Impact of “analysis period’ on “required PPA” results

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Impact of “analysis period’ on “required PPA” results

Modeling a PV system
I’ve been surprised by the new version of SAM determining that a higher PPA rate is required to solve for a given IRR when the analysis period is longer.
All other things being equal, I expect a lower required PPA rate to meet financial requirements when the initial costs are constant and the income is derived over a longer period. I find the expected result using the previous version of SAM (2013.9.20), but I get the opposite result from the current version (2015.1.30).

Running the same inputs in both models, with PVWatts standard system design, this appears to occur regardless of whether or not financing is assumed (20 year debt, analysis period of 20 years vs 25 years). The required PPA output is expressed as both first year PPA and levelized nominal PPA, each indicating the same discrepency between the old and new versions, with the new version indicating an increase in the required PPA for the longer analysis period.

Can anyone confirm this and explain it?

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