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Home » Power inverters » The baseline of standard inverter performance
The baseline of standard inverter performance
There are some data that do not stand up to even the slightest scrutiny and should be made available to anyone who reads it.
The study references a 2009 paper written by Dr. Gostein, who happily sent Gozuk a copy of the actual study, as the baseline of standard inverter performance. The paper shows that PVWatts slightly overstates the predicted output performance of PV installations. So when someone brags about their system outperforming PVWatts my response is, "Who doesn't?"
Link to his paper: http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=5411283
* The Enphase study does not point out that PVWatts predictions depend on assumptions about not only inverter performance, but also on all sorts of other things including the weather, PV module performance, and the quality of the installations.
In fact, my understanding is that one individual installer accounted for 90% of the installs in the Gostein paper – and this installer is no longer in business.
* The study goes to great lengths to demonstrate the +/- 5% meter accuracy of Envoy meter. The truth is that any paper which relies on the accuracy of the Envoy meter will result in a measurement error band of +/- 5%. This error band puts into perspective the claimed +8% upside in actual performance compared to PVWatts.
* Enphase acknowledges that the power measurement at their inverters are likely 1-3% higher than the meter reading due to line losses, but then disregard those losses in the results.
* It is commonly assumed that actual weather (esp. irradiance) in any particular year can vary +/- 10% from “average” or “typical” used in production estimation tools like PVWatts. Since the Gostein and Enphase studies are based on data collected from different time periods and different locations, there is no assurance that the actual irradiance during the times of data collection was comparable relative to that assumed by PVWatts.
* Finally, even if I could get past the poor methodology, it is simply wrong on its face to conclude that if PVWatts overstates predicted performance for one system by 8% and understates predicted performance for the Enphase-based system by 8% that “Enphase Micro inverters can improve the performance of solar installations by 16% on average versus standard inverter technology.” One could only conclude that the median predicted performance of the systems measured in the two studies vary by 16% -- a very different conclusion.
The study references a 2009 paper written by Dr. Gostein, who happily sent Gozuk a copy of the actual study, as the baseline of standard inverter performance. The paper shows that PVWatts slightly overstates the predicted output performance of PV installations. So when someone brags about their system outperforming PVWatts my response is, "Who doesn't?"
Link to his paper: http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=5411283
* The Enphase study does not point out that PVWatts predictions depend on assumptions about not only inverter performance, but also on all sorts of other things including the weather, PV module performance, and the quality of the installations.
In fact, my understanding is that one individual installer accounted for 90% of the installs in the Gostein paper – and this installer is no longer in business.
* The study goes to great lengths to demonstrate the +/- 5% meter accuracy of Envoy meter. The truth is that any paper which relies on the accuracy of the Envoy meter will result in a measurement error band of +/- 5%. This error band puts into perspective the claimed +8% upside in actual performance compared to PVWatts.
* Enphase acknowledges that the power measurement at their inverters are likely 1-3% higher than the meter reading due to line losses, but then disregard those losses in the results.
* It is commonly assumed that actual weather (esp. irradiance) in any particular year can vary +/- 10% from “average” or “typical” used in production estimation tools like PVWatts. Since the Gostein and Enphase studies are based on data collected from different time periods and different locations, there is no assurance that the actual irradiance during the times of data collection was comparable relative to that assumed by PVWatts.
* Finally, even if I could get past the poor methodology, it is simply wrong on its face to conclude that if PVWatts overstates predicted performance for one system by 8% and understates predicted performance for the Enphase-based system by 8% that “Enphase Micro inverters can improve the performance of solar installations by 16% on average versus standard inverter technology.” One could only conclude that the median predicted performance of the systems measured in the two studies vary by 16% -- a very different conclusion.
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