Summer is the time for planting gardens, landscaping and construction. Do you find yourself asking, are my Raspberries getting too much sun? Are my Tomato’s getting enough sun? Where am I going to put my house to maximize passive solar heating? How many trees do I have to cut down to put up photovoltaic panels? Only by knowing the path of the sun across the sky can these questions be answered (Fig. 1). Perhaps the simplest method of determining sun exposure at a site is to sit and observe, from dawn ‘til dusk, the spot where you are. As this method of determining shading requires a day of patient observation for every potential site, I highly recommend it. A day of quiet reflection in nature is good for the soul. For those with no time and lots of money, there are several tools commercially available for instantly generating this information at a glance. My favorite tool is the Solar Pathfinder. The Solar Pathfinder gives a detailed picture of how much shade a spot gets for every hour of the day, every day of the year but costs around $200. However, for the home gardener or do-it-yourselfer, a close approximation can be created for nearly free with a plastic protractor and a compass. The protractor method provides a relatively quick answer without spending $200 for the Solar Pathfinder. All that is required is a plastic protractor, 2 feet of string, a small fishing weight, pocket compass, pencil, paper and a little patience.
The first requirement of solar site selection it to put away the topographical map. Pull on your boots and go out to your potential site with your compass in hand. Find south; around here magnetic south and true south are close enough for most applications. Farther west, or up north, a local airport or weather authority should be consulted for the “magnetic deviation”. Magnetic deviation is the correction angle to add to compass reading to get true south. Scan the southern horizon from east to west. Are there any obvious permanent obstructions to the sun? Is the Victorian Bed and Breakfast next door completely blocking your sun? O.K., determine the borders of your project. If all the corners and southern edges are getting sun then the center of the plot will also. If you are building a home with a long southern exposure, be sure to check many points along the proposed southern edge. I often find that spots only 30 feet apart can have radically different sun exposure. Tie one end of the string to the fishing weight. Tie the other end of the string to the hole in the middle of the straight edge of the protractor. I prefer clear protractors so I can highlight my desired angle with a permanent marker. Holding the protractor at your desired angle, look up the straight edge of the protractor. Be sure to practice sighting a few times, holding the protractor high enough above eye level that you are actually sighting the line determined by the protractor (Fig. 2). For summer exposure, an angle of 32 to 37 degrees is adequate. To capitalize on the winter path of the sun, for year round exposure, the protractor should indicate 57 to 63 degrees. Face due east and look at the horizon. This is roughly where the sun rises. Any thing blocking the horizon is shade at sunrise. However, there is not much energy available in the sunlight at sunrise, so don’t worry about early morning shade. The energy available in full sunlight really gets going by about 9:00-9:30 a.m., when the sun gets up into the sky. So, turn about 20 degrees to the south (your compass probably has a degree ring) and sight up the protractor. Be sure your string is indicating the correct angle. Do you see unobstructed sky? Whatever is blocking your view is also shading the spot you are standing at 9:00 a.m. Make a note as to what is shading you, “Large poplar tree” or “West corner of house”, for example. Turn another 20 degrees south, approximately the suns position at 10:00 a.m., and sight up the protractor again. Repeat all the steps until you are facing due west.
You now have a rough idea of what is going to shade that corner of your project and at what times of the day you will have full sun. The period of the day when the most energy is available is called the ideal solar window. The ideal solar window is the six-hour period from 10:00a.m. to 4:00p.m. and is the most critical to the gardener or homebuilder. The notes you made give you an idea of the obstructions you would have to remove to achieve the ideal solar window. Repeat the sighting and recording process at all corners and southern edges of your site to get a clear picture of sun exposure.
The sun is the ultimate renewable energy source to power our nation and the world. To give you an idea of how much power is available from sunlight, consider the following. An average American home uses 30 Kwh/day. Asheville, NC gets an average of 1400 Btu/sq.ft. per day from the sun. This is approximately 0.5 Kwh/sq.ft. per day . Because an acre of land is 43,560 sq. ft., it will receive 21,780 Kwh a day. Therefore, an acre of land in Asheville, NC receives enough energy from sunlight to power 726 average homes. This is an enormous untapped source of clean, renewable energy. Please preserve our shared environment and harness the sun to meet your energy needs wherever possible.

Arthur Dougherty attended UNCA and recieved a BS in Environmental Science. He worked in air emissions testing until starting Solar Guys, a local renewable energy dealer/installer. Contact him at 828-273-4000.

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