What size solar system for an off-grid cabin?
Panel watts, battery bank, inverter, and charge controller, from daily watt-hours.
The formula, once
Everything flows from one number: daily watt-hours. Array watts = daily watt-hours ÷ worst-month sun hours ÷ 0.86 (NREL PVWatts' default 14% system losses: soiling, shading, mismatch, wiring, availability; not inverter or battery losses). 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.
The refrigerator preset, through the calculator
The refrigerator row in assets/app.js is 150 W × 8 h = 1,200 Wh. At 4 sun hours, 1,200 ÷ 4 ÷ 0.86 = 348.8 W, which sizeSystem prints as 400 W. At 2 days and 80% on a 24 V bank, 1,200 × 2 ÷ 0.8 = 3,000 Wh, and 3,000 ÷ 24 = 125 Ah, which the 10 Ah step prints as 130 Ah. The controller line is 400 ÷ 24 ÷ 0.85 = 19.6 A, listed as 20 A. A cabin day is the sum of the loads you run. This preset is one row, not that sum.
Common sizing mistakes
Heat loads: keep space heat, water heat, and dryers on propane or wood unless you have measured their watt-hours. A heat-pump annual-kWh example is on how many solar panels for a heat pump. Oversized banks, undersized arrays: the array has to replace daily use. Summer-sized arrays in winter: size to worst-month sun hours, or plan generator charging. The battery bank sizer and GeneratorSizer cover the storage and fuel-backup sides.
A flat 5 kWh/day total, without the cabin load list, is sized on what size solar system for 5 kWh per day.
Rack tilt, rear clearance, and who certifies the mount are on how to mount solar panels. How a published degradation rate eats into a sized array is on solar panel degradation.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- SolarSizer calculator, assets/app.js — refrigerator preset 150 W × 8 h = 1,200 Wh. Array watts = daily watt-hours ÷ sun hours ÷ 0.86, rounded up to the next 100 W. Bank watt-hours = daily watt-hours × days ÷ depth of discharge, then amp-hours rounded up to the next 10 Ah. Controller amps = array watts ÷ bank volts ÷ 0.85, then the first of 10, 20, 30, 40, 60, 80, and 100 A at or above that current. The form's depth-of-discharge options are 80% and 50%. Accessed Oct 6, 2026.
- NREL PVWatts Version 5 Manual, A. Dobos, September 4, 2014 — Table 6, 14% default system losses; 1 − 0.14 = 0.86. Accessed Oct 3, 2026.
1,200 ÷ 4 ÷ 0.86 = 348.8 W, printed as 400 W. 1,200 × 2 ÷ 0.8 ÷ 24 = 125 Ah, printed as 130 Ah. 400 ÷ 24 ÷ 0.85 = 19.6 A, listed as 20 A.