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Off-Grid Battery Bank Calculator
Building an off-grid solar system and have no idea how many batteries you actually need? Let's do the math so you don't end up in the dark with a dead battery bank three months in.
Daily Power Usage
Battery Requirements
Sizing Your Battery Bank Right
Most off-grid newbies make one of two mistakes: 1) Buy way too many batteries (waste $5k+), or 2) Buy too few and kill them in 18 months. The formula is simple, but the real world adds "fudge factors" nobody tells you about.
💡 Real Talk from Tom Baker, Off-Grid 8 Years
I killed my first battery bank in 14 months. Bought cheapo flooded lead-acid, ran them down to 80% DoD regularly thinking "I'll recharge tomorrow." Sulfation destroyed them. Round 2: proper AGM batteries, strict 50% DoD limit, temperature-compensated charging. Those lasted 7 years. Now I'm on lithium—expensive up front ($6k for 48V 400Ah bank) but I'll never replace them. Worth it.
The Battery Bank Formula
(Daily Wh × Days of Autonomy) ÷ (DoD × Efficiency)
- Daily Wh: Total watt-hours you use per day
- Days of Autonomy: How many cloudy days you can survive
- DoD (Depth of Discharge): Lead-acid 50%, Lithium 80-100%
- Efficiency: ~0.85 (losses in wiring, inverter, charging)
Example: Off-grid cabin, 24V system
- Daily use: 3,500 Wh
- Autonomy: 3 days (PNW cloudy winters)
- Battery type: AGM lead-acid (50% DoD)
Capacity needed: (3,500 × 3) ÷ (0.50 × 0.85) = 24,706 Wh
At 24V: 24,706 Wh ÷ 24V = 1,029 Ah
Using 12V 200Ah batteries in series/parallel: need 12 batteries (6 in series for 24V, 2 parallel strings for 1,200 Ah)
⚠️ Common Mistakes to Avoid
- Forgetting inverter efficiency loss: Inverters aren't 100% efficient. They waste 10-15% of power converting DC to AC. Always multiply your AC loads by 1.15. If you skip this, your "3 days of autonomy" becomes 2.5 days.
- Mixing old and new batteries: Never add a new battery to an old bank. The old batteries will drag the new one down to their level, killing it in months. Replace the entire bank at once.
- Under-sizing cables: Low voltage (12V/24V) means HIGH current. Using thin wires causes voltage drop and fire risk. A 2000W inverter at 12V pulls 166 Amps—you need 2/0 welding cable, not jumper cables.
- Ignoring temperature: Lead-acid batteries lose 50% capacity at 32°F. Lithium batteries can't be charged below freezing (permanent damage). Insulate your battery box!
Lead-Acid vs Lithium: The Real Comparison
Lead-Acid (Flooded/AGM/Gel):
- Cost: $150-250 per 100Ah (12V)
- Depth of Discharge: 50% max (go deeper = early death)
- Lifespan: 500-1,000 cycles at 50% DoD (3-7 years)
- Weight: HEAVY (65 lbs for 100Ah battery)
- Maintenance: Flooded needs water refills. AGM/Gel sealed.
- Temperature sensitive (cold kills performance)
Lithium (LiFePO4):
- Cost: $400-600 per 100Ah (12V)
- Depth of Discharge: 80-100% safe
- Lifespan: 3,000-5,000 cycles (10-15 years)
- Weight: 50% lighter than lead-acid
- Maintenance: Zero. Set it and forget it.
- Works fine in cold (just can't charge under 32°F without heater)
Cost comparison (10-year ownership):
- Lead-acid 400Ah bank: $1,200 initial + replacements every 5 years = $2,400 total
- Lithium 400Ah bank: $4,800 initial, lasts 10+ years = $4,800 total
Lithium costs 2x but lasts 2x+ longer AND gives you twice the usable capacity (80% DoD vs 50%). Math says lithium wins for permanent off-grid setups.
Days of Autonomy: How Many?
This isn't about apocalypse prepping. It's about: How many consecutive cloud y days before I'm willing to run my generator?
- 1-2 days: Sunny climates (AZ, NM). Risky—one cloudy week and you're screwed.
- 3 days: Sweet spot for most climates. Covers typical weather patterns.
