Shading derate factor values

Once again, thank you Paul.
Is there some approximate method which you would recommend for PVWatts v1 for calculating shading derate factor?

You could use the graph in this online documentation for PVWatts V1:
rredc.nrel.gov/solar/calculators/pvwatts/version1/change.html
Best regards,

Paul.

Thank you Paul.

I am actually looking for a way do calculate the PVWatts v1 shading derate factor for shading from buildings and objects. Sorry for not stating this from the very beginning of the topic.
The graph in the mentioned PVWatts v1 online documentation deals with shading from one PV row to another.

Mentioned online documentation says that I need to use tools such as Solar Pathfinder. Isn't there some way of using a regular irradiance applications to determine this derate factor (in PVWatts v1) instead? You already told me that I can't calculate the shading derate factor by calculating annual reduction in incident beam irradiance.
But can't this annual reduction in incident beam irradiance somehow be connected with the reduction of the array's electrical output?

Thank you once again, and I appreciate your patience and effort.
NREL online community truly has an amazing support because of you.

I think you'll have to use your judgement to determine the relationship between the reduction in incident beam irradiance to the array's electrical output.
You may want to read the reference cited in the PVWatts V5 reference manual with the passage you originally quoted in your first message. That report describes an NREL study that shows the reduction in output for different shade conditions: 
Deline, C.; Meydbray, J.; Donovan, M.; Forrest, J.; Photovoltaic Shading Testbed for Module-level Power Electronics. NREL Technical Report NREL/TP-5200-54876, May 2012. ( PDF 1.2 MB )
Best regards,

Paul.