The most common professional siting tools—the Solar Pathfinder and Solmetric SunEye—can help determine a particular site’s solar access or solar “window”—the average amount of sun that falls on a particular site, usually between 9 a.m. and 3 p.m. Maintaining shade-free access during this time is necessary to maximize system production while sunlight is at its peak. Solar siting tools allow quick comparisons between multiple locations on a property to determine the optimal location for system production, reliability, and overall aesthetics. Note that there are also some smartphone apps available, such as Solmetric’s IPV (for iPhone), Comoving Magnetics’ Solar Shading (for Android), and Onyx Solar Energy’s app (iPhone & Android).
Both of the pro tools, however, have optional software for analyzing the data and creating a detailed report. Standard software features include the ability to overlay multiple points of data, like a “four corners” evaluation to determine the overall solar access for the area. The data can generally be presented several ways, graphical or numerical. The software also allows users to digitally remove obstacles, such as trees or chimney vents, from the captured image and will recalculate the solar window and estimated energy production.
Evaluating a site’s solar access requires capturing data points for the four corners of the proposed PV array. You can either use a tripod or a beanbag to create a level surface for the siting tool. Obstacles and trees adjacent to the building may require additional readings along the perimeter of the proposed array location. Protrusions, such as chimneys and vents, on the roof can be quickly evaluated using the siting tool to determine a “shade-free” distance from the obstacle. Using a piece of roof chalk, the shade-affected area can be marked around the protrusion. The resulting shaded area can then be recorded on the roof layout part of the site survey. Mapping roof obstacles beforehand can minimize or eliminate having to make expensive redesign decisions during the system’s installation.
While on the roof, analyze the condition of the roof surface and structural integrity of the rack attachment area. A roof’s estimated age and thickness should be documented. Identifying signs of deterioration and damage varies depending on the roofing material. Having an experienced roofing contractor evaluate the roof can be money well-spent.
Noticeable waviness or spongy spots in the roof warrant further evaluation of the structure. Installing PV arrays on older roofs can later result in expensive removal and reinstallation of the array. Failure to recognize potential issues, such as compromised structural elements, can pose significant hazards if attachment points are stressed during large windstorms and other weather-related events. When attic access is available, inspecting and mapping the rafters is recommended.
Local weather considerations can also influence design choices and should be recorded during the evaluation process. In heavy snow areas, knowing the expected snow loads on the roof is necessary to select an appropriate racking product rated for the conditions, as well as determining the rail span distance between attachment points. Snow loads can also dictate module selection, since modules have specific static ratings. Wind-loading also must be considered for equipment placement and selection for many of the same reasons. Most rack and module products have wind load ratings that range between 90 mph and 120 mph. The perimeters of most residential roofs are exposed to greater wind forces than interior portions of the roof and may not be usable in many locations with high wind loads. This can limit array placement and size, as can required roof setbacks for firefighters (see “PV Array Siting & Mounting Considerations” in HP155). The local building department is a good place to glean information on expected snow and wind loads.