TL;DR — Battery Sizing for Off-Grid Solar
Battery bank size is calculated from three numbers: daily load, days of autonomy, and usable depth of discharge. The formula is simple: (Daily load × Days of autonomy) ÷ Usable DoD = Battery bank capacity. LiFePO4 has 80% usable DoD. AGM has 50%. Flooded lead-acid has 50%. A battery cycled regularly below its rated DoD loses capacity fast and fails early. Size correctly the first time. Battery replacement is the single most expensive repair in an off-grid system.
A homesteader in rural Tennessee used an online battery sizing chart. The chart asked for his daily usage and gave him a bank size. It did not ask about battery chemistry or depth of discharge. He bought AGM batteries. The chart had sized for 100% usable capacity. AGM's usable capacity is 50%. He ran his bank below 50% every night for three months before the batteries started failing. He had used a tool designed for lithium batteries with lead-acid chemistry. The misapplication cost him $4,200 in year one.
Table of Contents
- The three inputs every battery sizing calculation requires
- The formula — and why simplified charts get it wrong
- Depth of discharge by battery chemistry
- Days of autonomy — what number to use
- Temperature derating — the cold-weather penalty
- The real-world sizing example
- When to choose LiFePO4 vs lead-acid
- FAQ
The three inputs every battery sizing calculation requires
Input 1: Daily watt-hour load. How much energy your home consumes per day in the season you are sizing for. Use winter load for northern locations. Use the load calculation methodology from the off-grid power requirement guide.
Input 2: Days of autonomy. How many days your system must operate with zero solar input. Two days minimum for a primary residence. Three days for locations with regular multi-day overcast.
Input 3: Usable depth of discharge. The percentage of total battery capacity that can be used before recharging. This varies dramatically by chemistry. Every sizing calculation that omits this input is producing a wrong answer.
"Lithium iron phosphate (LiFePO4) battery cells demonstrated 80% or greater capacity retention after 4,000 full charge-discharge cycles at 80% depth of discharge under controlled laboratory conditions — compared to 60% capacity retention for valve-regulated lead-acid batteries after 600 cycles at the same discharge depth."
— National Renewable Energy Laboratory, Energy Storage Technology and Cost Characterization Report, 2019
The difference is not minor. A lead-acid bank cycled to 80% DoD daily will show significant capacity loss in 600 cycles — roughly eighteen months of daily cycling. A LiFePO4 bank cycled to 80% DoD daily retains 80% capacity at 4,000 cycles — roughly eleven years of daily cycling. This is why battery chemistry is the most consequential decision in an off-grid system — and why the sizing formula must account for chemistry from the start.
The formula — and why simplified charts get it wrong
Correct formula:
Battery bank capacity (Wh) = (Daily load × Days of autonomy) ÷ Usable DoD
Example: LiFePO4, 5,000 Wh/day, 2 days autonomy: (5,000 × 2) ÷ 0.80 = 12,500 Wh → size to 15kWh
Example: AGM, 5,000 Wh/day, 2 days autonomy: (5,000 × 2) ÷ 0.50 = 20,000 Wh → size to 20kWh
The same load requires 60% more battery capacity in AGM than in LiFePO4. Simplified charts that skip the DoD factor usually assume 100% usable DoD — a number that applies to no real battery chemistry. Their output is always optimistic. Usually by 25–100%.
For AGM the simplified chart says 10,000 Wh (load × days). The correct answer is 20,000 Wh (load × days ÷ 0.50). That is a 100% undersizing error on a 20kWh bank.
Depth of discharge by battery chemistry
| Battery Chemistry | Usable DoD | Cycle Life at Rated DoD | Consequence of Exceeding DoD |
|---|---|---|---|
| LiFePO4 | 80% | 4,000–6,000 cycles | Minimal damage below 20% SoC threshold |
| AGM lead-acid | 50% | 400–600 cycles | Accelerated sulfation, rapid capacity loss |
| Flooded lead-acid | 50% | 1,200–1,500 cycles | Sulfation, active material shedding |
| Lithium NMC | 80–90% | 500–1,000 cycles | Thermal risk at deep discharge depths |
For off-grid applications, LiFePO4 and flooded lead-acid are the practical choices. LiFePO4 is the correct choice if budget allows — longer life, higher usable DoD, no maintenance, better temperature range. Flooded lead-acid is the least expensive chemistry that still provides adequate cycle life, but requires monthly water level checks and good ventilation.
AGM is frequently sold to off-grid buyers as a maintenance-free upgrade over flooded. At off-grid cycling depths, its cycle life is dramatically shorter. AGM belongs in backup and seasonal applications — not in primary off-grid systems that cycle daily.
