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If you put your solar powered math calculator in your backpack it will turn off from the lack of sunlight needed to power the device. As you slowly open your backpack and begin to let sunlight in, the calculator will eventually turn on when the amount of sunlight is enough to power the calculator. Similarly, peak sun hours refer to the hours of they day where the sunlight is strong enough to power a solar panel. This is different from total sunlight hours, which is simply the amount of hours in a day when there is any sunlight.

The latitude of the location is important for measuring peak sun hours. The latitude determines how much overall sunlight there will be in a day. With a given latitude, time and date, one can accurately determine when sunrise and sunset will occur. Areas with latitudes closer to the equator will have a more consistent range of solar insolation throughout the year. Whereas areas closer to the poles will have a greater variance during the summer and winter months due to their higher latitudes.

Although most things are out of our control, like the sunrise, sunset or the weather, we can control the angle and the kind of mount we use for a solar panel. The angle of the solar panel, if using a fixed mount, should generally be around the angle that is equal to the latitude of the location to maximize output. In the northern hemisphere, panels angles should be lower in the winter months and higher in the summer months As a fixed mount is set, it is ideal to place it at an angle that will capture the most sunlight during the year. A 1-axis tracking solar mount will track the sun across the sky from sunrise to sunset, but the angle will remain the same throughout the day. A 2-axis solar mount will track the sun throughout the day from East to West, but the angle will also change automatically as the seasons change and the angle of the sun in the sky changes. Thus, a 2-axis will have a higher rating of peak sun hours compared to the 1-axis or fixed.

Climate in your geographical region is a major factor that will influence average peak sun hours per year. If you live in a region that does not have a lot of completely sunny days, then cloud coverage will greatly influence solar insolation on any given day. Mountains and trees may also contribute to lower solar insolation if they block the sun from your panels at any given point of the day.

By taking the latitude of Marfa one can get a close estimate of the amount of average peak sun hours per day for the geographical area. It varies with technology and the type of solar panel mount you use, but for a fixed mount solar panel in Marfa one can expect close to 5.3 average peak sun hours per day. With a 1-axis tracking mount you would get 6.9 hours per day, and 7.6 hours per day with a 2-axis tracking mount that tracks the sun everywhere in the sky.