
Proper battery bank sizing is critical for reliable off-grid and backup power systems. Our comprehensive Battery Bank Calculator determines the exact capacity needed based on your daily energy consumption, desired autonomy days, and battery chemistry—ensuring you have sufficient power without overspending on unnecessary capacity.
In 2026, lithium battery prices have dropped dramatically, with LiFePO4 (lithium iron phosphate) pack prices reaching $70-108/kWh wholesale and complete home storage systems costing $200-400/kWh installed. This represents a 40-50% decrease from just two years ago, making battery storage more accessible than ever. However, proper sizing remains essential—an undersized bank leads to frustrating power shortages and premature battery failure, while an oversized bank wastes thousands of dollars.
This calculator accounts for depth of discharge (DOD) limitations, system voltage selection, battery chemistry differences, and efficiency losses to provide accurate sizing for off-grid homes, backup systems, RVs, boats, and other applications.
Trusted Sources
How to Use the Battery Bank Calculator
- 1Enter your daily energy usage in kWh (use our Off-Grid Load Calculator for detailed analysis).
- 2Set desired days of autonomy—how long you need backup without any charging source.
- 3Choose your system voltage: 12V for small systems, 24V for medium, 48V for large.
- 4Select your battery chemistry: LiFePO4 (lithium), AGM, flooded lead-acid, or gel.
- 5Review the calculated total capacity required (kWh and Ah).
- 6Enable Advanced Mode for custom DOD, individual battery sizing, and cost estimates.
- 7Check the battery configuration showing series and parallel requirements.
- 8Compare chemistry options using the cost and lifespan analysis.
Formula
Total Capacity (kWh) = Daily Usage × Autonomy Days ÷ DOD ÷ EfficiencyTotal required capacity equals daily energy consumption multiplied by the number of autonomy days, divided by depth of discharge percentage (to protect battery life), and divided by system efficiency (typically 85-95%). The result in kWh is converted to amp-hours by dividing by system voltage: Ah = kWh × 1000 ÷ Voltage.
Understanding Battery Bank Sizing
Proper sizing balances capacity, cost, and battery longevity:
Key Sizing Formula:
Total Capacity = (Daily Usage × Autonomy Days) ÷ DOD ÷ Efficiency
Where:
- Daily Usage = kWh consumed per day
- Autonomy Days = days without charging
- DOD = depth of discharge (decimal)
- Efficiency = inverter/wiring efficiency (0.85-0.95)
Example Calculation:
| Parameter | Value |
|---|---|
| Daily usage | 10 kWh |
| Autonomy days | 2 days |
| DOD (LiFePO4) | 80% (0.80) |
| Efficiency | 90% (0.90) |
| Required capacity | 10 × 2 ÷ 0.80 ÷ 0.90 = 27.8 kWh |
Converting to Amp-Hours:
Ah = kWh × 1000 ÷ System Voltage
27.8 kWh at 48V = 27,800 ÷ 48 = 579 Ah
Why DOD Matters:
| Battery Type | Recommended DOD | Usable from 100Ah |
|---|---|---|
| LiFePO4 | 80-90% | 80-90 Ah |
| AGM | 50% | 50 Ah |
| Flooded Lead-Acid | 50% | 50 Ah |
| Gel | 50% | 50 Ah |
Critical: Exceeding recommended DOD dramatically shortens battery life. A lead-acid battery regularly discharged to 80% may last 200-300 cycles; discharged to 50% it may last 1,000+ cycles.
