Off-Grid Battery Bank Size Calculator

Why battery sizing is critical (and easy to get wrong)

An undersized battery bank means you lose power during cloudy days. An oversized one wastes thousands of dollars on capacity you’ll rarely use. The goal: enough energy to cover your needs during days without sufficient solar charging, with safety margin but not excess.

The basic formula:

Required capacity (kWh) = (Daily use × Days autonomy) ÷ (Depth of Discharge × Round-trip efficiency)

Then convert to amp-hours: Ah = kWh × 1000 ÷ System voltage

The four key parameters

Daily energy use (kWh): total electricity consumed in a typical day. Most off-grid homes use 3-15 kWh/day depending on lifestyle.

Days of autonomy: how many consecutive cloudy days your battery bank must support before charging resumes. Typical values:

• 1 day: minimal; requires reliable daily solar
• 2 days: standard for most off-grid systems
• 3 days: conservative; northern climates with frequent cloudy stretches
• 5+ days: paranoid; usually unnecessary if you have a generator backup

Depth of Discharge (DoD): how much of the battery’s rated capacity can be used without damage:

The fundamental difference: lead-acid batteries have finite cycle life that decreases dramatically with deeper discharge. A 50% discharge gives 800-1,500 cycles; an 80% discharge gives only 300-600 cycles. Lithium-iron-phosphate doesn’t have this problem — 80% discharge gives 3,000-5,000 cycles.

Round-trip efficiency: energy lost during charge/discharge:

A 90% efficient battery losing 10% per cycle eats into your useful storage.

The system voltage choice

Battery banks operate at 12V, 24V, 48V (or higher for utility-scale). The choice affects everything:

For new off-grid systems, 48V is standard. The higher voltage means less current at the same power, allowing thinner wires (savings) and better inverter efficiency. Almost all serious off-grid inverters support 48V.

Worked example — modest off-grid cabin

Daily energy use: 5 kWh Days of autonomy: 2 Battery type: LiFePO4 (DoD 85%, efficiency 95%) System voltage: 48V

Required capacity = (5 × 2) ÷ (0.85 × 0.95) = 10 ÷ 0.81 = 12.4 kWh

In amp-hours at 48V: 12.4 × 1000 ÷ 48 = 258 Ah

To build this with 100Ah/48V LiFePO4 batteries: round up to 3 batteries (300 Ah total, 14.4 kWh).

If we used flooded lead-acid (DoD 50%, efficiency 82%) instead: Required capacity = 10 ÷ (0.50 × 0.82) = 10 ÷ 0.41 = 24.4 kWh

That’s twice the lithium requirement. Plus lead-acid weighs 5x more for the same usable capacity.

Battery cost comparison (2024)

Per kWh stored, lithium is now cheaper than lead-acid over the system lifetime. This wasn’t true 10 years ago — lead-acid was dramatically cheaper. Modern LiFePO4 prices have dropped 80% since 2015, making lithium the clear choice for most new systems.

Why lithium dominates new installations

Beyond cost, lithium batteries have:

• 80-90% usable capacity vs 30-50% for lead-acid
• 5-10x cycle life
• 1/4 the weight for the same usable kWh
• No outgassing (can be installed in living spaces)
• No watering or maintenance
• Higher charging efficiency (faster recharge from solar)
• Built-in BMS (battery management system) for safety

Drawbacks of lithium:

• Higher upfront cost
• Strict temperature limits (no charging below 0°C / 32°F for most models)
• Needs proper BMS integration with inverter and solar charge controller
• Battery cells can ignite (rare with LiFePO4; more common with NMC)

Climate considerations

Battery performance and life are heavily affected by temperature:

Cold climates require either:

• Insulated battery enclosure with internal heater
• Climate-controlled mechanical room (basement, dedicated space)
• Self-heating batteries (some newer models have built-in warming)

Hot climates require ventilation and shading. Direct sun on a battery in summer can shorten life by 50%.

Charge controller and inverter matching

A battery bank doesn’t exist in isolation. You need:

• Solar charge controller (MPPT preferred): 30-100A typical
• Inverter (sine wave): 1,500W to 12,000W depending on house size
• Battery management system (BMS): built into lithium, separate for lead-acid

The whole system must match in voltage and current capacity. Common mismatches:

• 48V battery with 24V inverter (won’t work)
• Inverter undersized for surge loads (well pump start-up, refrigerator compressor)
• Solar array too small to recharge battery in cloudy weather

Solar array sizing

A battery bank without enough solar to recharge it is useless. Rule of thumb:

Daily solar generation should equal 1.5-2.5x daily use to allow for:

• Inefficient (cloudy) days
• Battery charging losses
• Future load growth
• Battery longevity (avoiding deep discharges)

For 5 kWh/day use, plan 7-12 kWh/day solar generation = roughly 2-3 kW of solar panels in moderate climates.

The generator backup question

Almost all off-grid systems benefit from a generator backup:

• Diesel or propane generator (3-10 kW)
• Used 50-200 hours/year typically
• Saves you from massive over-sizing of battery and solar

The “all renewable” zero-generator system requires 4-5 days of autonomy and 200%+ oversized solar, costing 2-3x as much as a system with generator backup. For most homesteads, a small generator is the economical choice.

Realistic off-grid budget

For a typical off-grid home using 8 kWh/day:

Larger homes (15-20 kWh/day) scale up to $40,000-$80,000. Compare to monthly utility bill of $200-$400 for the same usage — payback is 8-20 years depending on location.

Bottom line

Battery bank size = (daily use × autonomy days) ÷ (DoD × efficiency). Modern lithium (LiFePO4) is the clear winner for new systems: higher usable capacity, longer life, lower lifetime cost. 48V is standard system voltage. Plan 2-3 days of autonomy with backup generator. Total off-grid system: $18,000-$40,000 for typical home. Solar array should generate 2x daily use to allow for cloudy days and longevity.