Off-Grid Battery Bank Calculator: Sizing Your System Right
April 6, 2026
Quick Answer
Off-grid battery bank sizing requires covering your full daily energy consumption for multiple days without any solar input. For a typical off-grid home using 20-30 kWh per day, you need a 40-90 kWh battery bank with 2-3 days of autonomy. Lithium iron phosphate (LFP) batteries are the standard choice for new installations, offering 10-15 year lifespans, 90%+ depth of discharge, and virtually no maintenance compared to lead-acid alternatives.
Key Takeaways
- Off-grid battery banks are 3-5x larger than grid-tied systems because they must cover full daily consumption for multiple cloudy days.
- Plan for 2-3 days of autonomy in sunny climates, 3-4 days in cloudy regions, sizing for your worst-case consumption month.
- LFP (lithium iron phosphate) batteries are the recommended choice for new off-grid installations due to longer life, deeper discharge, and lower maintenance.
- A backup generator is more cost-effective than adding battery capacity for rare extended cloudy periods beyond your autonomy target.
- Seasonal solar variation can reduce winter production by 40-60% compared to summer — your battery must bridge this gap.
- The whole-home battery sizing calculator covers grid-tied sizing; this guide focuses specifically on off-grid requirements.
Off-Grid vs Grid-Tied Battery Sizing: Key Differences
Understanding why off-grid battery sizing is fundamentally different from grid-tied helps set realistic expectations:
| Factor | Grid-Tied | Off-Grid |
|---|---|---|
| Battery purpose | TOU savings + backup | Total energy independence |
| Daily cycling | Partial (peak periods) | Full (entire daily consumption) |
| Days of autonomy | 0-2 days | 2-4 days |
| Worst-case planning | Utility outage | Seasonal low-solar periods |
| Battery utilization | 20-40% of daily consumption | 100% of daily consumption |
| Solar dependence | Supplemental (grid backup) | Total (sole power source) |
| Typical bank size | 10-27 kWh | 30-100+ kWh |
The scale difference is dramatic. A grid-tied home might need a single 13.5 kWh battery for TOU savings and backup. An off-grid home with the same consumption needs 3-6 batteries to survive cloudy stretches.
The Complete Off-Grid Battery Sizing Calculation
Step 1: Calculate Your Daily Energy Consumption
Be thorough and honest. Off-grid systems have no grid to fall back on when you underestimate.
Method A: Utility Bill Analysis (if currently grid-connected) Pull 12 months of bills and find your highest month. Divide by 30 for your worst-case daily consumption.
Method B: Load-by-Load Calculation
| Load | Watts | Hours/Day | Daily kWh |
|---|---|---|---|
| Refrigerator (Energy Star) | 150W | 8 (cycling) | 1.2 |
| Chest freezer | 100W | 6 | 0.6 |
| LED lighting (whole home) | 100W | 6 | 0.6 |
| Well pump (1 HP) | 1,000W | 1.5 | 1.5 |
| Water heater (efficient) | 1,500W | 3 | 4.5 |
| Washing machine | 500W | 0.5 | 0.25 |
| Laptop + monitor | 150W | 8 | 1.2 |
| WiFi + networking | 30W | 24 | 0.72 |
| Phone chargers (×4) | 40W | 3 | 0.12 |
| Microwave | 1,200W | 0.25 | 0.3 |
| Coffee maker | 900W | 0.25 | 0.23 |
| Toaster | 850W | 0.15 | 0.13 |
| Ceiling fans (×3) | 75W | 12 | 0.9 |
| TV + streaming | 120W | 4 | 0.48 |
| Power tools (occasional) | 1,500W | 0.5 | 0.75 |
| Total | 13.48 kWh |
This example home uses approximately 13.5 kWh/day without HVAC. Adding a mini-split heat pump adds 5-15 kWh/day depending on climate.
