This same issue has come up over the years for us......first with PV Powered inverters and then noticed in Fronius as well......We have noticed in most of our grid tied systems that are connected thru a "revenue grade meter" for Oregon Energy Trust production reporting, that the inverter always has a higher performance than indicated on the "utility grade meter". We have seen the discrepency as high as 10%. Over time this adds up to significant kWh differences. In our experience the inverter always has the higher kWh reporting, we have attributed this to the inverter wanting to report a good production number, to boost their efficiency claims......maybe even more than is accurate.....I have asked PV Powered and Fronius about this and their line is that to put a "revenue grade meter" into the inverter would be cost prohibitive......interesting as the readily available revenue grade meters are only $30-$60. I would gladly pay that much extra if I didn't have to wire in an additional meter. I don't think this problem is unique to the Enphase units (i haven't installed any of these), I think all inverters should be required to install the revenue grade meters to give accurate reporting of actual production. Similar to how states have a "weights and scales" accuracy certification, energy consumption and production meters should be similarly calibratable.
-- Sunny Regards, Kirpal Khalsa NABCEP Certified Solar PV Installer Renewable Energy Systems www.oregonsolarworks.com 541-218-0201 m 541-592-3958 o
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