Aerial Photo Based Solar Site Survey Service

Will the survey typically pick up shading from something like plumbing vents that come up through the roof?

• Plumbing vents: The bottom line is that small 1.5" - 2" plumbing vents can usually be cut down to 6" high so they're flush with the top of the panels. For larger vents, we recommend maintaining at least a 2:1 ratio (distance to height) from modules. We've looked into ways to simulate the effect of partial shading on system output and the truth is that partial shading effects are too complex to model accurately. Even the industry standard PV modeling programs like PV*Sol and PVSyst (over $1,000 each) cannot do this... and these are the tools used for predicting system output on multi-million dollar projects.
  
  NREL did a great study (http://www.nrel.gov/docs/fy09osti/46001.pdf) which attempts to tackle this issue. They point out that in a worst case scenario, partial shade on a module can reduce output by up to 30 times the size of the shaded area but this is not typical or representative. They suggest a value of 2 may be more appropriate. They also acknowledge that looking at a single module or even a single string in isolation is an over-simplification, concluding that further study is needed on systems with multiple strings in parallel. When connected to a string inverter, the non-shaded strings and the inverter affect the functioning of bypass diodes in the partially shaded string and are in turn affected because the shaded string throws off the MPPT algorithm, etc.

If a panel is partially shaded, is the output from the un-shaded part counted?  How do you handle shading from partial shading objects like deciduous trees?

• Partial shading: We estimate the effects of partial shading by analyzing 8 points around the perimeter of each PV panel and use only the worst point. If the difference between the worst point and the best point is small, it indicates that the panel is either fully shaded or completely unshaded most of the time, spending little time in a partially shaded state. However, if there's a significant difference between the worst and best point, it indicates the panel is too close to an obstruction such as a dormer, chimney or vent. By using the worst point, we are slightly under-estimating production during the partially shaded time. We then average all the panels in an array together to come up with an array shading average. On a string inverter, this is slightly over-estimating production since the more shaded modules affect the output of the less shaded ones. We've concluded that these two effects tend to cancel each other out. Comparing our estimates to real-world systems has shown that our approach is valid.
• Deciduous trees: We treat all trees as solid objects for every month of the year. This is an industry standard practice. For example, this is how the Solmetric SunEye handles trees. While it's true that some light gets through between the branches when the leaves are off the tree, this happens during the lowest producing months of the year and typically affects panels early and late in the day. The partial sunlight that does get through is most similar to the worst case partial shading scenario I described above and has a small effect on total annual production.

There was one small shading event missed on the survey: http://www.builditsolar.com/Projects/PV/EnphasePV/Shading.htm
I'm not saying that this is even remotely a big deal as its a very minor amount of shading, but just wondering if: One should expect that maybe very subtle shading might be missed?  And, if someone alerts you to look for a hard to spot feature like this, will you try to include it in the survey?

• I did notice the fence to the east of your array when I first did your report. As it appeared to be maintaining a 2:1 distance from the array, I omitted it. Just out of curiosity, I put it in now and re-ran the numbers. As you can see in the revised report (attached), our algorithm did pick up the fence as evidenced by the two modules closest to the fence dropping from 98% to 97%. When the array is averaged and rounded off to the nearest percent, it still gives a 98% shading derate confirming that the effect from the fence is negligible.

I take it that if you do a survey for someone, and they point out that a significant shading item was missed that you would redo the survey for them?

• If a client is able to provide photos of the site, we can certainly use these to improve the quality and accuracy of the report. In fact, we prefer to have site photos. We include one revision in the price of the report and this is typically used to add or remove modules or to show how production would be impacted by the removal of a tree. If we missed a significant shading item, we would redo the report at no charge.