When a person seeks to install solar panels on an existing roof, there are several factors to consider.
I have attached what I used to evaluate my installation.
(attachment 1) Diagram with My parameters.
(attachment 2) Blank form for anyone wanting to try it. [I used PAINT to draw and move things around].
(attachment 3) The source worksheet I used to develop the Bullseye Diagram.
STEP 1
On attachment 1 (my roof) , I first entered the required data.
Longitude - used to determine time of day performance and Solar Noon variations inside a time zone
Latitude - used to determine seasonal performance
Roof orientation - used to adjust of day performance
Roof pitch - used to adjust seasonal performance.
STEP2
calculate pitch offset from latitude, mark it on the small vertical scale and move it to line that mark up with 90°
STEP3
calculate your time zone adjustment and move the time zone scale to the right by that amount.
STEP 4
move bottom most scale to align 00 with your roof orientation degrees on the time zone scale.
STEP 5
draw the horizontal & vertical lines as indicated and align the seasonal chart to align with the horizontal.
PRESS SAVE and Print.
My install shows a slight favoritism to spring/fall. It also shows a deficiency in AM performance. That is why I just recently added additional panels facing east. (see my previous post)
I hope this might be of assistance to someone else. Comments welcome.
(I do realize this does not account for temp extremes . It is intended to be a "ball park" guideline)
Just an example I did for a neighbor to show the difference between mounting the panel flat on his inclined roof VS adding some more inclination.