How does pressure affect PVWatts output?

Hi Paul, 

I've been trying to format a weather file; I added all elements mentioned in the  weather data elements  page, but noticed the output of pvwattsv8 was not quite right, after playing around with the weather file I found that if I added the pressure column on the weather file the output was correct. What confuses me is that the  weather data elements  page says that PVWatts doesn't use the atmospheric pressure from the weather file, but the pressure seems to affect the output.
Here's my output and code:
 

Hi Alejandro,

I tested PVWatts in SAM 2023.12.17, which runs the pvwattsv8 module with the attached test weather file that contains no pressure data.

A also ran your test script with the weather files you provided, and pvwattsv8 seemed to run with both weather files you provided:

 

The slight difference between the annual energy for the two cases is due to the code that calculates plane-of-array irradiance from the input DNI and DHI data: When the weather file does not include atmospheric pressure data, it sets the pressure to 1013.25 mb (sea level pressure).

Best regards,
Paul.

p.s. If you want to include an image in your posts on this forum, please attach the image as a file and then insert it into the message. Copying and pasting the image directly into the message does not work.

Thanks Paul!