ENERGY BASICS: Shading and Solar-Electric Systems

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A solar array with partial shading
A solar array with partial shading, which reduces energy production.

There are many factors that can reduce the production of a solar array. Shade is often the biggest.

A PV system’s yield is a large factor in whether the financial investment pays off. Anything that blocks the sun, even if partial shading, will affect output. That’s why accurate shade measurements and system planning are essential to maximizing the system’s production.

Shade can be caused by obstructions such as chimneys, trees, or nearby buildings. The sun’s trajectories through the sky (sun paths) are determined by the array site’s latitude/longitude and the season. The shape and location of a shadow is determined by the shape of the obstruction, the location of the array, and the location of the sun.

A small amount of shade may, at first, seem like a small problem, but because cells within a PV module are wired in series, this enables shade on a cell to block the path of current flow. Thus, a little shade can cause a disproportionate reduction in PV energy production. Bypass diodes are used within modules to limit the impact to a portion of a module, and microinverters and power optimizers can confine the impact to the individual shaded module(s). However, the reduction in energy is still typically many times higher than the percentage of shaded area.

Solar professionals use shading analysis tools, such as a Solar Pathfinder (manual) or Solmetric SunEye (digital), to assess a site’s shading for the entire year. These tools capture an image of the sky, including the horizon and any obstructions, as seen from the perspective of the array. They superimpose the sun paths for that latitude/longitude, identify obstructions in the image, and calculate the solar access.

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