Pvwattsv8 results in SAM and PySAM

  • pgilman
31 Jul 2023 15:49 #12361 by pgilman
Replied by pgilman on topic Pvwattsv8 results in SAM and PySAM
Hi Javier,

This one is a little tricky.

The documentation page for the Single owner module lists several interdependent variables :

The construction financing cost and total installed cost both depend on the system capacity, so if you change the system capacity, you must also change the construction financing cost and total installed cost.

(The cp_battery_nameplate and cp_system_nameplate variables are part of SAM's user interface and are not PySAM inputs. These two variables should not be in the list of interdependent variables -- we have to fix a problem with the code that generates these variable lists.)

On the Installation Costs input page in SAM for the default PVWatts / Single Owner case, you can see that there are several cost categories that use the $/Wdc option. These are costs that SAM calculates based on the system capacity. These calculations are in the SAM user interface. They are not part of the PySAM Singleowner module. To make your Python script consistent with SAM, you could either simplify the inputs in SAM so that you use a single cost category to represent the total installed cost, or add the cost calculations to your script.

One way to to simplify the cost inputs in SAM is to use the Module cost to represent the system cost. For example, for the 100 MW system in your analysis, the total installed cost in SAM is $106,498,000.00 or $1.06/Wdc. You could set all of the cost inputs to zero except for the Module cost to zero like this:


I used the "$/Unit" option for the module cost and typed 106498000 for the input in SAM to make it consistent with the 'total_installed_cost' input to Singleowner in PySAM.

You can see the construction financing cost on SAM's Financial Parameters input page. The calculation is described in the Financial Parameters Help topic . For your analysis, you could either set the construction financing inputs on the Financial Parameters page in SAM to zero (and adjust the total installed cost accordingly), or you could calculate the construction financing cost in your Python script.

Best regards,


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  • jbonilla
  • Topic Author
01 Aug 2023 10:02 #12362 by jbonilla
Replied by jbonilla on topic Pvwattsv8 results in SAM and PySAM
Hi Paul,

Thanks a lot for this detailed description and solutions. I am managing now such costs in my Python script and the PySAM results are consistent with those from SAM.

I am testing now with the TcsmoltenSalt module in the same terms (changing the nominal power) and considering this:  sam.nrel.gov/forum/forum-sdk/3422-central-tower-csp-plant-in-pysam-tcsmoltensalt.html?start=0 , but I am having also issues getting consistent results between PySAM and SAM.

Anyway, I will write another forum post if I cannot figure this out.


Best regards,

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