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IAM (incidence angle modifier)


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I have noticed modules have vastly different IAM (incidence angle modifier) values. Ranging from 95 to 99 to 99,9 to 100.

This has a fairly significant impact on the yield. How reliable are the IAM values provided by the manufactures and what level of auditing is done? These values are obviously not shown on the module datasheet and it seems suspicious that there is such a big range in values provided. 

I see for example that PVSyst requires a third party report from manufacturers if they claim non-standard IAM values for their modules.

Does PVSol team also apply this kind of due diligence or are manufacturers free to provide whatever database entries they chose?  

Any insight in this regard would be greatly appreciated!


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Hi Josh,

the module data entered by the manufacturers is only checked against min/max values, we do not require a third party report. The manufacturers are responsible for the data they enter. But if we encounter irregularities, we usually contact them to clarify the issue.

The model we use to calculate the reflexion losses is the one from AHSRAE:


Hope that helps, kind regards,


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  • 8 months later...


I share the exact same concern of Josh. If the IAM parameter isn't stated in datasheet and isn't subject to third pary auditing, manufacturers tend to give the highest value possible to gain a "virtual" competitive advantage. From what I have observed from your database, when manufacturers add/update their modules, they either increase slightly their IAM values, or aren't able to do so because they have reached 100%.

In that sense, I have some questions regarding the assessment of this parameter in your software:

  • What are exactly the min/max values that you consider for this parameter (is 100% the max value?)
  • What value(s) do you consider for the ASHRAE IAM model's parameter? And do you consider that this parameter depends on the module manufacturing (e.g.: single glazed, AR coating,...)
  • Have you consider using other models to define the IAM value?
  • Do you require manufacters to deliver results from the IAM measurements, and does these measurement procedures follow the IEC 61853-2 directive?

Can you please help me clarify this matter?

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  • 2 years later...


Hi I was wondering if the Questions of Miguel already have been answered because I also would like to know.

Currently I have a project whereby the subcontractor designed the PVsol and manually adjusted the IAM value ofthe module from 100% to 97% which resulted in a -4,48% reflection loss. I took the PVsyst and looked into the IAM model and indeed, according to PVsyst module data the IAM is not 1. Furthermore the PVsyst results of the system also indicate a 4.6% loss due to IAM.

The subcontractor , correctly in my eyes, added this factor to guarantee the PR of the system he wants to build. 4,5% yield reduction due to IAM is fairly significant.

(the PV module in question is the Jinko Solar Tiger Neo JKM565N-72HL4-BDV)


I havent checked but if the IAM is often considered to be too optimistic I wouldn't mind to have a button or parameter box in which you can set the IAM factor. example: you can set it at 98% of the Value given in the PV module parameters. I prefer this over having to check manually if the IAM is correct and having to make a dulpicate of the panel.


Hope you can clarify

thanks in advance,











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ok,  so I just found out that I can add the IAM %  in the 'choose module table', this already helps me out a lot with filtering unrealistic high values.

it seems both Trina and Jinko solar are both having very high values in comparison to other brands.... which have a more realistic value between 0.95~0.98.






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