Solar Battery Bank Sizing Calculator: How Much Storage Do You Need?

Calculator

Use this calculator to estimate your battery bank requirements.

Step-by-Step Battery Bank Sizing

Step 1: Determine Your Daily Energy Use (kWh)

Your daily energy use is the foundation of everything. There are two ways to find this number:

From your electric bill: Take your monthly kWh and divide by 30. The average US household uses about 30 kWh per day, but off-grid systems are typically designed for much less -- 5 to 15 kWh per day after efficiency improvements.

From a load audit: List every device you will run, its wattage, and hours of daily use. Multiply watts by hours to get watt-hours per device, then add them all up.

This is a modest off-grid home without air conditioning or electric heating.

Step 2: Multiply by Days of Autonomy

Days of autonomy is the number of days your battery bank must supply full power without any solar input. See our days of autonomy calculator for detailed guidance on choosing the right number.

For this example, we will use 3 days:

4.7 kWh x 3 days = 14.1 kWh total energy needed

Step 3: Divide by Depth of Discharge (DoD)

You cannot use 100% of a battery's rated capacity. Depth of discharge limits:

• LiFePO4: 80-100% DoD (we will use 80% for longer cycle life)
• AGM: 50% DoD
• Flooded lead-acid: 50% DoD

LiFePO4: 14.1 kWh / 0.80 = 17.6 kWh total battery capacity

Lead-acid: 14.1 kWh / 0.50 = 28.2 kWh total battery capacity

Notice lead-acid requires 60% more total capacity for the same usable energy.

Step 4: Convert to Amp-Hours at System Voltage

Ah = kWh x 1000 / System Voltage

At 48V (recommended for this system size):

• LiFePO4: 17,600 Wh / 48V = 367 Ah at 48V
• Lead-acid: 28,200 Wh / 48V = 588 Ah at 48V

At 24V:

• LiFePO4: 17,600 / 24 = 733 Ah at 24V
• Lead-acid: 28,200 / 24 = 1175 Ah at 24V

Higher system voltage means lower Ah requirements, which means fewer parallel strings and simpler wiring.

Step 5: Calculate Number of Batteries

For 48V LiFePO4 (using 12V 100Ah cells):

• Series: 4 batteries in series to reach 48V (4 x 12V = 48V)
• Each series string: 100Ah at 48V = 4.8 kWh
• Strings needed: 17.6 kWh / 4.8 kWh = 3.67, round up to 4 parallel strings
• Total batteries: 4 series x 4 parallel = 16 batteries
• Total capacity: 4.8 kWh x 4 = 19.2 kWh (17.6 kWh needed, slight surplus)

For 48V lead-acid (using 6V 225Ah golf cart batteries):

• Series: 8 batteries in series to reach 48V (8 x 6V = 48V)
• Each series string: 225Ah at 48V = 10.8 kWh
• Strings needed: 28.2 kWh / 10.8 kWh = 2.6, round up to 3 parallel strings
• Total batteries: 8 series x 3 parallel = 24 batteries
• Total capacity: 10.8 kWh x 3 = 32.4 kWh

Recommended Battery Bank Sizes by Daily Usage

Cost ranges reflect DIY assembly on the low end and professional installation with premium components on the high end. Lead-acid looks cheaper upfront but must be replaced every 3 to 5 years, while LiFePO4 lasts 10 to 15 years. Over a 15-year period, LiFePO4 is almost always cheaper.

Series vs Parallel Wiring

Understanding series and parallel wiring is essential for building a battery bank correctly.

Series Wiring: Increases Voltage

Connecting batteries positive-to-negative in a chain adds their voltages together. Capacity stays the same.

• 4 x 12V 100Ah in series = 48V, 100Ah (4.8 kWh)
• 2 x 12V 200Ah in series = 24V, 200Ah (4.8 kWh)
• 8 x 6V 225Ah in series = 48V, 225Ah (10.8 kWh)

All batteries in a series string must be identical: same chemistry, same capacity, same age, same manufacturer. A weak battery in a series string limits the entire string.

Parallel Wiring: Increases Capacity

Connecting batteries positive-to-positive and negative-to-negative adds their capacities together. Voltage stays the same.

• 3 x (48V 100Ah strings) in parallel = 48V, 300Ah (14.4 kWh)
• 4 x (24V 200Ah strings) in parallel = 24V, 800Ah (19.2 kWh)

Each parallel string should have its own fuse or breaker for safety. If one string develops a short, the fuse isolates it from the rest of the bank.

Combined Series-Parallel: The Standard Approach

Most battery banks use both. First, wire batteries in series to reach your system voltage. Then, wire multiple series strings in parallel to reach your capacity target.

Example: 48V 400Ah bank (19.2 kWh) using 12V 100Ah LiFePO4 batteries:

• 4 batteries in series = one 48V 100Ah string
• 4 strings in parallel = 48V 400Ah
• Total: 16 batteries

LiFePO4 vs Lead-Acid Sizing Differences

The choice between LiFePO4 and lead-acid fundamentally changes how large your battery bank needs to be.

Depth of Discharge

This is the single biggest difference. Lead-acid batteries degrade rapidly when discharged below 50%, so you can only use half their rated capacity. LiFePO4 can be discharged to 80-100% with minimal impact on cycle life.

