Off-grid solar size calculator

Add the watt-hours of everything you run per day, divide by your location's worst-month sun hours, then add 25% for panel derating (heat, dust, wiring losses). A typical minimalist cabin using 3 kWh/day needs about 1,200–1,500 W of panels in most of the US, paired with a 400–600 Ah 12 V LiFePO4 bank (or the 24 V/48 V equivalent) to ride through cloudy days.

A 200 Ah 12 V battery stores 2,400 Wh. To recharge it in one day of sun you need roughly 400–600 W of panels: 2,400 Wh ÷ 4.5 average sun hours ÷ 0.85 charge efficiency ≈ 630 W, so a 2×300 W array covers it in a day and a 400 W array in about a day and a half of good sun.

MPPT harvests 20–30% more power than PWM in cold or partly cloudy conditions and lets you use a higher-voltage array with thinner wire, but costs 2–3× more. For arrays above 200 W, or in winter-heavy climates, MPPT pays for itself. Size it: array watts ÷ battery voltage ÷ 0.85 efficiency, rounded up to the next standard rating (20 A, 30 A, 40 A, 60 A, 80 A).

Total the continuous watts of everything that could run at once, then add the surge of the biggest motor or compressor load (3–4× its running draw). Buy 25% above the continuous total, pure sine wave only. A fridge (150 W running, 600 W surge) plus lights and electronics usually lands at a 1,000–1,500 W inverter; add a microwave or well pump and you're at 2,000–3,000 W.