Discrepancy in Results Between SAM pvwattsv8 Module (2024.12.12) and PVWatts API

Details:I have encountered an issue where the results from the pvwattsv8 module in SAM (Version 2024.12.12) using "ssc.dll" and C# differ from the results obtained using the PVWatts API v8 (URL: [url] developer.nrel.gov/api/pvwatts/v8.json [/url]).Here are the specifics:

1. SAM Version: 2024.12.12
2. Library Used: "ssc.dll" with C#
3. API: PVWatts API v8
4. API-EndPoint:
   GET:  

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   " developer.nrel.gov/api/pvwatts/v8.json?api_key=DEMO_KEY&system_capacity=4&module_type=0&dc_ac_ratio=1.2&inv_eff=96&losses=14.08&array_type=0&tilt=20&azimuth=180&lat=24.7170641465&lon=46.6710757479&gcr=0.4 "
5. Parameters: I ensured that the same parameters were passed in both cases, please check the attached excel file "SAM_Inputs".
6. Weather File: I used the following weather file for the desired location, downloaded from the PVWatts module:
   • Latitude: 24.71706415
   • Longitude: 46.67107575
   • File-Name: 24.73_46.66_nsrdb-msg-v1-0-0-tmy_60_tmy.csv (File Attached)

Despite the identical input parameters, the results are inconsistent.Question:Could there be a difference in the underlying calculations or assumptions between the SAM pvwattsv8 module and the PVWatts API? If so, what adjustments can be made to align the results, or what might I be overlooking?

Thank you for your time.

Hi Odai AL-Omari,

Sorry for the dely in my reply. We had a technical issue with the forum that prevented me from seeing your post until now.

The likely cause of the difference is that the PVWatts V8 API is using different weather data than your C# code.

SAM 2024.12.12 uses SSC Version 298. For that version variable 'adjust:constant' should be 'adjust_constant'. (Older versions use a colon ':' in the 'adjust' variable names, and newer versions use underscore '_'.).

You can use SDKtool program that comes with SAM 2024.12.12 to see a complete list of inputs and outputs for the pvwattsv8 module. For more about SDKtool, see sam.nrel.gov/software-development-kit-sdk.html .



Best regards,
Paul.