Shading derate factor values

I am wondering how would one interpret the shading derate factor values for PVWatts v1?

It ranges from 0.00 to 1.00, where 1.00 means that pv array is not shaded.
But what about an intermediate values?
What would for example shading of 0.66 mean? That 2/3 of the time during year, that particular pv array is in a shade (from some ?tree, or other nearby objects)?
Or that 2/3 of the array's area through out the whole year are always in shade?

Thank you for the reply.

Hello,

PVWatts V1 is an obsolete version of PVWatts. The current version of PVWatts, both in SAM and on the PVWatts website is PVWatts V5. In V5, instead of derating factors, you account for shading and other losses using a loss percentage.

In both versions as they are implemented in SAM, you can account for shading losses in two different ways: Either using the system loss input (similar to the DC to AC derate factor in V1) or a set of beam shading losses. If you include shading in the system loss input, then a shading loss of 34% (or shading factor of 0.66) would mean that shading reduces the system's electrical output by 34%. If you use beam shading losses, then the value would mean that the beam incident irradiance on the entire array is reduced by 34%.

Note that the beam shading factor option is only available in SAM's implementation of PVWatts, not in the online version.

Best regards,
Paul.

Thank you for the reply Paul.

I am aware that version 1 is absolete, but it's the most documented one, at least when it comes to all the calculation process. I am trying to understand the v1 calculation first, then I will continue to the newer version.

Would you be so kind to tell me whether the same principle you mentioned, could be applied for PVWatts v1?

For example you said:
"If you include shading in the system loss input, then a shading loss of 34% (or shading factor of 0.66) would mean that shading reduces the system's electrical output by 34%."

Does this mean that shading loss of 34% - basically sets the overall "DC to AC derate factor" (0.77 by default) to 0.66?

Please see Chapter 11 of the PVWatts V5 reference manual ( PDF 741 KB ) for an explanation that should help answer your question.
Best regards,

Paul.

Thank you Paul, I a took a look at the PVWatts V5 reference manual you suggested.

May I quote a part from it:
"For a system experiencing shading due to nearby trees or structures, this loss value should be increased appropriately by using external shading software to predict shading losses, or with a onsite survey."

Would it be possible to use some general beam incident irradiance software instead of shading software? For example I would measure the beam incident irradiance on particular PV array with nearby buildings and objects included. Then measure the beam incident irradiance on the same PV array but without the nearby buildings and objects. Then check for the difference between these two measurements. For example in first case I got 1000 kWh/m2, and in second one 1200 kWh/m2. The difference would be 20%. And this would mean that my PVWatts v1 shading derate factor is 0.80? Did I got that right?

No. Your method determines the reduction in incident beam irradiance, not the reduction of the array's electrical output. From your description, it appears also to only determine the reduction for a particular moment in time, not over the entire year.

The "system losses" input for PVWatts V5 represents losses in the array's DC output.

The implementation of PVWatts V5 in SAM 2015.1.30 allows you to enter shading losses that represent a reduction in the beam irradiance as a set of beam irradiance shading losses.

Best regards,
Paul.