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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.

The latitude at the equator of the earth is zero degrees. This is where sunlight strikes the earth most directly. Due to the earth's curved shape, sunlight hits at a various angles depending on location. As latitude increases, the further you are located from the equator and more variance you see in sunlight hours. The latitude of Friday Harbor is 48.5.

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.

The sunrise and sunset is very predictable every day, but things like cloud coverage and weather are less predictable and vary daily and annually. If a location that is usually very sunny experiences a huge increase in storms and cloud coverage in a given year than the average peak sun hours for that day or year will probably decrease. Tall objects such as trees and buildings can block out the sun during high solar insolation times, so be sure to strategically place your solar panels to get better efficiency.

Since we know the latitude of Friday Harbor we can take the average amount of total sunlight hours and estimate that with a fixed solar panel there would be an average of 4.2 peak sun hours per day. 5 hours per day with a 1-axis tracking mount that tracks the sun from sunrise to sunset, and 6.4 hours with a 2-axis tracking mount that tracks the sun everywhere in the sky.