For the same 10 kWh of usable energy:

• LiFePO4 at 80% DoD: 12.5 kWh total capacity needed
• Lead-acid at 50% DoD: 20 kWh total capacity needed

Cycle Life

• LiFePO4: 3,000 to 5,000 cycles at 80% DoD (10-15 years of daily cycling)
• AGM: 500 to 800 cycles at 50% DoD (2-4 years of daily cycling)
• Flooded lead-acid: 800 to 1,200 cycles at 50% DoD (3-5 years of daily cycling)

Over a 15-year system life, you would replace lead-acid batteries 3 to 5 times. That replacement cost almost always exceeds the upfront premium for LiFePO4.

Charging Efficiency

• LiFePO4: 95-98% round-trip efficiency
• Lead-acid: 75-85% round-trip efficiency

This means lead-acid wastes 15-25% of the solar energy used to charge it. Over a year, that is a meaningful amount of lost production.

Weight

• LiFePO4: approximately 13 lbs per kWh
• Lead-acid: approximately 55 lbs per kWh

A 20 kWh LiFePO4 bank weighs about 260 lbs. The equivalent lead-acid bank (40 kWh total for 20 kWh usable) weighs over 2,200 lbs. This matters for floor load capacity, especially on upper floors or in RVs.

Cost Comparison: DIY Bank vs Tesla Powerwall

For a target of 13.5 kWh usable capacity (matching one Powerwall 3):

DIY LiFePO4 Bank

• 16 x 12V 100Ah LiFePO4 batteries (budget brand like Ampere Time): ~$160 each = $2,560
• Wired as 48V with 4 parallel strings: 19.2 kWh total, 15.4 kWh usable at 80% DoD
• Add BMS, fuses, bus bars, wiring, enclosure: ~$500-$800
• Total: $3,100-$3,400 (~$220-$250 per usable kWh)

Tesla Powerwall 3

• 13.5 kWh capacity, integrated inverter, gateway, and battery management
• Installed cost: approximately $9,200 (as of 2025)
• Total: $9,200 (~$680 per usable kWh)

The DIY bank is roughly one-third the cost per usable kWh. However, the Powerwall includes an integrated hybrid inverter, professional installation, a 10-year warranty, and smartphone monitoring. If you value your time and want a turnkey solution, the Powerwall premium buys significant convenience. If you are comfortable with electrical work and want maximum value, DIY LiFePO4 is hard to beat.

Lead-Acid Equivalent

• 8 x 6V 225Ah golf cart batteries (Trojan T-105): ~$180 each = $1,440
• Wired as 48V: 10.8 kWh total, 5.4 kWh usable at 50% DoD
• Need 3 strings for 16.2 kWh usable: 24 batteries = $4,320
• Total: ~$4,500-$5,000 with wiring and enclosure
• Must be replaced every 3-5 years: $13,500-$25,000 over 15 years

Lead-acid looks cheap initially but is the most expensive option over a system's lifetime.

Temperature Derating for Cold Climates

If your batteries are in an unheated garage, shed, or outdoor enclosure, cold weather reduces available capacity.

Lead-Acid Temperature Derating

A lead-acid bank sized for 20 kWh usable at 77 degrees F delivers only 12 kWh at 0 degrees F. In cold climates, multiply your required capacity by the derating multiplier.

LiFePO4 Temperature Effects

LiFePO4 handles cold better than lead-acid for discharge, losing about 10-20% of capacity at freezing. However, most LiFePO4 batteries cannot be charged below 32 degrees F (0 degrees C) without risking permanent damage from lithium plating.

Many quality LiFePO4 batteries (Battle Born, Victron, SOK) include built-in low-temperature cutoff switches that prevent charging below freezing. Some premium models include internal heaters that use a small amount of battery energy to warm the cells above the charging threshold.

If your batteries will be in an unheated space in a cold climate:

• Insulate the battery enclosure
• Consider batteries with built-in heaters
• Multiply LiFePO4 capacity by 1.1 for mild cold (down to 20 degrees F)
• Multiply LiFePO4 capacity by 1.2 for severe cold (below 0 degrees F)
• Multiply lead-acid capacity by 1.3 to 1.7 depending on minimum temperature

Common Mistakes to Avoid

Sizing for average daily use instead of peak daily use. If you run a washing machine, well pump, and oven on the same day, that day's usage is much higher than average. Size your bank for your highest-usage day, not your average.

Forgetting the inverter's self-consumption. Your inverter draws 10 to 50W just being on. Over 24 hours, that is 0.24 to 1.2 kWh of battery drain that produces no useful work. Add this to your daily load calculation.

Mixing battery types or ages. Never combine old and new batteries, or different capacities, or different chemistries in the same bank. The weakest cell limits the entire system and degrades faster, creating a cascading failure.

Ignoring temperature effects. Batteries in an unheated Northern garage will deliver 25 to 40% less capacity in January than in July. If you sized for summer conditions, you will come up short in winter -- exactly when outages are most likely.

Skipping fuses on parallel strings. Without individual string fusing, a short circuit in one string draws current from all other strings, potentially causing a fire. Every parallel connection needs a properly rated fuse.

Keep Reading

• Days of Autonomy Solar Calculator
• Lithium vs AGM vs Lead-Acid Batteries
• Battery Runtime Calculator
• Solar Charge Controller Size Calculator
• Inverter Size Calculator
• What Size Solar Panel to Charge a 100Ah Battery
• Peak Sun Hours by State