- 4-5 days: PNW, Alaska, mountain areas with week-long cloud cover.
- 7+ days: Overkill unless you're off-grid in the Arctic or hate generators with a passion.
Every extra day of autonomy = 33% bigger battery bank. 3 days → 4 days = add $1,500-2,000 in batteries. Balance cost vs. generator runtime.
System Voltage: 12V, 24V, or 48V?
12V: RVs, boats, small systems (<1,500 Wh/day). Wire size gets ridiculous above 100A draw.
24V: Cabins, tiny homes (1,500-5,000 Wh/day). Most common for off-grid. Good balance.
48V: Full homes (5,000+ Wh/day). Thinner wires, less loss. Professional-grade.
Higher voltage = lower amps = thinner/cheaper wire. A 2,400W inverter at 12V pulls 200A (needs 4/0 cable, $$$). At 48V? Only 50A (needs 6 AWG, way cheaper).
💡 Tom's Battery Lifespan Hack: Temperature Control
Heat kills batteries. Every 15°F above 77°F cuts lifespan in half. My first battery bank was in an un-insulated shed—summer temps hit 100°F. They died in 3 years instead of 7. Now I keep batteries in an insulated, vented box. Temp stays 60-75°F year-round. Batteries are happy, I'm happy. Simple as that.
Real-World Sizing Examples
Weekend Cabin (Minimal Use):
- Lights (LED): 200 Wh/day
- Water pump: 300 Wh/day
- Phone charging: 50 Wh/day
- Total: 550 Wh/day
- 12V system, 2 days autonomy, lead-acid (50% DoD)
- Needed: (550 × 2) ÷ (0.50 × 0.85) = 2,588 Wh ≈ 215 Ah at 12V
- Solution: Two 12V 120Ah AGM batteries
- Cost: ~$600
Full-Time Off-Grid Home:
- Fridge: 1,200 Wh/day
- Lights: 800 Wh/day
- Well pump: 600 Wh/day
- Computers/TV: 1,000 Wh/day
- Misc: 400 Wh/day
- Total: 4,000 Wh/day
- 48V system, 3 days, lithium (80% DoD)
- Needed: (4,000 × 3) ÷ (0.80 × 0.85) = 17,647 Wh ≈ 367 Ah at 48V
- Solution: Four 12V 100Ah LiFePO4 in series = 48V 100Ah, then 4 parallel strings = 48V 400Ah
- Cost: ~$5,000-7,000
Reviewed by Tom Baker
Off-Grid Homesteader (8 Years, Montana)
Tom powers a 1,200 sq ft cabin with 3kW solar and 48V 600Ah lithium bank. Zero grid connection since 2017. His advice? Size batteries for 3 cloudy days, then add 20% buffer. You'll thank yourself.
Frequently Asked Questions
How big should my battery bank be?
Rule of thumb: (Daily Wh usage × Days of autonomy) ÷ (Depth of Discharge × 0.85 efficiency). Example: 3,000 Wh/day, 3 days autonomy, 50% DoD → 3,000 × 3 ÷ (0.50 × 0.85) = 21,176 Wh or 1,764 Ah at 12V. Most cabins need 800-2,000 Ah. Don't cheap out—undersized batteries die fast.
What's depth of discharge (DoD)?
How much of the battery's capacity you use before recharging. Lead-acid: max 50% DoD (use only half) or they die in 2 years. Lithium (LiFePO4): 80% DoD safe, some go 100%. Why it matters: 400Ah lead-acid at 50% DoD gives you 200Ah usable. 400Ah lithium at 80% DoD gives 320Ah. Lithium costs 2-3x more but lasts 10x longer.
How many days of autonomy do I need?
Depends on cloud cover and your paranoia level. Sunny climates (AZ, CA): 2-3 days. Pacific Northwest/Alaska: 4-5 days minimum (weeks of cloudy winters). Most people do 3 days as a sweet spot. More autonomy = bigger battery bank = more money. Balance cost vs. how often you'll run a generator.
Can I mix old and new batteries?
NO. Never mix batteries of different ages, brands, or capacities. The weak/old battery drags down the whole bank and dies faster. Always replace the entire bank at once. I've seen people add one new battery to 5 old ones—it died in 6 months. Batteries are like chain links: only as strong as the weakest one.