Days of autonomy — what number to use
Autonomy is the number of consecutive days your system must operate with zero solar input. This covers extended overcast periods, storms that deposit debris on panels, and seasonal conditions where daily production falls below daily consumption.
Minimum by location:
| Climate / Region | Recommended Days of Autonomy |
|---|---|
| Desert Southwest (AZ, NM, NV) | 1.5–2 days |
| Southeast and Gulf Coast | 2 days (hurricane and storm periods) |
| Midwest plains | 2–3 days (winter overcast) |
| Pacific Northwest | 3–4 days (extended overcast seasons) |
| Upper Midwest / New England | 3 days (winter) |
| Northern Rockies and Great Plains | 2–3 days |
These are minimums for year-round primary residence. For a system you depend on as your sole power source, size conservatively. The cost difference between two and three days of autonomy is one additional battery module. The cost of running out of power on day three of a Montana blizzard is considerably higher.
Temperature derating — the cold-weather penalty
Cold temperatures reduce battery capacity. All battery chemistries are affected, though to different degrees.
| Battery Chemistry | Capacity at 32°F (0°C) | Capacity at 14°F (-10°C) |
|---|---|---|
| LiFePO4 | ~90% of rated | ~80% of rated |
| AGM | ~75% of rated | ~60% of rated |
| Flooded lead-acid | ~70% of rated | ~55% of rated |
For off-grid systems in climates that drop below freezing, add a temperature derating factor to your battery bank sizing. If winter temperatures regularly hit 14°F in your battery location:
LiFePO4 derating: divide by 0.80 = 25% more battery capacity required. AGM derating: divide by 0.60 = 66% more battery capacity required.
This is a major practical advantage of LiFePO4 in cold climates. AGM in an unheated battery shed in Montana loses 40% of its capacity at -10°F. A 20kWh AGM bank becomes effectively 12kWh — not enough for the two days of autonomy you designed for.
LiFePO4 should not be charged at temperatures below 32°F (0°C) without a battery management system (BMS) that prevents charging at sub-freezing temperatures. Most quality LiFePO4 batteries include this protection. Verify before purchasing.
The real-world sizing example
Location: Rural Tennessee
Season for sizing: Winter
Daily load (winter): 6,200 Wh
Battery chemistry: LiFePO4
Days of autonomy: 2
Winter temperature: Minimum 20°F in battery location (mild derating)
Step 1: Base battery bank size (6,200 × 2) ÷ 0.80 = 15,500 Wh → 16kWh
Step 2: Temperature derating at 20°F LiFePO4 at 20°F retains approximately 85% of capacity. 16,000 ÷ 0.85 = 18,823 Wh → round to 20kWh
Step 3: Growth margin (20%) 20,000 × 1.20 = 24,000 Wh → size to 24kWh
Result: A 24kWh LiFePO4 battery bank at 48V system voltage.
At 48V, 24kWh = 500Ah. Five 100Ah LiFePO4 batteries wired in series-parallel configuration. The Battle Born 100Ah LiFePO4 at $949 per battery = ~$4,745 for the battery bank — before wiring, BMS integration, and mounting. Check current pricing on Amazon, as battery prices have been declining.
This bank, properly sized and maintained, will deliver ten to fifteen years of daily service before showing meaningful capacity loss.
When to choose LiFePO4 vs lead-acid
Choose LiFePO4 when:
- Budget allows the upfront investment
- You are located in a climate with cold winters
- You expect to cycle the bank daily as a primary residence
- You want the lowest ten-year cost per kilowatt-hour
- You want zero maintenance
Choose flooded lead-acid when:
- Budget is the primary constraint
- You have ventile battery storage space
- You are building a seasonal or backup system, not a primary residence
- You are comfortable with monthly maintenance checks
- You plan to upgrade to LiFePO4 in three to five years
Never choose AGM for primary off-grid: AGM's short cycle life at off-grid discharge depths makes it the most expensive choice over a ten-year period despite the lower upfront cost. AGM belongs in RVs, boats, and short-term backup applications — not in a system you will cycle daily for a decade.
🦍 WATTSON ON BATTERY CHEMISTRY: "I ran flooded lead-acid for my first two years off-grid. Every month I was checking water levels, equalizing, and watching state of charge like a hawk. When I upgraded to LiFePO4 the monitoring requirements dropped by 80%. The batteries do not care if I leave them at 60% state of charge for a week. They do not sulfate. They do not need equalization. The ten-year math is not even close. If the budget is there, buy lithium. If it is not — plan the upgrade from day one."
Get Your Exact Battery Bank Size
The Solar Power Estimator calculates your battery bank size from your load, location, and autonomy target — including chemistry-specific depth of discharge factors. Free. Takes ten minutes.
Frequently Asked Questions
How do I calculate the battery bank size I need for off-grid solar?
