calculating north tilt vs afternoon tree shading

[bcroe]

Shade can easily cost 90% loss of output from the shaded cells, and any directly feeding through them. To maintain reasonable efficiency
under partial shade, your system needs to have means to bypass the shaded cells, such as optimizers or use of micro inverters. Accurate
simulation of shading is going to be difficult, except assuming zero output for the interval. Bruce Roe

[ButchDeal]

Any idea what percentage of energy an average tree without any leafs might block?

a leaf off tree is going to block nearly all production for the modules it shadows. With optimizers you will get a little but I would for planning consider it zero for the effected modules.

[S0larAlex]

first time posting on this website, thank you all for the stickies and such it was very helpful in my research.

My house has low pitch 10% degree roof in both south and north orientation. South orientation does not have enough space to fit all the panels and part of it gets tree shading in the afternoon (probably after 2pm, in the winter and probably none in the summer month since the tree is 35' south, 30 feet west' and about 30-35 feet high)
The design I got from the installer had 6 panels (out of total of 21) on the north facing slope, and part of the south roof not filled out completely.
I'm questioning their design because they do it remotely and obviously don't see the real conditions as google earth is outdated.

So I did a design using http://pvwatts.nrel.gov/ for both north and south facing slopes and would like to do an excel calculation sheet to determine whether filling up the south roof completely with some shading is more beneficial than putting 6 panels on the roof facing north.

My main question would be, how can I accurately account for tree shading in the afternoon.
In the excel I was planning to take south facing design and north facing design and mold each one to the two options I have. I planned to take off some percentage points for the tree shading in the winter - for example 20% off total December energy production.

Any idea what percentage of energy an average tree without any leafs might block?

Thanks