Off-Grid Solar Calculator: Size a Cabin, RV, Shed, or Tiny Home System

Start with a load list, not a kit size

The best off-grid system is built backward from your loads. List each appliance, the watts it uses, and how many hours per day it runs. Then add a margin for real life. A cabin with a refrigerator, lights, router, water pump, microwave, and occasional tools is very different from a weekend hunting cabin with only lights and phone charging.

Common loads people forget

• Well pumps, pressure pumps, and sump pumps
• Refrigerator and freezer startup surge
• Internet modem, router, cameras, and device chargers
• Propane heater fans and control boards
• Microwaves, coffee makers, induction plates, and power tools
• Idle draw from inverters and always-on electronics

Small continuous loads can quietly become large daily energy users because they run all day.

Panel sizing for off-grid systems

Your panels must do two jobs: cover daily use and recharge the battery after cloudy periods. A system that barely covers average use may work in perfect weather but struggle in winter or after several rainy days.

Use a higher multiplier if panels will be flat mounted, shaded part of the day, used in winter, or installed in a lower-sun location.

Battery autonomy matters

Autonomy is how long the system can run without meaningful solar recharge. One day of autonomy may be acceptable for a portable setup. Two to three days is more comfortable for cabins and critical backup. More autonomy increases cost quickly, so it is worth deciding what loads are truly essential.

Charge controller and inverter basics

An MPPT charge controller should be matched to the solar array and battery voltage. The inverter should be pure sine wave for most household appliances and sized for both continuous wattage and startup surge. If your system is large enough to need a 48V inverter, plan the battery bank and charge controller around that voltage from the beginning.

Best next step

Use the SolarKitGuide calculator to create a first sizing range, then refine the load list. Once the range is clear, compare equipment by voltage class, battery kWh, inverter surge rating, and panel expansion room instead of shopping only by kit headline watts.