An off-grid system must meet the load during low solar generation as well as during sunny hours. Solar array capacity, battery usable energy, inverter power and any generator input need to be designed together. Battery capacity alone does not establish autonomy.
Measure daily consumption
Estimate each load in watts multiplied by hours of operation. Separate continuous loads from occasional and high-starting-current equipment.
Plan low-generation periods
Use site-specific solar data and seasonal conditions. Define the required autonomy and whether a generator or another supply will be available.
Design for recovery
Check that the system can run the loads and recharge the battery after an extended discharge, within charge-current and temperature limits.