Take the case of Ted Weiler of Kenmore, WA, who sent me a note that other day wondering why he sometimes gets higher peak solar output from his panels during partly cloudy days than clear ones.
Here is a plot he sent me for his system output for a few days during the past week. The green line shows the power produced on June 4, a day that was nearly clear. Nice cosine shape. The blue line/area shows the output on June 9th, a partly cloudy day---something very strange is going on! At some times the solar power output was GREATER on the partly cloudy day than on the clear day. How could that be? And an even cloudier day is shown with the red line...even THAT day had a period with greater output than the clear day.
Let's check out a different trio of dates (June 6, 7, and 8). You can see the spikes above clear sky values again! What is going on?
The answer is obvious if you think about it. On partly cloudy days, we have periods with breaks in the clouds when you get direct solar radiation from the sun. But you ALSO get solar radiation that is reflected off nearby clouds...solar radiation that is not getting to someone else because the cloud is intercepting it. So you start with the clear-sky value direction from the sun, but you are also getting the radiation reflecting off clouds....the sum is clearly more than the clear sky value.
Here is the view from the top of my department at 2:30 PDT on June 9th. It was partially cloudy at that time, and you can see some of the bright clouds that were giving the direct sunlight an extra boost.
Although you might get a boost above clear skies values for a short while, inevitably the direct solar radiation will get blocked by a cloud--thus, your total power output will be less than the clear sky total over the entire day. If you can find a way to stay in the direct sun, while clouds surrounded you and never blocked the sun...you would have a really sweet situation. My god, I know such a place.!..it is known as Sequim and its famous "blue hole." No wonder so many California retirees are heading there, solar panels in tow.