Step 2: Add System Losses
Your battery bank must account for inefficiencies throughout the system:
| Loss Factor | Efficiency | Impact |
|---|---|---|
| Battery round-trip | 90-95% | 5-10% loss |
| Inverter DC to AC | 93-97% | 3-7% loss |
| Wiring and connections | 97-99% | 1-3% loss |
| Charge controller | 95-98% | 2-5% loss |
| Combined system efficiency | 80-88% | 12-20% loss |
Adjusted Daily Consumption = Raw Daily kWh ÷ System Efficiency
Example: 13.5 kWh ÷ 0.85 = 15.9 kWh/day after losses
Step 3: Determine Days of Autonomy
Days of autonomy (DoA) is how many days your battery bank can power your home with zero solar input. Choose based on your climate:
| Climate Type | Examples | Recommended DoA |
|---|---|---|
| Desert/Southwest | Phoenix, Las Vegas, Albuquerque | 2 days |
| Moderate/Mixed | Denver, Nashville, Atlanta | 2.5-3 days |
| Cloudy/Pacific NW | Seattle, Portland, coastal areas | 3-4 days |
| Extreme winter | Montana, Minnesota, Alaska | 3-5 days |
Step 4: Calculate Battery Bank Capacity
Battery Bank (kWh) = Adjusted Daily kWh × Days of Autonomy ÷ Depth of Discharge
For our example home in Colorado (moderate climate):
| Parameter | Value |
|---|---|
| Adjusted daily consumption | 15.9 kWh |
| Days of autonomy | 3 days |
| Depth of discharge (LFP) | 90% |
| Required battery bank | 15.9 × 3 ÷ 0.90 = 53 kWh |
This home needs approximately 53 kWh of rated battery capacity — roughly four Tesla Powerwalls or a custom LFP bank.
Step 5: Verify with Solar Production
Your solar array must be large enough to recharge the battery bank even during low-production months. Calculate your worst-case solar month:
Required Solar Array (kW) = Daily Consumption ÷ Peak Sun Hours (worst month) ÷ System Efficiency
| Location | June PSH | December PSH | Array Needed for 15.9 kWh/day |
|---|---|---|---|
| Phoenix, AZ | 7.5 | 5.5 | 3.8 kW (Dec) |
| Denver, CO | 6.5 | 4.5 | 4.7 kW (Dec) |
| Portland, OR | 6.0 | 1.8 | 11.7 kW (Dec) |
| Seattle, WA | 5.5 | 1.5 | 14.0 kW (Dec) |
In cloudy winter climates, you may need a much larger solar array than summer demands, which means significant overproduction in summer. This excess can be used for water heating or other flexible loads.
Lead-Acid vs Lithium: The Off-Grid Battery Decision
Lithium Iron Phosphate (LFP)
Pros:
- 10-15 year lifespan (4,000-6,000 cycles at 80% DoD)
- 90-100% depth of discharge
- 95% round-trip efficiency
- Zero maintenance
- Consistent voltage output
- Built-in BMS (Battery Management System)
- Lighter weight (50-70% less than lead-acid)
Cons:
- Higher upfront cost ($400-$700/kWh)
- Performance degrades in sub-freezing temperatures (needs heaters)
- More complex electronics (BMS required)
Lead-Acid (Flooded, AGM, Gel)
Pros:
- Lower upfront cost ($150-$300/kWh)
- Proven technology with decades of off-grid track record
- Performs well in cold temperatures
- Recyclable (99% of lead is recycled)
- Simple electronics — no BMS needed
Cons:
- 3-7 year lifespan (500-1,500 cycles at 50% DoD)
- Only 50% depth of discharge
- 80-85% round-trip efficiency
- Requires regular maintenance (flooded cells need watering)
- Heavier and bulkier
- Voltage sag under heavy loads
- Ventilation required (flooded cells produce hydrogen gas)
Lifetime Cost Comparison
For a 50 kWh bank over 15 years:
| Factor | LFP | Lead-Acid |
|---|---|---|
| Initial cost | $25,000-$35,000 | $10,000-$15,000 |
| Replacements needed | 0 (maybe 1 at year 12) | 2-3 |
| Replacement cost | $0-$35,000 | $20,000-$45,000 |
| Maintenance (time + materials) | $0 | $1,000-$3,000 |
| Efficiency losses (extra solar needed) | 5% | 15-20% |
| 15-year total cost | $25,000-$70,000 | $31,000-$63,000 |
The lifetime costs are similar, but LFP offers better predictability, no maintenance, and consistent performance. For new installations, LFP is the clear recommendation.
