Off-Grid Cabin System Design: From Load List to Array Size

Every off-grid system that disappoints was sized from an optimistic load list. Build the list in watt-hours per day for each appliance, then multiply by the days of autonomy you actually want. A refrigerator rated 1.2kWh/day in a 21°C kitchen can be 2kWh/day in a 30°C cabin - the difference decides whether you need another 2 panels or not.

At northern latitudes, December production can be a quarter of June. The array has to be sized for the worst month, not the average, and the battery has to carry the difference through the worst week - which is why off-grid systems look oversized to anyone sizing from annual averages.

1. 1

   Build a seasonal load list

   List every load in Wh/day, then adjust for the season of highest use - often winter for heat and lighting.

2. 2

   Choose autonomy days deliberately

   2-3 days is common where a generator exists; 5+ where it does not. Autonomy is the single biggest cost driver.

3. 3

   Derate the array for the worst month

   Use a production model, not a rule of thumb; tilt, shading and snow all matter more off-grid than on.

4. 4

   Size the controller on output current

   A 60A MPPT at 48V supports far more array than at 12V. Current defines the controller, not watts.

5. 5

   Design the generator as backup, not primary

   Choose a charger that can carry the daily load efficiently; running a generator at 20% load wastes fuel and shortens engine life.