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Peak sun hours is one of the most important criteria to examine when considering installing a solar power system. Peak sun hours are different than total sunlight hours in a day because the strength of the radiation of the sun varies throughout the day. For example, during sunrise and sunset the solar insolation from the sun is less powerful than at noon. For a moment of sunshine to be considered a peak sun hour the intensity needs to be at least 1 kilowatt per square meter. This means that a square meter of your solar panel should be receiving 1 kilowatt of energy from the sun. This number is used because it is an amount of light that allows a solar panel to produce output efficiently and not under perform due to lack of sunlight power.

Although you can easily predict sunrise and sunset hours each day to the minute, looking at latitude can help with your solar planning. The closer you get to the equator the closer your latitude gets to zero. Sunlight hours on the equator are consistent throughout the entire year. Places further from the equator can have large variance in daily sunlight. For example higher latitudes can have very long summer days with lots of sunlight and very dark winters. The latitude of Fairfax is 41.9.

Depending on your output needs, to get more out of your solar panels you can either upgrade your technology, buy more panels, or buy different tracking type panels. A fixed solar panel remains fixed at one angle throughout the year. A 1-axis panel will produce more output because it follows the path of the sun from sunrise to sunset to maximize sun exposure. Even more productive is a 2-axis panel that not only follows the sun's path throughout the day, but also accounts for the more subtle sun changes throughout the year with the different seasons.

Peak sun hours are greatly affected by weather patterns. Cloud coverage is a huge factor in peak sun hours per day because heavy cloud coverage will diminish the power of the solar insolation. You can use historical climate data to estimate average cloud and weather coverage, but it will obviously vary slightly from year to year.

By taking the latitude of Fairfax 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 Fairfax one can expect close to 4.8 average peak sun hours per day. With a 1-axis tracking mount you would get 6.6 hours per day, and 6.8 hours per day with a 2-axis tracking mount that tracks the sun everywhere in the sky.