This article is focused on residential solar electric (PV/photovoltaic) systems in Minnesota and other northern states in the U.S. Some of these principles also apply to solar hot water and solar air heating systems.
If you are wondering about a solar energy system for your home, business, or cabin, a great place to start is to understand some of the features that make one panel location and position more productive than another. Of course, the more productive a system is, the more cost-effective it will be. In this two part blog I briefly cover six “rules of thumb” good Upper Midwestern installers use when evaluating a site. Part 1 (Solar Panel Positioning) focuses on the best orientation and tilt for your panels. I’ll also show you a few mounting approaches than can improve panel positioning in cases where a roof is less than ideal. In Part 2 (Solar Panels vs. Shade) of this blog, I’ll give you some rules of thumb about shading—in some ways, even more critical than panel positioning. Together, these two articles can help you confirm that you have a great site for solar!
Rule of Thumb #1: Panels are best oriented within 45 degrees of south.
Panel Orientation: “Orientation” is the east/west direction a panel faces (or “azimuth”). Because a solar panel is most effective when the sun’s rays strike it at a perpendicular angle (that is, square-on), the very best panel orientation is straight south. That way, the panel experiences the sun square-on during the brightest part of the day (noon). It turns out that panels can actually be oriented off of south by as much as 45 degrees with only minor reductions in annual production (1-3%). They receive almost as many hours of the sun’s rays striking the panel square-on as panels oriented directly south. However, when panels are turned to a position more than 45 degrees away from south, production begins to drop off more. At 90 degrees off of south (straight east or west), reductions can be 15-30%. This is because during most of the day these panels are lit by the sun at a glancing angle and experience fewer hours of the sun’s rays striking the panel square-on. When the sun actually is square-on to these east or west facing panels, its rays are weakest. These panels will certainly produce some energy--just less so than if oriented more ideally.
Rule of Thumb #2: Best panel tilt angle is “latitude” (but aesthetics usually comes first).
Panel Tilt: Of course “tilt” describes how steeply the panel is mounted (slope, pitch or “altitude”). As I mentioned in the panel orientation paragraph above, the closer a PV panel surface is to perpendicular to the sun’s rays, the greater the energy harvest. Since the sun is high in the summer and low in the winter, a rule of thumb is to consider your latitude (in degrees) as your ideal tilt angle because that happens set the panel halfway between the ideal winter and summer sun angles. In the Minneapolis area, that makes it about 45 degrees. One of the advantages of a steep tilt like that in our area is that the panels will naturally shed snow better and will suffer smaller losses in the winter. But aesthetics strongly comes into play here: Most people agree that panels on a sloped roof look the best when “flush mounted” (parallel to the roof). Since we expect PV panels to be up there for 25-40 years or more, they do need to look good and the reality is we usually install them flush mounted on sloped roofs. This means if your system is flush mounted on your shallow sloped roof (say, 4:12 or 5:12 pitch), compared to a “latitude tilt” your production will be slightly less on an annual basis—not a disaster but something to be aware of. In this case, your production will be higher in the summer than a latitude tilt system, but lower in the winter.
Rule of Thumb #3: To get the best sun, consider all your mounting options.
Alternate Mounting Solutions Can Improve Production: If your roof is oriented the wrong way or has shading issues, there are sometimes other options.
On an east or west facing roof, panels can be mounted on racking that lifts one side up so they are tilted toward the south (See Example B in the Mounting Solutions Diagram). It's not the prettiest configuration but if located on a less visible part of the roof or with owners for whom production trumps aesthetics, this can be an effective solution. A nice alternative for some sites is to mount panels in an "awning style" on the south side of the building (Example C). And don't forget, solar panels don’t have to be mounted on the building at all--sometimes there is another sunny spot on the property that can work well. One popular approach is to create an attractive shading structure (pergola or car port) (Example F) using solar panels as the roof/covering. Another is to mount panels in groups on poles or horizontal racking in the yard or nearby field (Examples D and E).
Using these three rules of thumb above will give you a good start in determining if there a great spot on or near your building for panels. The other critical aspect in your evaluation has to do with shading of the panel location. In Part 2 (Solar Panels vs. Shade) of this blog I’ll give you three rules of thumb about what to look for regarding shading at your site.
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