In my Solar Array Surprises post about the SAS solar farm, one of the surprises was a midday dip in the power output, for which commenters supplied several possible explanations.
That data was from April, and we could only speculate what the summer data would look like. But now summer is over (by some accounts), and we can look at eight months of solar output data.
Looking at the summer data brings a new surprise: It's very noisy.
I know we had a fair amount of rain this summer, but I remember some dry spells, too. I guess the clouds were always around. At least July 14 looks clear and sunny, which is better than I can say for any day in June or August.
Getting back to the midday dip, here's a Graph Builder plot using the Data Filter to show only the most sunny day in each month (hand-picked).
A few observations from the plot:
Power output is greater in the summer (height of the curve).
Daylight lasts longer in the summer (width of the curve).
The midday dip is more pronounced in the winter.
Summer output is noisier.
Points 1 and 2 are obvious. The third point supports the idea that the horizontal axis of the panel rotation is causing the dip, since it is more pronounced when the sun is lower in the sky. The fourth point could just be the weather, but it might be exacerbated by the power output being near capacity for the array.
Ambient temperature was also mentioned as a possible factor in the power output, but I haven't analyzed it. I was hoping to find at least one cool sunny day and one warm sunny day from each season for comparison. It's still possible the temperature accounts for the slightly higher output in the morning than the afternoon since solar cells are more efficient at lower temperatures.
UPDATE (09-09-09): The solar array data is now available from the JMP File Exchange. Scroll down to the bottom of my author page to find the file titled "Solar Array Data Jan - Aug 2009."