Battery Bank Sizing for Off-Grid Solar

Daily Load and Days of Autonomy

Battery sizing starts with two numbers: your daily energy consumption in watt-hours and the number of days you want the battery to carry you without any solar input. Daily consumption comes from your load audit — sum up every appliance's wattage times hours of use. A typical off-grid cabin might use 3,000–5,000 Wh per day. A full-time off-grid home with a well pump, refrigerator, and washing machine might use 8,000–12,000 Wh.

Days of autonomy is how long the battery can power your loads during consecutive cloudy days or a system outage. Two days of autonomy is the minimum for most locations. Three days is common for northern climates where consecutive overcast days are frequent in winter. One day is acceptable if you have a reliable backup generator.

Multiply daily consumption by days of autonomy to get your gross storage requirement. For 5,000 Wh/day with 3 days of autonomy: 5,000 × 3 = 15,000 Wh = 15 kWh of gross storage. This is not your final battery size because you cannot use 100% of the battery capacity without destroying it.

Depth of Discharge: Why You Cannot Use 100%

Depth of discharge (DoD) is the percentage of total capacity you actually use before recharging. Discharging a battery deeper shortens its cycle life dramatically. This is the most important concept in battery bank sizing because it determines how much of your purchased capacity is actually usable.

Flooded lead-acid batteries should not be discharged below 50% state of charge (50% DoD). Doing so regularly will reduce cycle life from 1,500–2,000 cycles to 500 or fewer. AGM batteries are similar. Gel batteries tolerate slightly deeper discharge but at higher cost.

LiFePO4 (lithium iron phosphate) batteries can routinely discharge to 80% DoD (20% remaining) with minimal impact on cycle life. Many manufacturers rate them at 80% DoD for 3,000–5,000 cycles. Some allow 90% DoD with reduced cycle life. This deeper usable capacity is why lithium batteries need a smaller gross capacity than lead-acid for the same usable storage.

The formula: Usable capacity = Gross capacity × DoD. To find the required gross capacity: Battery Bank Size = Gross Storage Requirement ÷ DoD. For the 15 kWh example with lead-acid at 50% DoD: 15 / 0.50 = 30 kWh of battery capacity. With LiFePO4 at 80% DoD: 15 / 0.80 = 18.75 kWh. The lithium bank is 37% smaller for the same usable energy.

Lead-Acid vs LiFePO4: A Realistic Comparison

Flooded lead-acid (FLA) batteries have been the off-grid standard for decades. They are cheap upfront ($150–$250 per kWh), widely available, recyclable, and well-understood. The downsides: they need regular watering (every 2-4 weeks), must be kept in a ventilated enclosure (hydrogen gas during charging), weigh roughly 60 lbs per kWh, and last 5–7 years with proper maintenance at 50% DoD.

LiFePO4 batteries cost more upfront ($400–$600 per kWh) but last 10–15 years, require zero maintenance, tolerate 80% DoD, weigh 25-30 lbs per kWh, and produce no gas. Over their lifetime, the cost per cycle is typically lower than lead-acid. A $6,000 LiFePO4 bank lasting 10 years replaces two $3,000 lead-acid banks over the same period, plus you avoid the labor and risk of replacing lead-acid batteries in a remote location.

AGM (absorbed glass mat) batteries split the difference. No watering needed, sealed and spillproof, but still limited to 50% DoD and 3-5 year life at that discharge depth. They cost more than flooded lead-acid ($200–$350 per kWh) without the longevity of lithium. AGM makes sense for small systems where maintenance access is difficult and the upfront cost of lithium is prohibitive.

For new off-grid installations in 2025 and beyond, LiFePO4 is the default recommendation unless budget is an absolute constraint. The total cost of ownership is lower, the usable capacity per dollar is better, and the elimination of maintenance is significant for remote properties.

Temperature Effects on Capacity and Lifespan

Battery capacity is rated at 77°F (25°C). Below that temperature, available capacity decreases. At 32°F (0°C), a lead-acid battery delivers only 70–80% of its rated capacity. At 0°F (-18°C), capacity drops to 50–60%. This means a battery bank sized for summer conditions may not carry you through a cold winter night.

LiFePO4 batteries perform better in cold than lead-acid but have a critical limitation: most cannot be charged below 32°F (0°C) without risk of lithium plating, which permanently damages the cells. Some manufacturers include built-in heating elements that activate before charging begins. If your battery enclosure is unheated and temperatures drop below freezing, verify your specific battery supports cold-weather charging or add a thermostatically controlled heater.

