Site Survey: 2 Roof Surfaces, Different Orientations.

A Site Survey is an estimate that contains the following information:
  • The size of a system, in Kw, based on the number of solar panels that can fit in the available area.
  • The type of recommended system components that will work with that number of panels.
  • The annual power produced by that system, based on regional factors. 
Here is an example of a site survey, which contains all of that information.
This image was made after all of the roof measurements were made.  This particular roof contained two separate roof surfaces, one facing southeast, and the other southwest.

There were areas of shading, one from a chimney and the other from an adjacent house.  Those areas were marked with a darker color.  This image was produced using Macromedia Flash 8, but the measurements were produced with a ladder and a tape measure.  The orientation of each roof surface was also noted, and the shading was determined by using a pathfinder.

Here are the actual pathfinder readings.  As you can see here, the perimeter indicates that during the shorter months of the year (November - February), there were some issues in the afternoon with the next door house, and the chimney.  That's why a 10% shading derating factor was utilized in the total estimate.  The Pathfinder is a useful device and they're relatively expensive, for a dome-shaped piece of plastic, although they are cheaper compared to electronic devices which produce the same information.

Once the size of the system was determined using the available area on the roof, the next step was to estimate the power that the system would produce over the course of a year.

Since the system was facing southeast and southwest, respectively, each array faced a .96 derating factor (for not being perfectly south at 30°), and then an additional 10% was subtracted from the panel STC for shading.

The wattage rating of the selected panels (Sunpower 230's) were multiplied by their number (there were 18 on one roof, 24 on the other).

Other factors, such as irradiation in the region (in CT, it's 4.5) and the standard 75% derating factor was applied.  The result was an annual estimate of 28 kwh per day, saving the homeowner an average of $180 per month.

One Line Diagram.  Grounding Not Displayed.