Off-Grid Solar Battery Sizing: How Many Batteries You Need

Three numbers determine your battery bank size: daily load in watt-hours, autonomy days, and usable depth-of-discharge. Nail those and the math takes ten minutes. Blow them and you’re either sitting in the dark on day two of a cloudy stretch or you’ve got $4,000 in capacity collecting dust.

I’ve been running off-grid setups at a remote base camp in the Ozarks for six years. I’ve oversized, undersized, and finally landed on a system that hasn’t left me dark once. Here’s the calculation, step by step.

Step 1: Calculate Your Daily Load

List every device you’ll run and how long you’ll run it. Multiply watts by hours to get watt-hours (Wh). Add everything up for your total daily load.

A realistic off-grid cabin example:

• LED lighting (8 bulbs × 10W × 5 hours) = 400 Wh
• Laptop (65W × 4 hours) = 260 Wh
• Phone charging (10W × 2 hours) = 20 Wh
• 12V refrigerator (45W × 24 hours × 0.35 run cycle) = 378 Wh
• Water pump (150W × 0.5 hours) = 75 Wh
• Router/modem (15W × 24 hours) = 360 Wh

Total daily load: ~1,493 Wh, call it 1,500 Wh/day.

Add 25% for inverter losses, wiring resistance, and temperature derating. Design load becomes 1,875 Wh/day. I round to 2,000 Wh/day — clean math and real headroom.

Step 2: Choose Your Autonomy Days

Autonomy days are how many consecutive days your bank can carry your load with zero solar input. Pacific Northwest winters or northern latitudes in December: three to five days. Southwest desert: two days is plenty.

Full-time off-grid home, plan for three days minimum. Seasonal cabin or weekend retreat, two days works. Medical equipment or livestock water — go five days. The cost argument doesn’t matter when the alternative is a dead pump in January.

At 2,000 Wh/day with three autonomy days, you need 6,000 Wh of usable storage.

Step 3: Apply Depth-of-Discharge

No battery chemistry gives you 100% of rated capacity without eating into cycle life. Depth-of-discharge (DoD) is the percentage you can pull safely.

• Flooded lead-acid (FLA): 50% DoD max. Push deeper and you cut cycle life in half — I’ve watched it happen.
• AGM/Gel lead-acid: 50–60% DoD. More forgiving than FLA, still lead chemistry with lead chemistry limits.
• Lithium iron phosphate (LiFePO4): 80–90% DoD. This is the number that justifies the price premium.

Divide usable storage by DoD to get required bank capacity:

Required bank capacity = Usable storage ÷ DoD

• Lead-acid at 50% DoD: 6,000 Wh ÷ 0.50 = 12,000 Wh (12 kWh)
• LiFePO4 at 80% DoD: 6,000 Wh ÷ 0.80 = 7,500 Wh (7.5 kWh)

That 4.5 kWh gap is real weight, real space, and real dollars. LiFePO4 at 2,000–5,000 cycles versus lead-acid at 300–500 cycles — the cost-per-cycle math lands in lithium’s favor by year four on most builds I’ve priced out.

Step 4: Convert to Battery Count

Match required bank capacity to individual battery specs. Most off-grid systems run 12V, 24V, or 48V bus voltage. Go 48V on anything serious — lower amperage means smaller wire gauges and less heat loss across every foot of cable.

The formula: Number of batteries = Required bank capacity (Wh) ÷ (Battery voltage × Battery amp-hours)

Running it for a 48V LiFePO4 system using Battle Born 100Ah LiFePO4 batteries (12V, 100Ah = 1,200 Wh each):

• Required capacity: 7,500 Wh
• Per battery: 1,200 Wh
• Batteries needed: 7,500 ÷ 1,200 = 6.25 → round up to 8 batteries (4S2P configuration for 48V)

Same load with 6V flooded lead-acid golf cart batteries (225Ah = 1,350 Wh each) at 50% DoD on a 24V system:

• Required capacity: 12,000 Wh
• Per battery: 1,350 Wh
• Batteries needed: 12,000 ÷ 1,350 = 8.9 → round up to 10 batteries

In practice, use 12 golf cart batteries in a 24V system — six series strings of two — to keep the configuration clean and symmetrical.

Battery Types: Which One to Buy

Lithium Iron Phosphate (LiFePO4)

LiFePO4 is the right call for any new permanent off-grid build. 2,000–5,000 cycles at 80% DoD versus 300–500 cycles for lead-acid at 50% DoD. I’ve run twelve Battle Born 100Ah batteries in a 48V bank at my Ozarks camp for three years. Zero failures, zero warranty calls.

The Renogy 200Ah LiFePO4 is a solid mid-tier option — built-in BMS, 4,000-cycle rating, and it handles cold better than most budget lithium cells. For a serious permanent installation, Battle Born is what I’d spec and what I’ve staked my own camp on.

