Residential Solar Site Measurements: Page 5 of 7

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Although analysis using aerial imagery is increasingly useful in the early sales process, site visits provide critical information for solar installation companies. Being on-site enables sales representatives, auditors and designers to capture accurate dimensions and spot obstructions that may not have been apparent from aerial photos. Vent pipes, for example, are difficult to resolve in most aerial images and can significantly impact where modules can be placed.

Sun Paths and Shade Impacts

The sun’s azimuth and elevation angle relative to the horizon vary with the time of day and year. Obstructions that overlap with the sun’s path cause shade during the time and month when that overlap occurs. Shading has a disproportionate impact on PV production, reducing a system’s output power up to 30 times more than the relative size of the shadow on the array, according to Chris Deline, an engineer at the National Renewable Energy Laboratory (NREL). The lopsided nature of this dependency comes from the fact that cells are connected in series and that shading a substantial portion of just one cell is enough to trigger the associated bypass diode, temporarily eliminating the production of that module substring.

Optimizing string configurations relative to shading objects can mitigate their effects to some degree. Microinverters and dc power optimizers provide MPPT at the module level, which helps reduce shade impacts. However, a shaded module, regardless of whether it has per-module MPPT, produces less energy and therefore is a less valuable asset. Ryan Mazelli, senior solar advisor for One Block Off the Grid, a collective system purchasing provider, comments: “Requirements for shade measurements should not change if systems utilize microinverters, power optimizers or ac modules. What these products achieve is slightly better performance in partial shade conditions. Shade is shade, and a panel in complete shade is not going to produce any power.” Installers and investors should not underestimate the importance of accurate shade measurements and mitigation approaches, regardless of the technology employed.

Once on-site, you can verify roof dimensions and the location of obstructions such as skylights and vent pipes by using a tape measure, wheel or laser range finder. Tilt angles can be verified with an inclinometer with accuracies within 1 to 2 degrees. On rough roof surfaces, such as architectural shingles, tilt measurement accuracy can be improved by extending the footprint of the inclinometer: Place it on a length of wood or measure tilt on a rafter extending under the eave.

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