Solar System Sizing for Off-Grid Cabins: How Many Panels, Batteries, and Watts You Actually Need

Most off-grid solar systems are undersized. The result: dead batteries on day three of cloudy weather, freezer thawing out, generator running constantly. Here's how to size correctly.

Step 1 — Calculate your daily kWh load

List every appliance you'll run and its wattage × hours per day. For example: fridge 100W × 8hrs (cycle) = 800Wh, lights 20W × 5hrs = 100Wh, laptop 50W × 4hrs = 200Wh, well pump 1500W × 0.5hr = 750Wh. Add 20% for inverter losses. Total: your daily kWh load.

Step 2 — Solar array sizing

Daily load ÷ sun-hours-per-day (regional, 3-5 hrs in winter for most US locations) = required panel wattage. Round up 30% for cloudy weather buffer. A 3 kWh/day cabin in winter needs ~1,200W of panels minimum.

Step 3 — Battery bank sizing

Plan for 2-3 days of autonomy (no sun). Modern lithium (LiFePO4) batteries handle 80-90% depth of discharge; lead-acid only 50%. For a 3 kWh/day load with 3 days autonomy on lithium: 9 kWh × 1.25 buffer = ~11 kWh battery bank. At 12V that's ~900 amp-hours. Lithium is now cost-competitive with lead-acid and lasts 4x longer.

Common mistakes

• Sizing for summer. Winter sun-hours are 30-50% of summer. Size for the worst month, not the best.
• Ignoring inrush current. Well pumps and refrigerator compressors draw 3-5x rated wattage on startup. Inverter must handle the surge.
• Cheap charge controller. PWM controllers waste 30% vs MPPT. Always go MPPT for off-grid.
• Mixing battery types. Never connect lithium and lead-acid in the same bank. The charge profiles are incompatible.

Generator backup

Even with a properly-sized system, plan for a propane or diesel generator for the once-a-year week of bad weather. Sizing: at least 25% of your battery bank capacity in kW. For an 11 kWh bank, ~3 kW generator minimum.

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