Enphase Claims Microinverter Outperforms NREL Estimates

A new study shows a 16% net benefit compared to traditional inverters—but do these results translate to the field?

MJ Shiao August 31, 2011

MJ Shiao

Head of Americas Research
GTM Research

MJ Shiao is Head of Americas Research at GTM Research and is a leading expert on PV inverters, electronics, and balance of system components. A 9-year veteran of the solar industry, MJ has experience ranging from fabrication of crystalline silicon solar cells to PV project development. MJ holds a bachelor's in electrical engineering from the University of Delaware, where he was named a National Truman Scholar. A believer in the social benefits of solar power, MJ has researched, installed and tested rural off-grid PV in India and Thailand and formerly served on the Steering Committee of SustainUS, a non-profit engaging U.S. youth in sustainable development advocacy.

In a shaky global solar market, leading microinverter manufacturer Enphase Energy keeps firing on all cylinders.

Earlier this year, Enphase filed an S-1, began raising another round of VC funding, and released a new line of third-generation microinverters that aim to increase performance while cutting manufacturing costs. Today, Enphase released results from a field performance study demonstrating that Enphase installations, on average, perform 8 percent higher than the PVWatts calculator predicts.

PVWatts, a PV performance calculator developed by the National Renewable Energy Laboratory (NREL), is popular among residential and small commercial installers, though it is usually cast aside in favor of more robust simulators like PVSyst and PV*Sol for larger systems. The field performance data was gathered from 143 Enphase installations spread across California and eastern states through partner installers Astrum Solar, Real Goods, and Solar Universe.

View the full study here.

Enphase also pointed to an analysis of 480 residential and commercial PV systems in Austin, TX from 2009 by Atonometrics, Inc. that showed most installed PV systems underperformed PVWatts' estimates by 8 percent on average. Enphase Marketing Manager David Briggs stated that Enphase dug through data provided by Austin Energy and found that most installers left PVWatts’ DC-to-AC derate factor -- a catch-all setting that factors in system losses like inverter efficiency, module shading, wire losses, and system uptime -- at the default level of 0.77. The studies coupled together would indicate that Enphase microinverters can improve the performance of solar installations by 16 percent on average.

“The Enphase Microinverter increases energy production versus standard inverter technology, and this study further confirms it,” said Raghu Belur, VP of products and co-founder of Enphase Energy. “This level of superior system performance is one of the key benefits of the Enphase Microinverter System enjoyed by system owners.”

However, the GTM Research team has been quick to point out a few limitations of the study. First, the study compares actual performance of two years (with an average of a single year0 against simulation data. Since PVWatts uses an average yearly output, year-to-year variations are not accounted for. Furthermore, both the time period (2005 to 2008) and location (Austin, TX) for the Atonometrics study are different from the analysis period and locations of the Enphase systems, which means other lurking variables may account for some of the +/-8 percent difference in the two studies compared to PVWatts simulations. Thus, the leap from the conclusions of this study to the affirmation of inherent microinverter benefits seems somewhat dubious. To Enphase’s credit, the study does take into account site error that could accrue from non-revenue grade metering from the Enphase Envoy metering device, as well as slight discrepancies between Enphase-reported and actual generation due to AC wiring losses.

Nevertheless, Astrum Solar, an East Coast residential and small commercial installer, indicated that customer feedback from Enphase users has been positive, especially with the ability to monitor individual panel output. Enphase has shipped 750,000 units as of early June and has ramped capacity due to strong demand.

GTM Research estimates that microinverters were installed in 16 percent of U.S. residential systems in the the first quarter of 2011 with a slight increase in the second quarter. According to GTM Research's latest inverter report, the distributed optimization sector, in which microinverters play a part, is expected to reach 4.6 gigawatts in annual installations by the end of 2015.

Still, the influx of new competitors (which have admitted to riding on Enphase’s coattails) may threaten Enphase’s dominance. SolarBridge recently announced commercially available AC modules with AUO Solar. In July, Enecsys released UL1741 certified micro-inverters for the North American market. Beyond microinverters, the successful use of DC optimization technologies like SolarEdge and Tigo by dominant residential installers could also reduce Enphase’s market share. The increased competition has caused some uncertainty and even confusion as installers and investors attempt to differentiate the technologies and assess the true reliability and benefits of each.

Other companies in the mix, each with their own technology, include eIQ, Azuray, the ever-mysterious Array Converter, and potentially dominant central inverter manufacturers SMA and Power-One.