Do I have a Good Site For Solar? Part 2

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.


Do you have a home, business or cabin that might be a good candidate for a solar energy system? There a number of easy things you can look for as you start sizing up your site for solar. This is our second installment of a two part blog discussing “rules of thumb” good Upper Midwestern installers use when analyzing a site for PV. In Part 1 (Solar Panel Positioning) we covered Rules 1, 2 and 3 focused on how panels are best positioned. Here we’ll give you three more rules of thumb on how to think about shading. Together, the six rules of thumb can help you confirm that you have a great site for solar!

Rule of Thumb #4: Panels are best shade-free from 9 am to 3 pm every day.
The Ideal Site: PV systems with panels oriented directly south and in a location that is shade free from 9 am to 3 pm every day of the year are the most productive and so are the most cost-effective. Some sites that do not quite meet this ideal may still be great for solar but since they will produce slightly less energy overall, they will be proportionally less cost-effective. Every hour your panels are shaded is an hour where you are not producing power. The equipment costs the same either way. A worthy goal is to locate panels where they will produce at least 90% of their annual potential compared to a 100% unshaded site. In fact, it’s common for utility companies in our area to require a system to be 90% “shade-free” before it can qualify for rebates. A system installed at almost any site will produce at least some power but for the most cost-effective results, good orientation and minimal shading are essential.

Rule of Thumb #5: There’s no such thing as “a little shade!”
Panel Shading: For PV panels, even a little shading can have a serious effect—more than you might think! A panel experiencing shading on only 1/10th of its surface may see its power harvest reduced to almost zero at that time. Even what may seem like an insignificant shadow from, say, a leafless branch in the winter, or a 2" rooftop vent pipe, can have a major effect. Remember, only while a panel is fully lit by the sun is it going to be productive.

Rule of Thumb #6: Minimal, occasional shading may not be a disaster.
Seasonal Shading: Most residential sites in the upper Midwest have at least periodic shading by one or several trees or a neighbor's roof, especially in the winter when the sun is low all day long. The good news is that if this condition is limited to only one or two winter months this occasional shading may not be too serious. It might only reduce your total annual production by a few percent. NOTE: If the offending tree is shading the panel location for more than an hour between 9 am and 3 pm during any non-winter months, that shading may be more significant than we are meaning here. But now we're getting into details best measured by shading analysis tools and an in-person site visit by a solar expert. The point is that panels need to be in clear sun to be productive but occasional and limited shading may be tolerable.


As you probably suspect, it's easy to get bogged down in the fine points of site evaluation (especially when it comes to shading). It's your installer's job to calculate the effect on production by things like your roof orientation or occasional shading by a neighbor's tree. Be aware, many sites are neither great nor terrible for solar—for those sites, our rules of thumb won't provide any clear direction. Typically, systems installed at marginal sites are not very cost-effective. But if you are lucky enough to have a great site, these six rules should affirm it and encourage you to get started on your solar energy project!

Do I Have a Good Site For Solar? Part 1

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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