2026 Battery Pricing and Options
Battery storage costs have dropped significantly, making proper sizing more affordable:
2026 Battery Prices (Complete Systems Installed):
| Chemistry | Price Range | $/kWh | Lifespan |
|---|---|---|---|
| LiFePO4 (LFP) | $200-400/kWh | Best value | 10-15 years |
| AGM Lead-Acid | $150-250/kWh | Budget option | 3-5 years |
| Flooded Lead-Acid | $100-180/kWh | Lowest cost | 3-7 years |
| Gel | $200-300/kWh | Special apps | 5-7 years |
Complete Home Battery Systems (2026):
| Capacity | LiFePO4 Cost | AGM Cost |
|---|---|---|
| 5 kWh | $1,200-2,000 | $800-1,250 |
| 10 kWh | $2,000-4,000 | $1,500-2,500 |
| 15 kWh | $3,000-5,500 | $2,200-3,750 |
| 20 kWh | $4,000-7,000 | $3,000-5,000 |
| 30 kWh | $6,000-10,000 | $4,500-7,500 |
DIY vs. Turnkey Systems:
| Approach | Cost/kWh | Pros | Cons |
|---|---|---|---|
| DIY cells + BMS | $100-200 | Lowest cost | Requires expertise |
| Pre-built batteries | $200-350 | Plug-and-play | More expensive |
| Turnkey systems | $300-500 | Warranty, support | Highest cost |
Popular LiFePO4 Products (2026):
| Product | Capacity | Price | $/kWh |
|---|---|---|---|
| 12V 100Ah server rack | 1.28 kWh | $250-400 | $195-312 |
| 48V 100Ah (5.12 kWh) | 5.12 kWh | $1,200-1,800 | $234-351 |
| 48V 200Ah (10.24 kWh) | 10.24 kWh | $2,200-3,500 | $215-342 |
| Stackable modules | 5-20 kWh | $300-400/kWh | Scalable |
Lithium vs. Lead-Acid: Complete Comparison
Choosing the right battery chemistry affects cost, performance, and total ownership cost:
LiFePO4 (Lithium Iron Phosphate) Advantages:
| Factor | LiFePO4 | Lead-Acid |
|---|---|---|
| Usable capacity | 80-90% | 50% |
| Cycle life | 3,000-6,000 | 500-1,000 |
| Lifespan | 10-15 years | 3-5 years |
| Weight | ~13 lbs/kWh | ~60 lbs/kWh |
| Maintenance | None | Monthly (flooded) |
| Charge efficiency | 97-99% | 80-85% |
| Self-discharge | 1-3%/month | 5-15%/month |
| Temperature range | -4°F to 140°F | 32°F to 113°F |
When Lead-Acid Still Makes Sense:
- Very tight initial budget
- Cold storage (AGM handles cold better)
- Simple weekend cabin use
- You can perform regular maintenance
- Short-term or temporary installation
Total Cost of Ownership (10 kWh system over 15 years):
| Factor | LiFePO4 | AGM | Flooded |
|---|---|---|---|
| Initial cost | $3,000 | $1,800 | $1,200 |
| Replacements | 0 | 2-3 | 2-4 |
| Replacement cost | $0 | $3,600-5,400 | $2,400-4,800 |
| Maintenance | $0 | $0 | $200-400 |
| 15-year total | $3,000 | $5,400-7,200 | $3,800-6,400 |
Key Insight: LiFePO4 has the lowest total cost of ownership for systems used regularly over 10+ years.
