Microinverters Are Changing Solar

Microinverters are getting a lot of buzz in the solar world right now, and use of this technology has increased tremendously this year. We thought now would be an appropriate time educate our readers on why Microinverters are changing the face of solar.

The short explanation is that Microinverters are helping raise power production of solar arrays substantially, while offering double the warranty period of standard inverters, and many other advantages.

When a company installs a solar array, they usually get an energy-production and financial performance model that shows how much energy the array will produce in a typical year. The industry standard modeling package for making this estimation is PVWatts – a computer program developed by the Department of Energy, National Renewable Energy Laboratory. Installers, bankers, and other industry insiders trust PVWatts as reliable and predictable. But how do solar arrays really perform compared to the modeled prediction by PVWatts?

Roof-fastened Racking with Enphase Microinverters in place

To answer that question, let’s look inside the solar array. An essential component of any solar installation is the inverter – the piece of electronics that converts direct current (DC) from the solar module into alternating current (AC), which is then fed into the building’s electrical system.

There are two different types of inverter architecture: “central inverters” and “microinverters”. Central inverters are connected to as many as 20 solar modules all wired together in a string like Christmas tree lights. In contrast, microinverters are configured with one inverter for every solar module. Traditional central inverters are cheaper on a per-watt basis, and offer good ease of access. But they do present some significant drawbacks.

Most importantly, like a string of tree lights, a central inverter is plagued by the “Christmas Light Effect” – whenever one panel drops in production, the entire system is limited to the output of that lowest-producing panel. So, for example, if a tree in the yard casts its shadow on one of the panels in a residential system for part of the day, the entire system will produce as if it were shaded, for as long as that one panel is shaded. This can happen when the shading is as small as a leaf that has fallen from a tree on to one of the modules.

Panels installed over microinverters and racking

Microinverters solve this problem, and more. Each microinverter is able to isolate and “tune” the output of its panel. So if one module is performing better than its neighbor, because of shading or just because it is a little bit stronger, its microinverter will produce more AC power independent of the rest of the modules in the array.

There have been two peer-reviewed scientific studies conducted on the performance of inverters compared to the industry-standard PVWatts program. The most extensive study to date was conducted by Gostein, et al. (2009); in which 480 sites with traditional string inverters were compared to their initial PVWatts estimates of energy production. The conclusion was that the systems using traditional string inverters delivered, on average, 8% less energy than predicted. The cause of this 8% shortfall is a combination of the irregular shading, mismatched power among the modules, and the power loss resulting from long electrical runs from the modules to the inverters.

More recently, a similar peer-reviewed study was conducted by Enphase Energy, comparing their microinverter energy production with PVWatts estimates.  The study evaluated 143 installations from California to the Eastern US, both residential and commercial, with a variety of shading conditions.[i] From this broad sampling, PVWatts calculations were made and then monthly performance data was collected for each site for an average of 12 months. The study even factored meter accuracy into the study’s calculations for each and every microinverter to ensure the most accurate data possible.

The results showed that Enphase systems on average performed 8% better than their PVWatts estimates across all the sites and shading conditions (76% outperformed PVWatts), and an average of 15% better on sites with little or no shading (95% outperformed PVWatts).  Compare this to the results from the Gostein study that shows string inverters underperformed PVWatts by 8%, and that results in a 16% improvement in energy production resulting from microinverters compared to string inverters. Let’s say that again: microinverters can be expected to deliver at least 16% more energy than string inverters connected to the same modules under the same conditions.

Now you would expect that when a company does its own study on its own products, that its products would outperform the others on the market. Otherwise why would they publish the study? Being conservative pragmatists here at Microgrid, we conducted our own informal and unscientific evaluation of half a dozen Enphase installations, and we found the same results: Enphase microinverters are outperforming PVWatts by as much as 30%.

This performance advantage really tips the scales toward microinverters. Particularly when you combine the performance benefit with the added safety, faster troubleshooting, higher reliability, and several other factors. Microinverters have proven themselves in the marketplace for the last 3-4 years, and we recommend them where appropriate for our clients’ needs. Please let us know if you would like to read the referenced report, or other white papers on microinverter reliability, or if you want to talk with us about what technology approach makes sense for your commercial building or home.

[i] David Briggs and Mark Baldassari, “Performance of Enphase Microinverter Systems v. PVWatts Estimates”,  enphase.com, 2011.