North, South, East, West - Which direction is best for solar production? Why?

Solar production varies based on where you are at in the country. In the United States (and the northern hemisphere), south facing panels perform best. The further north you go, the more pronounced this is. This is due to the angle of the sun throughout the day, averaged throughout the year. 

Typically, south will maximize your panel performance in your given geography. East and west facing panels will typically see a reduction of 15-20% when compared to south, and north being 30-50% reduction. 

Sometimes, strategy comes into play. If you are optimizing for time of use, south west might be a good orientation to optimize for, roof permitting. This is because the sun sets in the west, and most people use the bulk of their power in the late afternoon or evening.