Capacity Comparison for Same Usable Energy:
| Usable Energy | LiFePO4 Needed | AGM Needed | Weight Diff |
|---|---|---|---|
| 5 kWh usable | 6.25 kWh bank | 10 kWh bank | 400 lbs less |
| 10 kWh usable | 12.5 kWh bank | 20 kWh bank | 800 lbs less |
| 20 kWh usable | 25 kWh bank | 40 kWh bank | 1,600 lbs less |
Series vs. Parallel Connections
Understanding battery connections is essential for building your bank:
Series Connections (Increases Voltage):
Batteries: + ─┬─ - + ─┬─ - + ─┬─ - + ─┬─ -
│ │ │ │
12V 12V 12V 12V
└───────────────────────┘
48V total
100Ah (same)
- Voltage adds, capacity stays the same
- 4 × 12V batteries = 48V system
- All batteries MUST be identical
- Used to reach inverter voltage requirements
Parallel Connections (Increases Capacity):
┌── + Battery 1 - ──┐
│ │
Positive ───┼── + Battery 2 - ──┼─── Negative
│ │
└── + Battery 3 - ──┘
12V total
300Ah (3 × 100Ah)
- Capacity adds, voltage stays the same
- 3 × 100Ah batteries = 300Ah total
- Batteries should be matched
- Used to increase storage capacity
Series-Parallel Combinations:
| Configuration | Voltage | Capacity | Total Energy |
|---|---|---|---|
| 4S × 2P (12V 100Ah batteries) | 48V | 200Ah | 9.6 kWh |
| 4S × 3P (12V 100Ah batteries) | 48V | 300Ah | 14.4 kWh |
| 4S × 4P (12V 100Ah batteries) | 48V | 400Ah | 19.2 kWh |
System Voltage Selection Guide:
| Daily Usage | Recommended Voltage | Reason |
|---|---|---|
| <2 kWh/day | 12V | Simple, low-power |
| 2-8 kWh/day | 24V | Balance of cost/efficiency |
| 8-20 kWh/day | 48V | Standard off-grid |
| 20+ kWh/day | 48V (multiple banks) | High capacity |
Higher Voltage Advantages:
- Lower current = smaller wire sizes
- Less power loss in cables
- More efficient inverters available
- Required for larger inverters (3kW+)
Autonomy Days by Application
How many days of backup you need depends on your use case:
Grid-Tied Backup (Power Outages):
| Risk Level | Autonomy | Use Case |
|---|---|---|
| Low | 1 day | Urban, reliable grid, short outages |
| Medium | 2-3 days | Suburban, occasional outages |
| High | 3-5 days | Rural, extended outages common |
| Critical | 5-7 days | Medical equipment, remote locations |
Off-Grid Solar Systems:
| Climate | Autonomy | Reasoning |
|---|---|---|
| Southwest desert | 2-3 days | Consistent sun |
| Southeast | 3-4 days | Afternoon storms |
| Midwest | 4-5 days | Seasonal variation |
| Pacific Northwest | 5-7 days | Extended cloudy periods |
| Northern/Alaska | 7-14 days | Winter darkness |
Mobile Applications:
| Application | Autonomy | Daily Usage |
|---|---|---|
| Weekend RV/camping | 2 days | 2-3 kWh |
| Extended RV travel | 3-5 days | 3-5 kWh |
| Full-time RV | 2-3 days (with solar) | 5-10 kWh |
| Sailboat | 3-5 days | 2-4 kWh |
| Work truck | 1-2 days | 1-3 kWh |
Reducing Required Autonomy:
| Strategy | Impact |
|---|---|
| Add more solar | Reduces battery drain days |
| Generator backup | Provides emergency charging |
| Load management | Reduces daily consumption |
| Critical loads only | 50-80% usage reduction |
Critical vs. Total Loads:
| Load Type | Example | Daily kWh |
|---|---|---|
| Critical | Fridge, lights, phones | 2-3 kWh |
| Important | TV, laptop, small appliances | 3-5 kWh |
| Comfort | HVAC, cooking, full use | 10-30 kWh |
Sizing Tip: Size for critical loads with maximum autonomy, then scale for comfort loads if budget allows.
