How many Sun Hours do you average per day?
This is based on an annual average. Depending on your location and energy usage patterns, you might need a larger system to avoid using a backup generator excessively during the winter months.
You can also look at the Solar Map to find this number.
Typically 4-6 hours, most of the US receives an average of around 5 sun hours per day.
*An overestimation of sun hours will result in an incorrect system calculation.