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How tight is the correlation between % GHI decrease and % energy production decrease?


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My runs of PVsyst indicate that, for exactly the same system design inputs, a percent decrease in GHI levels causes about the same percent decrease in energy generation.

So far I have tested 5%, 10%, 11%, 12%, 15%, and 17% GHI decreases by editing my hourly GHI data in PVsyst standard format. In each case, these decreases produce a PVsyst-predicted decrease in solar plant energy generation of about 1% less than GHI was decreased (i.e. a 10% decrease in GHI results in a 9% decrease in energy generation).

Is this conclusion largely accepted by the community of solar energy plant analysts?

If so, and given the mathematical relationship between GHI and insolation, can I presume that the same conclusion can be stated for an insolation decrease by the same above percentages?

Thank you for any insights!

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There are more variables to consider on a site specific basis, and when comparing different locations. If you are looking at a single site, and comparing changes in GHI year to year, a decrease in GHI may correlate to an increase in diffuse, lower temperature, etc, which depending on system and equipment, may not give the relationship you've calculated. It also depends on system design. High DC AC ratio site may not give the same result, shading losses may be different, etc. So, not quite, at least we don't, due to the uniqueness of each project.
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