Battery Management Systems (BMS)
A BMS is essential for lithium batteries and beneficial for lead-acid:
What a BMS Does:
| Protection | Function |
|---|---|
| Overcharge | Disconnects at max voltage (3.65V/cell LFP) |
| Over-discharge | Disconnects at min voltage (2.5V/cell LFP) |
| Overcurrent | Limits discharge current |
| Short circuit | Immediate disconnect |
| Temperature | Protects from heat/cold |
| Cell balancing | Equalizes cell voltages |
BMS Types:
| Type | Best For | Price Range |
|---|---|---|
| Integrated | Pre-built batteries | Included |
| Smart BMS (Bluetooth) | DIY monitoring | $80-200 |
| Basic BMS | Budget DIY | $30-80 |
| High-current BMS | Large systems | $150-400 |
Sizing BMS Current:
BMS Continuous Rating ≥ Inverter Max Current × 1.2
Example: 3000W inverter at 48V = 62.5A
BMS should be rated ≥75A continuous
BMS Communication:
| Protocol | Features | Use |
|---|---|---|
| Bluetooth | Phone monitoring | Consumer systems |
| CAN Bus | Inverter communication | Professional |
| RS485 | Multi-battery systems | Large installations |
| UART | Basic data | DIY projects |
Lead-Acid Without BMS: Lead-acid batteries don`t require BMS but benefit from:
- Low voltage disconnect (LVD)
- Temperature-compensated charging
- Equalization charging (flooded)
- Voltage monitoring
Common BMS Issues:
| Issue | Cause | Solution |
|---|---|---|
| Random disconnect | Overcurrent trip | Upsize BMS or reduce load |
| Won`t charge | Low temp cutoff | Warm batteries above 32°F |
| Cell imbalance | Quality issues | Top-balance cells, replace outliers |
| Overheating | Undersized | Improve ventilation, upsize |
Charging Methods and Requirements
Proper charging extends battery life and ensures full capacity:
LiFePO4 Charging Parameters:
| Parameter | Value | Notes |
|---|---|---|
| Bulk voltage | 14.2-14.6V (12V system) | 3.55-3.65V per cell |
| Float voltage | 13.4-13.8V | Or no float |
| Absorption time | 0-30 minutes | LFP charges fast |
| Max charge rate | 0.5-1C | 100A for 100Ah battery |
| Min charge temp | 32°F (0°C) | BMS should block |
| Ideal charge temp | 50-86°F | Best efficiency |
Lead-Acid Charging Parameters:
| Parameter | AGM | Flooded | Gel |
|---|---|---|---|
| Bulk voltage | 14.4-14.8V | 14.4-14.8V | 14.1-14.4V |
| Float voltage | 13.4-13.8V | 13.2-13.5V | 13.5-13.8V |
| Absorption time | 2-4 hours | 2-4 hours | 3-5 hours |
| Max charge rate | 0.2C | 0.1-0.2C | 0.1C |
| Equalization | Not recommended | 15.0-15.5V | Never |
Charging Sources:
| Source | Pros | Cons |
|---|---|---|
| Solar MPPT | Free energy, efficient | Weather dependent |
| Shore power | Fast, reliable | Requires grid/hookup |
| Generator | On-demand | Noise, fuel, maintenance |
| Alternator | Mobile charging | Limited output, may need DC-DC |
| Wind | 24/7 potential | Location dependent |
Multi-Source Charging:
| Configuration | Notes |
|---|---|
| Solar + Shore | Most common for RV/off-grid |
| Solar + Generator | Off-grid backup |
| All three | Maximum flexibility |
Charger Sizing:
Minimum: Battery Capacity × 0.1 (10% charge rate)
Optimal: Battery Capacity × 0.2-0.3 (20-30% rate)
Example: 400Ah bank → 40-120A charger
Installation and Safety
Proper installation ensures safety and optimal performance:
Battery Location Requirements:
| Factor | Lithium | Lead-Acid |
|---|---|---|
| Ventilation | Minimal needed | Required (hydrogen gas) |
| Temperature | 32-95°F ideal | 50-80°F ideal |
| Humidity | Low to moderate | Low |
| Access | For monitoring | For maintenance |
| Fireproof enclosure | Recommended | Optional |
Wiring Requirements:
| System Voltage | Max Current | Wire Size (10ft) |
|---|---|---|
| 12V | 200A | 2/0 AWG |
| 12V | 300A | 4/0 AWG |
| 24V | 200A | 2 AWG |
| 48V | 100A | 6 AWG |
| 48V | 200A | 2 AWG |
Essential Safety Equipment:
| Equipment | Purpose | Cost |
|---|---|---|
| Battery fuse | Overcurrent protection | $15-50 |
