What size solar system for an off-grid cabin?
Worked sizing examples from weekend cabin to full-time homestead — panel counts, battery banks, and where the money goes.
The formula, once
Everything flows from one number: daily watt-hours. Array watts = daily watt-hours ÷ worst-month sun hours ÷ 0.75 (wiring, controller, battery round-trip, panel heat and dust). Bank watt-hours = daily Wh × days of autonomy ÷ max depth of discharge. LiFePO4's 80% usable depth is why it beats AGM's 50% on paper even before cycle life. Run your own numbers in the solar size calculator.
Example 1 — Weekend cabin
Loads: lights, device charging, water pump, internet ≈ 1,450 Wh/day. Array: 1,450 ÷ 4 sun hrs ÷ 0.75 ≈ 500 W (2×250 W). Bank: one day autonomy ÷ 80% DoD → 24 V 100 Ah LiFePO4. Inverter: no big motors → 1,000 W pure sine. Controller: 500 ÷ 24 ÷ 0.85 ≈ 25 A → 30 A MPPT. DIY parts: ~$1,200–1,800.
Example 2 — Full-time cabin (fridge, propane cooking)
Add an Energy Star fridge (1,200 Wh/day cycled), TV, fan, occasional washing machine ≈ 3,800 Wh/day. Array: ≈ 1,300–1,500 W (5×300 W). Bank: 2 days autonomy ÷ 80% DoD → 400 Ah at 24 V LiFePO4. Inverter: fridge surge + microwave → 2,000 W pure sine. Controller: ≈ 74 A → 80 A MPPT (or 2×40 A). DIY parts: ~$4,000–6,500. Cooking and water heating stay on propane — electrifying them adds ~4 kWh/day and thousands in panels.
Example 3 — Small homestead (well pump, workshop, electric cooking)
≈ 8,000 Wh/day. Array: ≈ 3,000 W ground-mounted. Bank: 2 days autonomy at 80% DoD → 48 V 400 Ah — 48 V halves DC amps and cable cost vs 24 V. Inverter: well pump startup over a 1,200 W base → 4,000–5,000 W pure sine. Controller: ≈ 74 A at 48 V → 80–100 A MPPT or a hybrid inverter/charger.
Common sizing mistakes
Heat loads: space heat, water heat, and dryers on solar means a 10+ kW system — keep those on propane or wood. Oversized banks, undersized arrays: a bank that never fully recharges degrades faster; the array must replace daily use plus recharge overnight draw. Summer-sized arrays in winter: size to worst-month sun hours, or plan generator/shore charging — the battery bank sizer and GeneratorSizer · WeldingCalc cover the storage and fuel-backup sides.