How to Position Your Solar Energy System for Maximum Output

Understanding how much sunlight falls on your solar system is the key to success. Positioning your solar collectors correctly in the solar window will ensure you get the most from your investment.

| May 2016

isogonic map

Isogonic map of the United States.

Illustration courtesy New Society Publishers

Of all the renewable energy options open to us, solar hot water heaters have been around the longest. There are several designs to choose from, and selecting the best solar collector can be confusing. Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems, by Bob Ramlow and Benjamin Nusz (New Society Publishers, 2010), is the definitive guide to this clean reliable technology. In this excerpt, the authors explain how to site your solar energy system properly by determining the solar window of your location using tools like the Solar Pathfinder and an isogonic chart.

You can purchase this book from the MOTHER EARTH NEWS store: Solar Water Heating.

Siting a Solar Energy System

Sometimes it seems silly to state the obvious, but here goes: solar collectors must be in the full, direct sun if they are going to work properly. This may seem obvious, but you wouldn’t believe how many times we’ve had prospective customers tell us there is lots of sun where they are thinking of placing the solar collectors, only to find the spot in considerable shade. So, what is the bottom line for choosing a location for solar collectors? Read on.

First, let’s review how the sun shines on the Earth. The relative movement of the sun across the sky determines our days as well as our seasons. The path of the sun changes every day. Here in Wisconsin, on the first day of spring (March 21), the sun rises directly in the east; at noon it is directly to the south and is above the horizon at about a 45 degree angle, and it sets almost directly west. On that day, the sun is up for 12 hours and down for 12 hours. On the longest day of the year, the first day of summer (June 21), the sun rises in the northeastern sky, is directly overhead at noon (68 degrees from horizontal) and sets in the northwest. The first day of fall (September 21) is exactly like the first day of spring. The shortest day of the year is the first day of winter (December 21), and the sun rises in the southeastern sky, is directly to the south at noon and is about 23 degrees above horizontal (low in the sky) and then sets in the southwest. The height of the sun, in degrees, will vary depending on your geographical location. The further north you go, the lower the sun will be during the winter months.

Generally, solar collectors should face within 30 degrees of south, be mounted at an angle to the sun that will maximize their performance and be in the direct sun (no shading at all) from 9 a.m. until 3 p.m. It is between these hours that a fixed point will receive 80–90 percent of all the solar radiation it receives over the whole day. Some solar installers advocate for full sun only from 10 a.m. until 2 p.m. In some cases, this will be suitable, but for optimal collection, you should try to have full sun between 9 a.m. and 3 p.m.

To get the maximum performance from your solar energy system, the solar collectors must face the sun at noon. Both early and later in the day, the energy coming from the sun must pass through more of the atmosphere than during the middle of the day. This atmosphere is full of dust, water vapor and moving molecules. All this stuff in the air interrupts and weakens the energy flow from the sun to the Earth’s surface. The less atmosphere the sun’s energy has to pass through, the stronger it will be. It may seem elementary, but solar collectors need to be in the full sun for most of the day. They will not work if the direct sun must pass through trees or other vegetation, even if the leaves are gone. A tree without leaves will block up to 75 percent of the sun’s energy.

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