Generator Backup for Off-Grid Systems
No matter how well you size your battery bank, there will be periods when solar production cannot keep up. A backup generator is essential insurance.
When a Generator Saves You Money
Adding battery capacity to cover a rare 5-day cloudy stretch is expensive. A generator handles these edge cases at a fraction of the cost:
| Approach | Cost | Covers |
|---|---|---|
| Add 30 kWh more battery | $15,000-$21,000 | 2 extra days of autonomy |
| 5 kW portable generator | $800-$2,000 | Unlimited (with fuel) |
| 10 kW standby generator | $4,000-$8,000 | Unlimited + auto-start |
Generator Sizing for Battery Charging
Your generator should be large enough to charge your batteries while simultaneously powering your home:
Generator Size (kW) = Home Load (kW) + Battery Charging Rate (kW)
For a home with 3 kW average load and a 50 kWh battery bank:
- Target recharge time: 6-8 hours
- Charging rate: 50 kWh ÷ 7 hours = 7.1 kW
- Generator size: 3 kW + 7.1 kW = 10.1 kW minimum
A 10-12 kW generator covers this scenario with a comfortable margin.
Generator Best Practices
- Use propane or diesel, not gasoline, for long-term fuel storage.
- Install an auto-start transfer switch that triggers the generator when batteries reach 20% state of charge.
- Run the generator during peak daytime loads so it powers the home while solar charges the batteries.
- Maintain the generator with monthly exercise runs and annual oil changes.
- Size the generator to handle battery charging efficiently — generators are most fuel-efficient at 50-75% load.
Off-Grid Battery Configuration: Series vs Parallel
Voltage Considerations
Off-grid battery banks operate at higher voltages than grid-tied systems to reduce current and wire sizing:
| System Size | Battery Voltage | Inverter Compatibility |
|---|---|---|
| Small (<5 kWh/day) | 12V or 24V | Small off-grid inverters |
| Medium (5-20 kWh/day) | 24V or 48V | Mid-range inverters |
| Large (20+ kWh/day) | 48V | Full-size off-grid inverters |
Most modern off-grid homes use 48V systems. Higher voltages mean lower current for the same power, which means smaller wires, less heat, and lower cost.
LFP Battery Bank Configuration
LFP cells are typically 3.2V nominal. Common configurations:
- 48V bank: 16 cells in series (51.2V nominal)
- 48V bank with capacity: 16S × multiple parallel strings
- Pre-built 48V modules: Most manufacturers sell 48V drop-in modules (100Ah, 200Ah, 300Ah)
For simplicity and reliability, use pre-built 48V LFP modules rather than assembling cells yourself.
Seasonal Adjustments and Load Management
Summer vs Winter Consumption
Off-grid homes often have significantly different consumption profiles between seasons:
| Load | Summer | Winter |
|---|---|---|
| Lighting | 2-3 kWh/day | 5-8 kWh/day |
| Refrigeration | 2-3 kWh/day | 1-1.5 kWh/day |
| Heating | 0 | 10-20 kWh/day (electric) |
| Cooling | 5-15 kWh/day | 0 |
| Water pumping | 1-2 kWh/day | 1 kWh/day |
Critical sizing tip: Size your battery for your worst-case month — the month with the highest consumption AND lowest solar production. This is often December or January in northern latitudes.
Load Management Strategies
Off-grid living requires more intentional energy use than grid-tied homes:
- Run heavy loads during solar hours. Washing machines, water pumping, and power tools should run when the sun is shining and the batteries are charging.
- Use propane for heating and cooking. Electric resistance heating is extremely battery-intensive. Propane furnaces, stoves, and water heaters dramatically reduce electrical consumption.
- Stagger major appliances. Do not run the well pump, washing machine, and microwave simultaneously.
- Use energy-efficient everything. The cost premium for efficient appliances pays for itself in reduced battery and solar requirements.
- Consider a solar water heater. Thermal solar for water heating is more efficient per square foot than PV and reduces your electrical load significantly.
