English: Edit-- The following information is no longer correct for 2020+ solar panels as their efficiency in lower light conditions have significantly improved. ) Optimally a south facing roof should be used for mounting photovoltaics, but if the gable runs north south, it does not matter if the panels are mounted on the east roof or the west roof, using net metering. If they are mounted on both roofs, though, the hourly output is more consistent with south facing panels. The output over the course of the year is about 75% of a south facing roof, and your power company will thank you if you mount them on the west facing roof because the output will occur closer to peak demand. The output in the winter is greatly reduced, about 40% of what a south facing roof would provide, but the summer output is close to what a south facing roof would provide. As the graph shows if panels are mounted on the east and west facing roofs, the output is greater in the morning and evening than for a south facing roof, and less at noon. In the winter there is very little additional output in the morning and evening from the east-west facing roofs. To draw the graph, the total output over a two week period in January was compared with a two week period at the end of July and the beginning of August.