Insolation

Insolation or sunlight intensity is measured in equivalent full sun hours. One hour of maximum, or 100% sunshine, received by a module equals one equivalent full sun hour. Even though the sun may be above the horizon, for example, 14 hours a day, this site may only receive six hours of equivalent full sun. Why? For two main reasons: One is reflection due to a high angle of the sun in relationship to your array. The second is also due to the high angle and the amount of the earth's atmosphere the light is passing through. When the sun is straight overhead the light is passing through the least amount of atmosphere. Early or late in the day the sunlight is passing through much more of the atmosphere due to its position in the sky.

Our sun trackers can help reduce reflectance but cannot help with the increased atmosphere in the sun's path.

Because of these factors our most productive hours of sunlight are from 9:00 a.m. to 3:00 p.m. around solar noon. Before and after these times we are making power but at much lower levels.

When we size solar modules, we take these equivalent full sun hour figures per day and average them over a given period. See the charts below.

Average and Winter Minimum Full Sun Hour Maps for the USA

We like to work with two figures here: average annual equivalent full sun hours and average winter equivalent full sun hours, In most locations in the United States winter yields the least sunlight because of shorter days and increased cloud cover, as well as the sun's lower position in the sky.

Path of the sun on winter and summer solstice

This diagram illustrates the path of the sun over varying seasons. Remember when selecting a site for your solar modules to pick a spot that is clear of shade from a minimum of 10 A.M. to 2 P.M. on December 21st. Even a limb from a deciduous tree will substantially reduce power output. These are averages, contact us for your exact insolation data.

Many solar sites are quite uncomplicated in terms of shading and aspect. You may already have a good idea of where the sun appears in the morning and disappears in the evening, as well as how low it swings in the winter sky. If your site is partially shaded, it may be necessary to determine exactly where the best placement of modules will be. We do have site analysis tools. If you need a more sophisticated site analysis, please contact us. We also have world-wide insolation data.

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