How to Size a Battery Bank for an Off-Grid Cabin
Battery sizing depends on two things: how much energy you use per day, and how many days of autonomy (cloudy stretches) you want to ride through. For a typical weekend cabin, 100-200Ah of LiFePO4 at 12V (roughly 1.3-2.6 kWh usable) is enough. For a full-time cabin, 400-600Ah (5-8 kWh usable) is more realistic. Below is the framework and specific sizing examples.
The battery bank is where most cabin owners either save too much or overspend. Undersized banks force generator use and shorten the cabin usability window during cloudy stretches; oversized banks cost more than necessary and don't deliver proportional benefit. Getting this right is one of the highest-impact decisions in cabin solar design.
The sizing framework
Battery capacity needed ≈ (Daily energy use in Wh) × (Days of autonomy) ÷ (Depth of discharge)
For LiFePO4 (which safely discharges to 90-95% depth), assume 90% depth of discharge. For lead-acid (which shouldn't regularly go below 50%), depth of discharge is 50%.
Example: Weekend cabin, 1,000Wh/day, 2 days autonomy, LiFePO4:
1,000 × 2 ÷ 0.9 = 2,222 Wh capacity. At 12V, that's roughly 185Ah. Round up to 200Ah.
Example: Full-time cabin, 2,500Wh/day, 3 days autonomy, LiFePO4:
2,500 × 3 ÷ 0.9 = 8,333 Wh capacity. At 12V, roughly 700Ah, or at 24V, 350Ah, or at 48V, 175Ah.
Why LiFePO4 wins for cabin banks
Comparison table for a typical cabin battery bank size (200Ah usable capacity):
| Type | Nominal capacity needed | Weight | Cost trajectory | Lifespan |
|---|---|---|---|---|
| LiFePO4 (90% DoD) | 220Ah | ~60 lbs | Higher upfront | 10+ years |
| AGM lead-acid (50% DoD) | 400Ah | ~250 lbs | Lower upfront | 3-5 years |
| Flooded lead-acid (50% DoD) | 400Ah | ~250 lbs | Lowest upfront | 3-6 years with maintenance |
Across 10 years of cabin service, LiFePO4 typically costs less total than lead-acid due to lifespan differences. It also delivers usable capacity that lead-acid alternatives can't match (a 200Ah lead-acid bank has 100Ah usable; a 200Ah LiFePO4 has 180Ah+ usable).
System voltage: 12V vs 24V vs 48V
For very small cabin systems (under 1,500W of loads), 12V is fine and matches most RV and marine equipment. For larger systems, 24V or 48V reduces current levels and allows smaller wire gauges throughout.
- 12V: Simplest, widest accessory availability (RV/marine ecosystem). Best for systems under about 2,000W total inverter capacity.
- 24V: The middle ground. Better wire efficiency than 12V. Some accessories available.
- 48V: The residential/light-commercial standard. Best wire efficiency. Server-rack battery banks and larger inverters dominate.
For most weekend/family cabins, 12V is the practical choice. For full-time cabins with well pumps, larger fridges, and Starlink, 24V or 48V is worth the small accessory constraint.
Days of autonomy: how many is right?
The autonomy question is really "how bad is it if the batteries run out." Answer varies:
1 day autonomy: Minimal margin. Assumes sun every day. Requires generator backup for cloudy stretches. Common for tight-budget systems.
2-3 days autonomy: Standard cabin choice. Rides through typical cloudy stretches. Comfortable for most cabin owners with occasional generator use.
4-5+ days autonomy: Cabins in cloudy climates (Pacific Northwest, Great Lakes region) or cabins that must avoid generator use for noise or emissions reasons. Higher upfront cost.
Most cabin owners land at 2-3 days autonomy plus generator backup for exceptional weather.
Real-world cabin battery examples
Small weekend cabin (1,000 Wh/day, 2 days autonomy):
200Ah LiFePO4 at 12V. Roughly 2,500Wh usable. Handles LED lights, phones, laptops, small radios for two days without sun.
Family weekend cabin (1,500 Wh/day, 2 days autonomy):
300Ah LiFePO4 at 12V. Roughly 3,600Wh usable. Adds a small compressor fridge to the small-cabin baseline.