| Battery disconnect | Emergency shutoff | $20-80 |
| Shunt monitor | Capacity tracking | $100-200 |
| Temp sensor | BMS integration | $10-30 |
| Cable lugs | Proper termination | $2-5 each |
| Heat shrink | Insulation | $10-20 |
Fusing Requirements:
Fuse Rating = Maximum Continuous Current × 1.25
Wire must be rated > Fuse rating
Example: 100A continuous load
Fuse: 125A
Wire: Rated for 150A+ at length
Common Installation Mistakes:
| Mistake | Consequence | Prevention |
|---|---|---|
| Undersized wiring | Fire risk, voltage drop | Calculate properly |
| No fusing | Fire risk | Always fuse battery positive |
| Poor connections | Overheating, failure | Torque to spec, use anti-corrosion |
| Mixed batteries | Accelerated degradation | Match all batteries |
| Poor ventilation (LA) | Hydrogen accumulation | Ensure airflow |
Monitoring and Maintenance
Proper monitoring extends battery life and predicts issues:
Essential Monitoring:
| Parameter | Target Range | Concern Level |
|---|---|---|
| State of Charge | 20-90% daily | <10% or >95% prolonged |
| Voltage per cell | 2.8-3.5V (LFP) | <2.5V or >3.65V |
| Cell balance | <50mV difference | >100mV difference |
| Temperature | 50-85°F | <32°F or >113°F |
| Current | Within BMS rating | Frequent limiting |
Monitoring Equipment:
| Type | Features | Price |
|---|---|---|
| Basic shunt monitor | SOC, voltage, current | $80-150 |
| Smart shunt (Bluetooth) | Phone app, history | $150-250 |
| Full BMS display | Cell-level data | Included with BMS |
| Victron/similar | Professional monitoring | $200-400 |
LiFePO4 Maintenance (Minimal):
| Interval | Task |
|---|---|
| Monthly | Check connections |
| Quarterly | Verify cell balance |
| Annually | Full capacity test |
| As needed | Update BMS firmware |
Lead-Acid Maintenance:
| Type | Interval | Task |
|---|---|---|
| Flooded | Monthly | Check water levels |
| Flooded | Monthly | Clean terminals |
| Flooded | Quarterly | Equalization charge |
| AGM/Gel | Quarterly | Check terminals |
| All | Annually | Capacity test |
Signs of Battery Degradation:
| Symptom | Possible Cause |
|---|---|
| Reduced runtime | Capacity loss, cell failure |
| Slow charging | Internal resistance increase |
| Quick voltage drop under load | Weak cells |
| Cell imbalance growing | Cells diverging |
| Excessive heating | Internal resistance, overcurrent |
Pro Tips
- 💡Start with critical loads only—refrigerator, lights, and phone charging typically need just 2-3 kWh/day.
- 💡Higher system voltage (48V) means lower current, smaller wires, less power loss, and more inverter options.
- 💡Keep batteries at room temperature (50-80°F)—extreme heat or cold significantly reduces capacity and lifespan.
- 💡For LiFePO4, avoid storing at 100% SOC for extended periods—80% is better for longevity.
- 💡If using lead-acid, check water levels monthly and perform equalization charges quarterly (flooded only).
- 💡Always fuse the battery positive cable as close to the battery as possible—this is critical for fire safety.
- 💡Size wiring for the fuse rating, not the expected load—wires must survive a fault condition.
- 💡When parallel connecting batteries, use identical batteries and equal-length cables to each battery.
- 💡Consider lithium self-heating batteries for cold-climate installations where heating enclosure isn`t practical.
- 💡Monitor cell balance monthly—diverging cells indicate problems that worsen over time.
- 💡For solar systems, size battery bank to absorb full solar production on a good day.
- 💡Keep 10-20% reserve capacity—never design to use 100% of calculated capacity.
Frequently Asked Questions
Most off-grid homes need 10-30 kWh of battery storage depending on daily usage and autonomy requirements. For a typical home using 10 kWh/day with 2 days autonomy and LiFePO4 batteries (80% DOD), you need approximately 25 kWh of battery capacity. Using 5.12 kWh 48V batteries, this is 5 batteries.