Check Price on Amazon → (paid link)

AGM Lead-Acid

AGM makes sense for budget builds, temporary setups, or systems that regularly see temps below 20°F — lithium loses serious capacity in hard freezes without a heating element. The Renogy 100Ah AGM is a reliable workhorse at a price that won’t hurt. Expect 500–800 cycles at 50% DoD with proper maintenance.

Skip the no-name AGM batteries. I’ve watched two separate off-grid setups fail inside 18 months because the owners went bargain-hunting. Renogy, VMAXTANKS, and Trojan are the names I trust. That’s the full list.

Check Price on Amazon → (paid link)

Flooded Lead-Acid (Golf Cart Batteries)

Lowest upfront cost per watt-hour, but you’re trading dollars for maintenance time. FLA batteries need monthly water top-offs, real ventilation for hydrogen off-gassing, and they’ll punish a deep discharge hard. Trojan T-105 6V batteries are the industry standard for a reason — 750+ cycles when treated right. If you’re building a budget cabin system and you’re comfortable with the maintenance schedule, FLA is still a legitimate choice.

Series vs. Parallel Wiring: Getting Your Voltage Right

Series wiring adds voltage, keeps amp-hours the same. Parallel wiring adds amp-hours, keeps voltage the same. Most banks need both.

48V system using 12V batteries: wire 4 in series (4 × 12V = 48V). That’s one series string. Need more capacity, add another series string in parallel alongside it.

24V system using 6V batteries: wire 4 in series (4 × 6V = 24V). Same logic for adding parallel strings.

Cap parallel strings at four for lead-acid. Uneven charging across too many parallel strings is a real failure mode — I’ve diagnosed it twice. LiFePO4 with individual BMS units handles parallel strings better, but I’d still keep it to four strings max without a dedicated bank-level BMS overseeing the whole thing.

Put a quality battery monitor on every system. The Victron BMV-712 is the one I’d install without a second thought — Bluetooth, accurate state-of-charge via coulomb counting, and it’ll alert you before you over-discharge. Around $100 to protect a $3,000+ battery bank. Buy it.

Check Price on Amazon → (paid link)

Temperature Derating: The Factor Most People Skip

Cold kills rated capacity. A lead-acid battery at 32°F delivers roughly 80% of what’s on the label. At 0°F, you’re down to 50–60%. LiFePO4 holds up better but still dips — expect 70–80% capacity at 14°F.

If your battery bank lives in an unheated space in a cold climate, multiply your required bank capacity by 1.25. That’s not conservative — that’s accurate.

At my Ozarks camp, the bank sits in an insulated, partially buried enclosure. It rarely drops below 40°F in there even when it’s 10°F outside — thermal mass from the earth does the work. If you’re in Minnesota or Montana, run a small thermostatically controlled heat tape on the enclosure. It draws minimal power and protects a serious investment.

Quick Reference: Battery Count by System Size

These are ballpark figures for 3-day autonomy at the DoD appropriate to each chemistry. Adjust for your actual load.

• Small cabin (1,000 Wh/day): 4× 100Ah LiFePO4 at 24V, or 8× 6V FLA golf cart batteries at 24V
• Medium cabin (2,000 Wh/day): 8× 100Ah LiFePO4 at 48V, or 12× 6V FLA at 24V
• Full home (4,000 Wh/day): 16× 100Ah LiFePO4 at 48V, or 24× 6V FLA at 48V
• Homestead with well pump + freezer (6,000+ Wh/day): 24× 100Ah LiFePO4 at 48V minimum

One More Thing: Match Your Charge Controller

Your battery bank is only as good as how it gets charged. Size your MPPT charge controller to your panel array — this is not the place to cut corners. The Victron SmartSolar MPPT 100/50 handles up to 1,400W of panels on a 24V system and talks directly to the BMV-712 via Bluetooth. I’ve run Victron components for four years across two separate installs. Haven’t filed a single warranty claim.

Check Price on Amazon → (paid link)

Match the charge profile to your battery chemistry. LiFePO4 wants absorption voltage around 14.2V (12V nominal) — not the 14.4–14.8V you’d run for lead-acid. Most quality MPPT controllers have a LiFePO4 preset. Use it. Wrong charge profile on a lithium bank is how you void a warranty and shorten a 10-year investment.

The Bottom Line

Four steps: daily load plus 25% buffer, times autonomy days, divided by DoD, divided by individual battery watt-hours. That’s your battery count. Apply temperature derating if your bank sees hard freezes, cap parallel strings at four without proper BMS oversight, and don’t buy cheap batteries — they’re the foundation the whole system sits on.

For a new permanent off-grid build, I’d spec LiFePO4 at 48V, a Victron SmartSolar MPPT charge controller, and a BMV-712 monitor. More upfront than lead-acid. Still the cheaper system by year four.

As an Amazon Associate I earn from qualifying purchases. Product links on this page are affiliate links — if you click and buy, I may earn a small commission at no extra cost to you.