Siting a Solar Energy System

— Bob Ramlow with Benjamin Nusz

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 prospective customers have told me that there is lots of sun where they are thinking of placing the collectors, only to find the spot in considerable shade. So, what is the bottom line for choosing a location for our solar collectors? Read on.

First, let's review how the sun shines down on the Earth. The movements of the sun across the sky determine our days as well as our seasons. The path of the sun changes every day. Here in Wisconsin (and everywhere on the planet), 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 around a 60 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 (90 degrees from horizontal) and sets in the northwest. The first day of fall (Sept. 21) is exactly like the first day of spring. The shortest day of the year is the first day of winter (Dec. 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 between only 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 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.

Collectors should face as close to true south as possible, but a variation of up to 30 degrees is generally acceptable and will not significantly reduce panel performance. To find south you first have to find north and then look the other way. However, compared to a compass reading of north, true north is not the same everywhere. To find true north at your particular location, drive a stake vertically into the ground and watch its shadow. When the shadow reaches its shortest length, it is pointing exactly north. Unless you have some time to sit around staring at a stake in the ground, you probably want an easier way to find true south.