When I put the solar electric array in, I knew that there would be some shading from an oak on my neighbor's property that I could have had removed, but didn't want to spend the additional money. Now that we're in winter solstice season, and because today is clear and sunny, I took some shading photos and looked at the output of the array. Here's a shot at about 10 am, when the tree is beginning to appear on the array:
At this point, the 4.76 kW array is putting out 2915W. The tree covers both strings of ten.
Here's a shot an hour later, around 11 am, and the output is 2844W:
Half an hour later, only the lower east edge has any shade, and the output is 3495W:
At 11:50 am, 18 minutes later, the array is unshaded and the output jumps to 3900W, which is 82% of the rated output, and typical of peak output. I have some data I took at 10:30 am on Dec 11th and with the tree smack dab in the middle of the array the output was 1 kW lower than what it was at 11:50 am eighty minutes later. So I conclude that during the roughly 1-1/2 hours that the tree crosses the array, I'm losing about 1 kW, or about 25% of the peak output.
My house faces about 11 degrees east of south, so 11 am this time of year is when the sun is perpendicular to the array. From here on out the output will drop. At 12:30 pm it's down to 3650W. At some point, the smaller trees to the west will start to shade the array and output will drop off quickly.