Evaluating Glare from Roof-Mounted PV Arrays: Page 3 of 3

This example illustrates that sunlight reflected off a residential roof-mounted array generally goes skyward, where it is most likely to travel over the top of neighboring buildings. The higher the sun is in the sky, the higher the angle of reflectance. Even if the solar altitude angle in Figure 4 were 0°, the minimum angle of reflection relative to an abutting property would be 50°.

Height of reflection. The distance between a PV array and neighboring properties also factors into a glare analysis. When the angle of reflectance is high, reflected sunlight travels above neighboring residences, even those located close to the array. When the angle of reflectance is at its lowest, the elevation of the reflected sunlight increases linearly with distance. Therefore, the more distance there is between a PV array and a neighboring home, the more likely the reflected sunlight is to travel over the top of that building.

Sunlight reflected off a PV array presents a problem only when the array directs it toward a neighboring property at a height that an observer can see from that property. Therefore, a thorough glare analysis for a PV array determines the height of reflected sunlight, either based on the distance of specific landmarks or at a representative set of distances. Once you know the angle of reflection, x, you can calculate its height, h, at any distance from the array, d, using Equation 1.

h = tan(x) × d (1)

When you apply this calculation at different distances, it quickly becomes apparent that sunlight reflected off a PV array is unlikely to adversely affect surrounding properties in a residential neighborhood. Even if a PV array generates some glare, a neighbor is unlikely to see that glare from the height of a residential building.

Case Study: Belmont, Massachusetts

Belmont, Massachusetts, is an inner-ring suburb of Boston comprised primarily of single- and two-family homes. The typical residence in Belmont is two stories tall. As such, Belmont is representative of many residential communities across the country. Based on local zoning rules and minimum setback requirements, structures on abutting properties are located at least 20 feet apart side to side and distances between structures are always larger front to front or front to back.

A simple two-step process evaluates the glare potential associated with a residential roof-mounted PV array in Belmont. In the first step, determine the altitude angle of the sun at representative times of the day and year and use these data to calculate the associated angles of reflection. Table 1 details the results generated in Excel.

In the second step, use Equation 1 to determine the height of the reflected sunlight at different distances from the array based on the previously calculated angles of reflection. Table 2 details the height of reflection seasonally for distances in the 20- to 50-foot range. For example, the height of sunlight reflected off a PV array at noon on the spring or fall equinox is nearly 80 feet above the array at a distance of only 20 feet from the array; by the time the reflection is 50 feet from the array, its elevation is nearly 200 feet. Note that the height of the reflected sunlight is not relative to ground level but originates at the installed height of the roof-mounted array.

These data clearly indicate that glare from roof-mounted PV arrays is unlikely to pose a problem for neighbors when the sun is highest in the sky. Based on the typical distances between residences in Belmont, Massachusetts, reflected sunlight is most likely to travel over structures on abutting properties. While this simple analysis may not rule out the potential for glare at sunrise or sunset, you can perform similar analyses that take azimuth angles into account. Generally speaking, if a PV array is facing south, incident sunlight at sunrise and sunset arriving from the east or the west does not directly strike the surface of a residential roof-mounted array. By the time sunlight does strike the array directly, its angle of incidence is high enough to preclude the visibility of any glare to neighbors.

Roger Colton / Fisher Sheehan & Colton / Belmont, MA / fsconline.com

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