Full-time cabin (2,500 Wh/day, 3 days autonomy):
400Ah LiFePO4 at 24V or 200Ah at 48V. Roughly 9,000Wh usable. Well pump, fridge, standard cabin loads.
Full-time cabin with Starlink (4,000 Wh/day, 3 days autonomy):
600Ah LiFePO4 at 48V. Roughly 13,000Wh usable. Everything plus continuous cellular/internet infrastructure.
Charging: the panel-to-battery match
Battery banks need daily recharge from solar to sustain autonomy. Panel array should recharge from 20% to 100% within a single sunny day, ideally.
Rough matching: for LiFePO4 at 12V, aim for panel wattage roughly 2-4x battery Ah capacity. A 200Ah bank pairs with 400-800W of panels; a 400Ah bank pairs with 800-1,600W.
For 48V systems, the current levels are lower for the same power, so the wattage-to-Ah ratio scales differently. Focus on charging within a day rather than specific ratios.
The BMS and battery communication
LiFePO4 batteries have a Battery Management System (BMS) that handles cell balancing, over-voltage cutoff, and low-voltage cutoff. Modern BMS systems include communication protocols (RS485, CAN bus, Bluetooth) that let the inverter and battery talk.
For cabin systems, look for LiFePO4 batteries with:
- Integrated BMS (not optional external)
- Bluetooth for state-of-charge monitoring
- Communication compatibility with your inverter (Victron, Renogy, and others each have preferred protocols)
- Adequate continuous discharge rating for your inverter
Expansion planning
Cabin battery banks often grow over years. Design the initial installation to support parallel additions:
- Buy matched batteries (same brand, same model, same age) for the initial bank.
- Wire the initial bank with proper busbars and cabling that support additional parallel connections.
- Size the charge controller and inverter for the future bank size.
Adding a second identical LiFePO4 in parallel to an existing bank is straightforward. Adding a different brand or a much older battery creates cell drift problems.
Common mistakes
Sizing for average day, not cloudy stretches. Sunny days aren't the design case; sequences of cloudy days are.
Skimping on batteries to save money. Undersized banks force generator use, shorten cabin usability, and get replaced sooner. Buy quality up front.
Mixing chemistries. Never mix LiFePO4 with lead-acid in the same bank. The charging voltages are different; both batteries suffer.
Ignoring temperature. LiFePO4 charging in below-freezing temperatures damages cells permanently. Keep the bank above 32°F during charging (see the cold-storage guide).
Bottom line
Battery bank sizing is the single most important number in a cabin solar system. Size for your daily energy use, multiply by desired days of autonomy, and buy quality LiFePO4 with room for parallel expansion. For weekend cabins, 100-300Ah at 12V is typical. For full-time cabins, 400Ah+ at 24V or 48V is more realistic. This is one place where buying quality on the first pass pays back for a decade.
Frequently Asked Questions
What size battery bank for a weekend cabin?
For a weekend cabin with lights, phone charging, laptops, and a small fridge, 200-300Ah of LiFePO4 at 12V is typical (2,500-3,800Wh usable). This handles 2 days of typical use without sun.
What size battery bank for a full-time off-grid cabin?
For a full-time cabin with well pump, fridge, Wi-Fi, and normal loads, 400-600Ah of LiFePO4 at 24V or 48V is typical (10-15 kWh usable). Full-time cabins with Starlink or larger appliances need more.
Should I use 12V, 24V, or 48V for my cabin?
12V for small cabins (under 2,000W inverter total). 24V or 48V for larger cabins with well pumps, larger fridges, and Starlink. 48V systems benefit from server-rack LiFePO4 batteries and the residential solar ecosystem. RV/marine accessories are mostly 12V, so that matters if your cabin has RV-style equipment.
How many days of autonomy do I actually need?
Most cabin owners land at 2-3 days plus generator backup for exceptional weather. Cabins in cloudy climates (Pacific Northwest) or without generator backup typically need 4-5 days. Cabins in sunny climates with generator backup can safely run 1-2 days.
Should I mix new and old batteries in the same bank?
No — never mix battery brands, models, or age significantly in the same parallel bank. Cell drift causes imbalance that damages both new and old batteries. Buy matched batteries for the initial bank, and add identical models for parallel expansion.