Enphase recently posted a study entitled "Performance of Enphase Microinverter Systems v. PVWatts Estimates" on the Enphase website. One of the technical considerations addressed in the report is the discrepancy between the measurement of power production by the Envoy and the power that reaches the utility meter. There seem to be two aspects to the discrepancy. The first is described in the report as follows: “To increase the precision of power measurement, the meter accuracy for each microinverter was factored into the study’s calculations (see “Site Bias” calculation below). The meter accuracy is determined by comparing the microinverter’s internal power measurement to its actual power output, as tested during the manufacturing process of the microinverter.” Does this mean that each individual microinverter has a measured bias that was recorded during the manufacturing process? If so, how can we find out what the bias is for each of the microinverters in our system, as measured during the manufacturing process?
A second loss of power is described in the report as follows: “The energy losses due to resistance in system wiring could not be accurately accounted for in this study. Because Enphase Microinverters are collecting energy production information at the module, the reporting by Enphase Microinverters could be 1-3% higher than the energy production registered at the meter, which is what the PVWatts calculator is designed to predict. These losses vary depending on wire length, conductor size and other system design factors.”
My electric utility installed its production meter in my system on 5 August 2011. In the month and a half since that date, the electricity production as measured by the utility meter has been one percent lower than the electricity production reported by the Envoy as presented in the data in the Enlighten system. This one percent discrepancy presumably accounts for the average bias of my particular set of microinverters and the wiring loss between my microinverters and the utility meter.
I think it would be interesting for system owners to report in this forum a comparison of the electricity produced as measured by their utility production meter and the Envoy data presented in the Enlighten system. It would also be interesting for system owners to report a comparison of the electricity produced to the estimate of production by PVWatts.
I used a DC to AC derate factor of .87 in my PVWatts calculations, based on claims by installers in my area. I also multiplied the PVWatts estimates by monthly shading factors for my system. The electricity production, as reported in the Enlighten system, was 1 percent higher during the second half of June than predicted by PVWatts (average per day compared to average per day for June predicted by PVWatts). The production during July was 16 percent higher than predicted, and for August it was 17 percent higher. So far, production for September is about 4 percent below predicted. Compared to predictions based on the default PVWatts DC to AC derate factor of .77, the production of my system was 14 percent higher in June, 31 percent higher in July, 33 percent higher in August, and so far 8 percent higher in September than PVWatts estimates using the default DC to AC derate factor and including monthly shading factors. (The predicted shading during these months is small, 1 percent in June-August and 2 percent in September.)