Sun Path for Portland, Maine; Latitude 43.7°N
The area of sky open to sunlight for a site is called the “solar window.” The upper limit of the solar window is the sun’s path on the summer solstice—the day of the highest arc that the sun will make. Assuming no obstructions, the path of the sun on the winter solstice is the lower limit of the solar window.
The east and west edges of the solar window are not defined by the horizons, as one might expect. While the sun travels through the width of the sky, the amount of radiation that reaches the earth during early morning and late afternoon is limited due to the increased thickness of atmosphere that it must pass through. Instead, the edges of the solar window are typically considered 9 a.m. to 3 p.m., solar time. This six-hour window contains a majority of a site’s solar radiation. It is preferable to have no shading within the solar window, since shading during this time will most adversely affect system production.