Sunlight is at its greatest intensity when it is incident directly upon a surface. In these conditions, solar panels generate more power than when the sun is incident at indirect angles. Hence, the ideal angle for a solar panel is perpendicular to incoming sunlight. This angle varies based on the time of day, time of year and location.
As the angle of the sunlight on a solar panel deviates from the perpendicular, the solar-panel power output decreases. This is because the average intensity of the light incident on a flat surface area decreases as the angle decreases from 90 degrees. You can visualize this by shining a flashlight on a piece of paper. If you shine the flashlight straight down on the paper, the paper is illuminated at the greatest average intensity. As you angle the flashlight, certain areas of the paper receive less light. On a solar panel, less light equals less power generated.
The sun's maximum elevation in the sky changes depending on latitude. As you get closer to the equator, the sun's maximum angle from the equatorial horizon increases. For example, if you travel south in the Northern Hemisphere, the sun will reach an increasingly higher maximum point in the sky. Consequently, the optimum angle for a solar panel will vary based on latitude.
The sun also changes its path across the sky depending on the season. Its maximum altitude is higher in the summer, lower in the winter. Because of this fluctuation, the optimum tilt angle from the horizontal is equal to the latitude minus 15 degrees during the summer. In the winter, the optimum tilt angle from the horizontal position is equal to the latitude plus 15 degrees.
A tracking system can maximize sunlight intensity over the course of both a day and a year. Such a system adjusts the panel angle based on the sun's position in the sky. While this can be an expensive aspect of a small-scale solar panel system, it can boost power output by more than 20 percent. Large-scale solar power farms with multiple arrays typically use tracking systems to automatically adjust angles for maximum output. Some tracking systems tilt panels along a single, east-west axis. Other systems have two axes in order to adjust for both the east-west movement of the sun as well as changes in the sun's altitude.
- University of Wisconsin-Stevens Point: Insolation
- Weber State University: The Sun and Seasons
- Rensselaer Polytechnic Institute: How Does the Tilt Angle and/or Orientation of the PV Panel Affect System Performance?
- Southeastern Louisiana University: Tracking Solar Panels
- University of Minnesota: Solar Power
- University of California, San Diego: Tilt Scenarios and Yield
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