Panels that appear dark over periods of the day are often caused by shading from clouds, buildings, poles, trees, dirty PV-Modules, birds or other light blocking obstacles.
Types of Shading
Shading can come from a variety of contributing factors, a few of which are highlighted below:
Pole or Isolated Obstacle Shading
If you see a set of PV-Modules that are darkened on a consistent basis from day to day, the lack of production is likely caused by shading from an obstacle that is in the way of the sun.
In the animated Gif below, the PV-Modules on the right side of the array are being darkened at a specific time as the sun progresses through the sky. This is likely due to a shadow that is casted from a stationary obstacle, that moves across the array (as the sun moves across the sky).
If you were to compare to the data from the day before, it probably shows a similar pattern of shading. This would be considered normal operation.
NOTE: Shading from obstacles can be a seasonal event. As the sun's angle changes, the patterns of shading will be different in the summer months (as opposed to winter).
The animated gif below, illustrates an example of cloud shading. Notice that this entire site darkens throughout the day, and does not have a smooth production curve.
When compared to a different day, the data reveals that the site's production does not follow the same pattern. This gif shows the same system, under the smooth production curve of a cloudless day. The PV-Modules are producing (bright green) as long as the sun is overhead.
Both of these shading events, would be considered normal operation
In the Gif above, you will also see a few of PV-Modules that remain slightly darker than others, throughout the day. Here's on from the top array:
A consistent lowered output of a single PV-Module could be an indication of a semi-permanent reduction of solar production, due to dirt, leaves or even bird droppings. In some cases, it can also indicate a panel degradation or diode failure (within the PV-Module, itself).
This would not be considered normal operation. Review the panel and see if it requires cleaning or if there are other visible (or testable) issues.
Summary and Takeaways
Shading generally follows a pattern from day-to-day. A quick way to compare days, is to look at the green production wave (at the bottom of the screen. A sunny day will look significantly different than a cloudy day.
- A Sunny Day
- A Cloudy Day