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 sun hour. Even though the sun may be above the horizon 14 hours a day, any one site may only receive six hours of equivalent full sun. Why? For two reasons. One is reflection due to a high angle of the sun in relationship to your array. The second also due to 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 its position in the sky.
The numbers listed on these maps are the average worst case insolation hours. Insolation hours provide a way of predicting the output of a solar module at a specific location. This data has typically been gathered over a number of years from weather stations located throughout the world. For example, if you have a 60W solar module that produces 3.5A peak power in a location that has 3 hours of insolation, it can be said that the 60W will produce 10.5AH a day. A word of caution. This insolation information should be used only for estimates. Solar systems should not have a final design based on this information. This map does take into account small climate changes and may not be 100% accurate for all locations. ASP has a vast resource of databases that have specific cities listed with accurate insolation data. Please allow us to verify any critical design before purchasing.
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 at 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 chart above. 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.