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What pvwatts parameters did you use - in particular did you tweak the AC->DC derate factor at all?

Marc Rosenbaum

This model was done with the default derate factor of 0.77.

Erik Haugsjaa

Using PVWatts 1 for Worcester, MA with 0.77 derate and our Canadian Solar 230W PVs and Enphase M190s, we have a similar 2012. (10% high so far) VS our total for 2011 was 99.6% of the PVWatts estimate.

All very inexact since we have some shading that I am not factoring in and the M190s clip occasionally in fall and spring. I suppose the low 0.77 is acting as our shading.


I did a similar post last week too ;) http://sandeen.net/wordpress/energy/comparing-pvwatts-estimates-to-actual-solar-production/ but used the 0.817 derate Enphase recommends. It's hard to tease out snow vs. shade vs. m190 clipping etc, but in the end PVWatts was reasonably close - I'm not overproducing like you are, though....

Marc Rosenbaum

Eric, with a 0.82 derate factor, using Boston weather data, the system is still out-performing the estimate, but not by as much of course - about 7.8%.

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