Solar modules perform best when perpendicular to the sun's rays. Because tracking the sun is not always possible, we typically mount the modules facing due south.
A common question is the effectiveness of facing one module to the southeast, one due south and another southwest. While this may sound like a good idea, it is not. All modules facing due south will net the largest amount of power of any other arrangement second only to a sun tracker.
Remember that the true south and magnetic south vary upon your site's declination. Call your local airport or us if you do not have this figure or browse the National Geophysical Data Center (external link).
Because the sun's position in the sky varies through the year (higher in summer and lower in winter), it's a good idea to provide for seasonal adjustment. The rule of thumb goes: latitude plus 15 degrees angle in winter and latitude minus 15 degrees in summer. Your latitude can be found on any good map of your area.
If you wish to permanently mount the modules and not seasonally adjust the structure, fix your mount at a winter (minimal sun period) angle. This is when sunlight is limited, days are shorter and you want the system maximizing the available power. We offer a wide variety of mounts both fixed and tracking.
To Track the Sun... or Not To Track...
Trackers are used to increase the daily output of PV modules by keeping them faced as directly as possible toward the sun. The sun sees a wider surface, and the increased reflectivity that occurs at low angles of incidence is avoided. During the long days of summer when the sun is rising north of east and setting north of west, a tracker can increase the daily output of modules by 25 to 40 percent (we can help determine what you can expect). During the winter when the sun takes a low, short arc above the horizon, the tracker will contribute much less, perhaps 10 to 15 percent. The output of a tracker remains much more constant throughout the year in tropical climates.
We generally recommend trackers for spring, summer and fall applications, such as water pumping for livestock summer pasture or small scale irrigation. For home power systems, we often do not recommend them because a household's power requirements are generally greatest in the winter just when the efficiency of the tracker is least. It often is a better choice to use a less expensive static mount and put the money into extra modules. In tropical and subtropical regions with less seasonal variation of sun and loads a tracker can make sense for a home system.
When calculating aiming error, rule of thumb is that a 10 degree aiming error will result in a loss of 2% of the solar module output, 20 degree-6%, 30 degree-14%, 40 degree-22%, 50 degree-35%, 60 degree-50%.
This table below compares insolation for fixed and tracking surfaces at three U.S, cities of varying latitudes. We have data for many locations broken down by the month, call if you would like the figures for your area.
|Fixed Array||Fixed Array||One Axis Tracking||One Axis Tracking||Two Axis Tracking|
|Summer position||Winter position||Summer||Winter|
|latitude -15 deg.
||latitude +15 deg.||latitude -15 deg.||latitude +15 deg||E&W, N&S|
|Great Falls, MT|
|Values are equivalent full sun hours per day.|
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