Quick Answer
Calculate your daily energy consumption in watt-hours, then multiply by the number of autonomy days you want (typically 2-3 days for off-grid). Divide by usable battery capacity — 80-90% for lithium, 50% for lead-acid. For example, if you use 5 kWh daily and want 3 days of autonomy: 5,000 × 3 = 15,000 Wh needed. With lithium at 90% depth of discharge: 15,000 ÷ 0.9 = 16,667 Wh total battery capacity. For a 48V system, that's about 350Ah. Always round up and account for efficiency losses in the inverter (roughly 10%).
How to Size Your Off-Grid Battery Bank
Step 1: Calculate Daily Energy Use
List every electrical device you use and estimate daily run time. Be honest — underestimating leads to a system that can’t keep up.
Common off-grid cabin loads:
- LED lighting (10 bulbs × 10W × 5 hrs) = 500 Wh
- Refrigerator (efficient DC fridge) = 500-1,000 Wh
- Water pump (1/2 HP × 1 hr runtime) = 400 Wh
- Laptop (65W × 4 hrs) = 260 Wh
- Phone charging (2 phones) = 30 Wh
- Router/modem = 200 Wh
- Miscellaneous (fans, small tools) = 500 Wh
Typical range: 2,500-5,000 Wh per day for a modest off-grid cabin without air conditioning or electric heating.
Step 2: Determine Autonomy Days
Autonomy days are how long your battery bank can power your home without any solar input. This covers cloudy stretches and winter low-production periods.
- 2 days: Minimum for moderate climates with reliable sun
- 3 days: Standard recommendation for most off-grid homes
- 5+ days: Northern climates with extended cloudy periods or winter
Step 3: Apply the Sizing Formula
Daily use × autonomy days ÷ usable depth of discharge ÷ inverter efficiency = required battery capacity
Example with 4 kWh daily use, 3 autonomy days, lithium batteries:
- 4,000 Wh × 3 days = 12,000 Wh needed
- ÷ 0.85 (85% usable DoD for LiFePO4) = 14,118 Wh
- ÷ 0.90 (inverter efficiency) = 15,686 Wh total capacity
- At 48V: 15,686 ÷ 48 = 327 Ah (round up to 400Ah for margin)
Step 4: Choose Battery Chemistry
Lithium Iron Phosphate (LiFePO4) — The modern standard. 80-90% usable capacity, 3,000-5,000 cycle life, no maintenance, lightweight. Higher upfront cost but dramatically lower cost per cycle over the battery’s lifetime.
AGM Lead-Acid — Lower upfront cost but only 50% usable capacity (you need twice the rated capacity), 500-800 cycle life, heavier, and requires more careful charge management. Only recommended for very tight budgets.
Flooded Lead-Acid — Cheapest per kWh but requires regular water maintenance, venting for hydrogen gas, and has the same 50% DoD limitation. Good for DIY builders on extreme budgets who don’t mind maintenance.
Step 5: Choose System Voltage
- 12V: Only for very small systems (RV, tiny cabin under 1 kWh/day)
- 24V: Small to medium systems (1-3 kWh/day)
- 48V: Standard for homes and larger cabins (3+ kWh/day) — most efficient, lowest wire losses
Common Mistakes
- Undersizing by using manufacturer’s rated capacity instead of usable capacity
- Forgetting inverter efficiency losses (typically 5-15%)
- Not accounting for battery capacity loss in cold temperatures (lead-acid loses 30-40% capacity at freezing)
- Mixing old and new batteries in the same bank
- Running lead-acid batteries below 50% regularly (dramatically shortens lifespan)
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