Use this formula: (Daily watt-hour load × Days of autonomy) ÷ Usable depth of discharge = Battery bank capacity in watt-hours. For LiFePO4, usable DoD is 80%. For AGM or flooded lead-acid, it is 50%. Add 20% growth margin. Apply a temperature derating factor if your battery storage reaches below freezing in winter.What is depth of discharge and why does it matter?
Depth of discharge (DoD) is the percentage of a battery's rated capacity that can be used without damaging the cells. A 100Ah LiFePO4 battery with 80% DoD provides 80Ah of usable capacity. Regularly discharging any battery below its rated DoD accelerates capacity loss and dramatically shortens cycle life. Most online battery sizing charts skip this factor, resulting in undersized banks.How many batteries do I need to go off-grid?
It depends on your daily load, system voltage, and battery capacity. Divide your battery bank size in watt-hours by your battery's watt-hour rating (voltage × amp-hours). A 20kWh bank at 48V requires 417Ah — approximately five 100Ah batteries wired in series-parallel. Use the sizing formula above and the Solar Power Estimator to generate your specific number.Is LiFePO4 worth the extra cost for off-grid?
Yes, when sized correctly and used in a daily-cycling primary residence application. LiFePO4 delivers 4,000–6,000 cycles at 80% DoD. AGM delivers 400–600 cycles at 50% DoD. Over ten years of daily use, you replace AGM four to six times — at equivalent total bank cost — for the same number of cycles LiFePO4 provides in one purchase. The upfront premium pays back within three to five years in most primary off-grid applications.Can I mix different battery types in my off-grid bank?
No. Never mix battery chemistries or batteries of different ages, manufacturers, or states of charge in the same bank. Mismatched batteries charge and discharge unevenly, causing individual batteries to work harder. The weakest battery degrades fastest and begins limiting the entire bank's usable capacity. If you need to add capacity, add identical batteries from the same manufacturer and production run.What battery voltage should I use — 12V, 24V, or 48V?
48V for any residential off-grid system. At 48V, your system runs at lower current for the same power level, allowing smaller, cheaper wire and reducing line losses significantly. A 3,000W load at 12V draws 250A. The same load at 48V draws 62.5A. The wire sizing difference is enormous. 12V is for RVs and small cabins. 24V is acceptable for modest loads under 3kW. 48V is the standard for any serious off-grid installation.How long do off-grid batteries last?
LiFePO4: 4,000–6,000 full charge-discharge cycles, typically 10–15 years of daily use. AGM: 400–600 cycles at 50% DoD, typically 2–4 years of daily use. Flooded lead-acid: 1,200–1,500 cycles at 50% DoD, typically 4–7 years of daily use. These are approximations — actual lifespan depends on cycling depth, temperature, charging practices, and maintenance.Do batteries lose capacity in cold weather?
Yes. All battery chemistries lose usable capacity in cold temperatures. LiFePO4 retains approximately 80% of rated capacity at 14°F. AGM retains approximately 60%. For batteries stored in unheated spaces in cold climates, apply a temperature derating to your bank sizing calculation. LiFePO4 must not be charged at temperatures below 32°F without a BMS that prevents sub-freezing charging.What is a battery management system (BMS) and do I need one?
A BMS monitors individual cell voltages, temperatures, and current in a lithium battery bank. It prevents overcharging, over-discharging, and charging at unsafe temperatures. Most quality LiFePO4 batteries include an integrated BMS. For DIY battery banks assembled from individual cells, an external BMS is required. For assembled batteries from established manufacturers, verify that a BMS is included before purchasing.How do I wire batteries in series vs parallel for off-grid?
Wiring in series increases voltage while keeping amp-hours the same. Four 12V 100Ah batteries in series = 48V 100Ah. Wiring in parallel increases amp-hours while keeping voltage the same. Four 12V 100Ah batteries in parallel = 12V 400Ah. For a 48V system, wire groups of four 12V batteries in series, then wire those groups in parallel to reach your target amp-hour capacity. Use identical wire lengths and gauges for all parallel connections to ensure balanced charging and discharging.The formula is three variables. The mistake costs four years.
Battery bank undersizing is the most common off-grid system failure. The formula is three terms. The math takes five minutes. The consequence of skipping the depth of discharge factor is a bank that fails years before it should — and a system that cannot deliver what you built it to provide.
Size it from the formula. Verify with the Solar Estimator. Buy the right chemistry for your climate and usage. Then move on to the rest of the system.
The Tennessee homesteader paid $4,200 in year one because a chart skipped the depth of discharge factor. The formula is three inputs: load, autonomy, DoD. That is the entire calculation. Run the Solar Power Estimator — it applies all three correctly for your specific chemistry, location, and seasonal load. Bring that output to every battery vendor conversation. Then verify the chemistry matches the numbers before you sign.