Have you noticed that lately your system isn't producing as much as it used to? Maybe as recently as a few months ago, it was performing much better than it is now. The answer could have to do with the changing of the seasons.
The first image below (Figure 1) is a screenshot of a particular system at 11am on the 14th of February, 2012. The second picture below (Figure 2) is a screenshot of the exact same system at the same time on the 27th of June, 2012. Why is the system producing so much more at the same time of day in the second picture?
It turns out that this system has a group of trees next to it. In the winter months, when the sun is lower in the sky, the lower right group of panels stays shaded for longer (in this case, until after 11am). In the summer months, when the sun is more directly overhead, the system is hardly shaded at all.
The third image below (Figure 3) is a screenshot of the same system at 11am on October 29, 2012, when the sun has once again started to sink back down in the sky. As you can see, the same lowered production occurs in the morning.
So if you start to notice your system producing less power, check and see if you notice trends in lowering and rising production throughout the year. It might just be the changing of the seasons.
Figure 1 - Heavy shade and low sun in February
Figure 2 - No shade and high sun in June
Figure 3 - Heavy shade and low sun in October