DC output calculation of PVWatts

Hi,

I have a question about the PVWatts section (SAM 2014.1.14), It will be nice of you to give some information.

I want to know what the formula or (Model type) is used in PVwatts to calculate the DC output?

I tried to figure out by substituting the SAM data mainly: T_cell and Plane of array (kw/m^2) into the below famous formula, it did not work.


P=P(dc_rating)*(Plane of array)*(1+(temp coef)*(Tcell-25))


When I changed the ac-dc derate factor to 0.92, the value by the above formula matches the SAM dc-output. I wonder if it is considered the wind speed, heat transfer and losses issue or another model type.



Sincerely,

Arian

Dear Arian,
Please see the manual for PVWatts on the Performance Documentation page of this website for those details.
I would recommend installing the current version of SAM for the latest version of PVWatts. SAM 2014.1.14 uses PVWatts V1, and the current version uses PVWatts V5.
Best regards,

Paul.

Dear Gilman

Thanks for your valuable information.

Sincerely,

Arian.

Dear Gilman

Thanks for your updated "Errata for SAM Photovoltaic Model Technical Reference" in April 2016. But I think, there are still some errors I found this 6 months ago, please check this one:

The parameter b for Perez model should be b=max(cos(85),cos(zenith angle)) not the incident angle. There are many papers for checking that.

Best regards,

Arian

Dear Arian,

Thank you for the reminder. I verified that the equation in the SAM code is correct and added the mistake to the errata sheet for the PV reference manual.

Best regards,
Paul.

Dear Gilman

I checked your errata. Unfortunately, you changed the a parameter as well. The a parameter should be AOI. No need to change that. Only the b parameter is a function of zenith angle. You can checked Perez paper in page 281:

Perez R, Ineichen P, Seals R, Michalsky J, Stewart R. Modeling daylight availability and irradiance components from direct and global irradiance. Sol Energy 1990;44:271–89.