When we do design work on photovoltaic (PV) solar power systems, one of the first questions asked is usually, “What is the best orientation for the solar panels?”

The rules-of-thumb in the solar industry are that the panels should face toward the equator and be mounted at an angle of inclination (angle between the ground and the panel) equal to the degree latitude of the installation site. Here in Seattle that would mean the panels face due south and they are mounted at an angle of inclination of 47.5°.

These rules-of-thumb provide the orientation that over the course of the year will result in the panels most frequently being perfectly normal to the sunlight. Normal here meaning the rays of the sun are perpendicular to the face of the panel.

HOWEVER, and this is a big however, this is assuming a random distribution of sunny and cloudy days. We all know that in Seattle our sunny days come clumped in the summer and our cloudy days in the winter (ok, let’s be realistic, they are pretty frequent in the spring and the fall too).  How does this weather pattern affect the ideal PV panel mounting orientation?

To investigate we used the PV WATTS tool developed by the National Renewable Energy Laboratory. This program lets you select a location and orientation for a PV system and predicts electricity production based on historical weather data.

First let’s vary the direction the PV system faces, but hold all other variables constant. North is considered 0°, east is 90°, south is 180°, and west is 270°. The vertical green line on the graph represents the “rule of thumb” orientation. 










As you can see in the graph above, this rule of thumb is pretty accurate. The maximium electricity production comes from the system is facing 180-190°, which is pretty much due south. The only reason you would likely vary from this orientation would be if you were in a location where there was consistently more sun during one portion of the day than during another (think morning fog in San Francisco). Evidently here in Seattle, we have on average a pretty even distribution of sun during the course of a day.

Next let’s vary the angle of inclination for the system, but hold all other variables constant. A panel mounted horizontally (flush on a flat roof) has an angle of 0° and a panel mounted vertically (flush on a wall) has an angle of 90°. Again, the green line represents the rule-of-thumb orientation. 


Here you can see that the rule-of-thumb is less accurate. The ideal angle of inclination in Seattle is about 35°. Logically this makes sense because it means the panels are tilted to be more ideally oriented during the summer, when the sun is high in the sky. There is, of course, a penalty to be paid during the winter months, but frankly, we aren’t missing much sun during that time anyway. 

The moral of this story is that in Seattle, a good starting point for design is to mount PV panels facing south at an angle of inclination of 35°.