High temperatures are equally destructive. Every 18°F (10°C) above 77°F roughly halves the calendar life of lead-acid batteries. A flooded lead-acid battery rated for 7 years at 77°F might last only 3.5 years in a 95°F shed. LiFePO4 tolerates heat better but still degrades faster above 95°F.

The sizing implication: if your battery enclosure reaches low temperatures, apply a temperature correction factor. At 50°F, multiply your required battery capacity by 1.11. At 32°F, multiply by 1.30. At 0°F, multiply by 1.60. This ensures you have enough usable capacity at the actual operating temperature, not just at the datasheet temperature.

Series and Parallel Configuration

Batteries in series add voltage while maintaining the same amp-hour capacity. Four 12V 100Ah batteries in series produce 48V at 100Ah (4,800 Wh). Batteries in parallel add amp-hour capacity while maintaining voltage. Four 12V 100Ah batteries in parallel produce 12V at 400Ah (4,800 Wh). Same total energy, different voltage and current characteristics.

Most modern off-grid systems operate at 48V because higher voltage means lower current for the same power, which allows smaller wire sizes and reduces losses. A 48V system delivering 5 kW draws about 104 amps. A 12V system delivering 5 kW draws 417 amps, requiring massive cables and producing significant heat losses.

Configuration rules: never mix battery types, brands, ages, or capacities in a bank. Every battery in a series string must be identical. Parallel strings should also be identical. Mixing creates imbalanced charging and discharging, which kills the weakest battery first and then cascades through the bank.

For lead-acid, keep parallel strings to 3 or fewer. More parallel strings create uneven current distribution that accelerates degradation. If you need more capacity, use larger individual batteries rather than adding more parallel strings. For LiFePO4, the built-in BMS (battery management system) handles parallel balancing better, but the same principle applies — fewer parallel connections means fewer potential failure points.

Matching the Charge Controller to the Battery

The charge controller sits between the solar panels and the battery bank. It regulates voltage and current to match the battery's charging profile. An undersized or improperly configured charge controller will overcharge or undercharge the batteries, either of which shortens battery life.

MPPT charge controllers are standard for any system above 200W. They convert excess panel voltage into additional charging current, improving harvest by 15–30% compared to PWM controllers. Size the controller for the maximum solar array current and the battery bank voltage. A 60A MPPT controller at 48V handles up to 60 × 48 = 2,880W of charging power.

Charging profile must match the battery chemistry. Lead-acid batteries require a multi-stage charge profile: bulk (constant current to ~80% SOC), absorb (constant voltage for 2-4 hours), and float (reduced voltage for maintenance). LiFePO4 batteries use a simpler profile: constant current to the absorption voltage (typically 14.2–14.6V per 12V nominal), then the BMS handles the rest. Using a lead-acid profile on LiFePO4 wastes energy and can damage cells. Using a lithium profile on lead-acid causes chronic undercharge and sulfation.

Every quality charge controller has selectable battery chemistry profiles. Verify the controller supports your specific chemistry and set it correctly during initial setup. This is a 30-second configuration step that prevents years of premature battery degradation.

Maintenance for Maximum Battery Life

For flooded lead-acid: check water levels every 2-4 weeks and top up with distilled water only. Inspect terminals for corrosion monthly and clean with a baking soda solution. Equalize (controlled overcharge) every 30-90 days to balance cell voltages and prevent stratification. Monitor specific gravity with a hydrometer quarterly — cells reading 1.265 SG are fully charged, below 1.200 indicates sulfation or cell damage.

For AGM: no watering needed, but monitor terminal voltage and avoid deep discharge. AGM batteries are less tolerant of overcharge than flooded, so verify charge controller absorb voltage matches manufacturer specs. Clean terminals annually.

For LiFePO4: maintenance is minimal. Monitor the BMS dashboard (if available) for cell balance and temperature. Keep the battery enclosure above 32°F during charging. Avoid storing at 100% SOC for extended periods — 50–60% SOC is ideal for long-term storage. Check terminal connections annually for tightness.

Regardless of chemistry, the best maintenance practice is proper system sizing and charge control configuration. A correctly sized bank that is never over-discharged or overcharged, kept at reasonable temperatures, and recharged fully on a regular basis will reach or exceed its rated cycle life. Most premature battery failures trace back to chronic undercharging, excessive depth of discharge, or temperature extremes — not manufacturing defects.