How to Properly Place an Array

Published December 29, 2010

To properly site an array, keep three things in mind.
1. The orientation (azimuth) of the array is ideally situated to True South here in Tennessee.  Modules facing north do not receive sufficient sunlight to make an adequate investment.
2. The tilt (pitch) of the mounting surface upon which the panels will rest.  A pitch between 20-40 degrees facing south will yield an efficient capture of solar radiation.
 
3. Obstructions to sunlight (shading) is detrimental to the amount of energy an array will generate.  During the winter months the sun is lower in the sky and will cast longer shadows from objects that do not shade your roof in the summertime.
When a LightWave representative performs a site evaluation, they have equipment that accurately determines how much shading an area will receive throughout the year.  This tool is known as a Solar Pathfinder.
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When building “Solar Ready,” keep these items in mind during design phase: 

Surface Area

Allow 100 square feet of unshaded south facing roof per kw of photovoltaics installed.  For a house this usually means around 200 to 600 feet. However remember to allow about 20% of the roof space for access and to pull back form the edge a couple of feet. This means you might consider 80% of available solar space usable under general conditions.  There are exceptions to this rule of thumb.  As noted above keep roof penetrations (plumbing vent stacks, fans,
exhausts, etc.) away from the designated solar south roof area.

 

Slope

A 30 degree slope is optimal for our location, that is a 7/12 pitch.  However 20 to 50 degrees is acceptable, which is a pitch between 4/12 and 12/12.  The shallow pitch will perform better in the summer and the steeper pitch will perform better in the winter.  Another rule of thumb is latitude (36 degrees for Nashville) plus or minus 15 degrees.  Provide a permanent anchor at the ridgeline for roofs over 8/12 pitch to attach a safety rope.  Sometimes panels are installed completely flat or vertical for various reasons.  In these instances the owner and design team should be informed of the appropriate correction factors.

 

Shading

Obviously, the less the better.  An instrument should be used to predict shading throughout the year.   A neighboring building that may not cast a shadow in December may create a problem in June. Also, remember the installation of solar panels can reduce heat gain to the building and this could be a significant contribution over the system life.  Sometimes panels are mounted in an awning type formation over second story windows (dorm of office type situation) which will prevent solar gain through the second floor windows in the summer but allow it in the winter while generating electricity all year round.  See http://www.susdesign.com/tools.php for some design tools on sun angles and shading overhangs.

 

Azimuth:

Production loss at 22.5° azimuth (SSW or SSE) is only 1%, at 45° azimuth (SE or SW) losses increase to 5%.  For solar purposes 180° is due south. Slight western orientation maximizes evening production which would be advantageous for TOU (time of use) metering when it is implemented in the future.

 

Conduit:

For residential provide a ¾” EMT conduit from near the eventual solar panel location (maybe in the attic, above a drop ceiling or up on a flat roof) to a metal junction box near the meter base.  From this junction box continue the conduit to the main panel.  If the system is expected to use more than 500 square feet of roof make it a 1” conduit.  Romex and PVC are not allowed within the house for a PV output circuit as per NEC 690.  For commercial provide a 1” EMT conduit or larger.

 

These guidelines are for general consideration and there are exceptions.  We are always happy to discuss your specific project and how to best plan for adding solar.