Real-World Off-Grid Sizing Examples
Example 1: Small Off-Grid Cabin (Colorado Mountains)
- Consumption: 8 kWh/day (propane heat/cooking, no AC)
- Solar: 3 kW array
- Battery: 24 kWh LFP bank (2 days autonomy)
- Generator: 3 kW portable propane
- Total system cost: ~$18,000
Example 2: Full-Time Off-Grid Home (New Mexico)
- Consumption: 25 kWh/day (efficient heat pump, well pump)
- Solar: 10 kW array
- Battery: 60 kWh LFP bank (2.5 days autonomy)
- Generator: 8 kW diesel auto-start
- Total system cost: ~$55,000
Example 3: Off-Grid Family Home (Pacific Northwest)
- Consumption: 30 kWh/day (propane backup heat, efficient appliances)
- Solar: 15 kW array (oversized for winter)
- Battery: 90 kWh LFP bank (3 days autonomy)
- Generator: 12 kW propane standby
- Total system cost: ~$85,000
Getting Started with Your Off-Grid System
- Complete a detailed load analysis listing every appliance with wattage and daily hours of use.
- Choose your autonomy target based on climate and willingness to run a generator.
- Calculate battery bank size using the formulas in this guide.
- Size your solar array for worst-month production.
- Budget for a backup generator unless you live in an exceptionally sunny climate.
- Work with an experienced off-grid installer — these systems require specialized knowledge that grid-tied installers may not have.
The solar plus storage payback period guide provides additional context on combined system economics, while the home battery cost per kWh analysis helps you compare battery pricing across chemistries and brands.
The Bottom Line
Off-grid battery bank sizing is a careful balance between consumption, solar production, climate, and budget. The key is honest load analysis and planning for your worst-case scenario — typically the darkest, coldest month of the year. While the upfront investment is significant ($30,000-$80,000 for a complete system), the independence and reliability of a properly designed off-grid power system provides value that goes beyond simple payback calculations.
FAQ
How do I calculate battery bank size for an off-grid home?
Multiply your daily energy consumption (kWh) by your desired days of autonomy (typically 2-3), then divide by the battery’s depth of discharge (90% for lithium, 50% for lead-acid). Add 10-15% for inverter losses and cold temperature derating. Example: 20 kWh/day × 3 days ÷ 0.90 = 67 kWh battery bank.
How many days of autonomy should I plan for off-grid?
Most off-grid systems target 2-3 days of autonomy (storage to cover consumption without any solar input). In cloudy climates like the Pacific Northwest, plan for 3-4 days. In sunny desert climates, 2 days may suffice. More days of autonomy means larger battery bank and higher cost.
Do I need a generator backup for my off-grid battery system?
Most off-grid homes include a backup generator for extended cloudy periods. A generator is cheaper than the extra battery capacity needed to cover rare 4-5 day low-solar events. A properly sized generator (3-5 kW) can recharge your batteries while also powering loads during extended storms.
Should I choose lead-acid or lithium batteries for off-grid?
Lithium (LFP) batteries are the better choice for new off-grid installations. They offer longer lifespan (10-15 years vs 3-7 years), deeper discharge (90-100% vs 50%), higher efficiency (95% vs 80-85%), and no maintenance. Lead-acid only makes sense for extremely tight budgets or very cold climates where lithium heaters add cost.
How does off-grid battery sizing differ from grid-tied sizing?
Off-grid batteries must cover your full daily consumption for multiple days, not just peak periods. Grid-tied batteries typically only need to cover 4-6 hours of peak consumption. Off-grid systems also need to account for seasonal solar variation, cloudy day reserves, and inverter surge capacity for motor starting.
What is the typical cost of an off-grid battery bank?
A typical off-grid battery bank (20-40 kWh usable capacity) costs $12,000-$30,000 for lithium or $5,000-$15,000 for lead-acid. However, lead-acid needs replacement every 3-7 years, making lifetime costs comparable or higher than lithium. Total off-grid systems (solar + battery + inverter + installation) typically run $30,000-$80,000.
How do I account for seasonal solar variation in battery sizing?
Calculate your worst-case solar month (usually December in the Northern Hemisphere) and size the battery to bridge the gap between that reduced production and your consumption. In many locations, December solar production is 40-60% of June production. Your battery must be large enough to cover cloudy stretches during this low-production period.