# Calculate Sun Position for Solar Gain

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In full sun, about 100 watts of solar energy per square foot reaches the Earth’s surface.
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As the sun's vertical angle changes with the seasons, its azimuth changes with the time of day.
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# On-Site Solar

When calculating solar energy, how much is available is determined by your position on the Earth and cloud cover.  There are tables that will tell you how many sunny days per year in your area.  Further calculations will tell you exactly how many hours of sunshine are available per year, when factoring in the sunrise and sunset times.  Next you must reduce this further by the angle that your solar collectors, either solar-electric or solar hot water, face towards the sun. There is an instrument, similar to a surveyor’s azimuth, called a Solar Pathfinder that will help you orient the collectors correctly.

The nominal angle for solar collectors is equal to your site’s latitude.  However, due to the tilt of the Earth’s axis, during the year the vertical angle to face the sun will change by plus and minus 23.5 degrees.  For example, if your site is located at 45 degrees latitude, then on December 21st (winter solstice) the angle to the sun will be 21.5 degrees and on Jun 21st (summer solstice) it will be 66.5 degrees, only on March 21st and September 21st (spring and fall equinoxes) will the angle be match the latitude (in this example 45 degrees).  That is a range of 47 degrees up and down throughout the year.  East and west, the solar azimuth varies 180 degrees each day during the equinoxes, and varies from non-existent to always on at the Poles and some where in-between for all the other latitudes during the solstices.

As the sun’s vertical angle changes with the seasons, its azimuth angle changes with the time of day.  The bad news is that when a solar collector is disoriented from the sun by 45 degrees either east or west or up or down, the energy received is 25% less.  Further, energy will be reduced by nearby structures or trees that overshadow the collector.

A sun tracking mechanism can be used to maximize the energy collected from solar collectors.  The benefit in energy gained by optimum sun tracking has to be weighed against its maintenance and the power to operate it.  Manual tracking is too labor intensive to capture all variations of the sun angle per the time of day, but it does minimize the loss per the seasonal angle changes.  A simpler idea is to hinge the collectors and manually adjust the angle weekly as the season changes.

A way to estimate total hours of collection per year and account for the loss due to time and seasonal misalignment between the sun and the collector is to only count the usable hours of solar collection where the collector is at 90% or more of its potential.  Going back to the example of a collector located at 45 degrees latitude and angled at the same degree with no tracking or seasonal adjustment, it will be effective about 9 hours per day on the summer solstice and 5 hours on the winter solstice, so average that over a year and multiply by the percentage of sunny days over cloudy days multiplied by 90% to get the total hours of collection per year.  Now multiply the total hours by the capacity of the collector and you’ll arrive at the KWH or BTU/hr that can be obtained per collector.