SMA America Responds to Article on Solar Inverters

Note: SolarTown recently ran a learning article on choosing between a central inverter and microinverter. The learning article received a lot of attention and one manufacturer of central inverters took exception to some of the facts and conclusion of the article. We offered space for SMA to air its views on central inverters. This is an unedited version of SMA's article in response to the learning article. The views expressed are exclusively those of SMA.

Updated: August 4, 2011 

Greg Smith
Technical Trainer, Solar Academy
SMA America, LLC

I recently read an online article in SolarTown Learning entitled "Choosing between a Central Inverter and a Micro Inverter" and wanted to clarify a few points made between the two technologies.

The first paragraph of the article incorrectly lumps micro inverters and central inverters in the same category when referring to the output of solar arrays on homes. Central inverters are typically used in commercial or utility grade installations as their capacities are very large, much more than a typical home owner could ever hope to have the roof space (or land) for.

Continuing with the article, and in the spirit in which it was written, there are a few statements concerning central inverters that need clarification. For example, the efficiency comparison of micro's and centrals at parity with 95% "maximum" efficiency is not entirely accurate. Most are above 95% peak although their CEC efficiency may be around 95%. Some central inverter manufacturers are well above 95% with the SMA SC800CP (for behind the fence applications in the U.S.) boasting a whopping 98.7% peak efficiency. These efficiencies are prominently displayed on manufacturer websites and can be verified with the slightest of effort.

"Some problems with central inverters" is an interesting section and paints a negative picture of centrals using 3 points. First is shading, where the strength of the micro inverter lies. When referring to both shaded module and inverter output, not all inverters are created equal and not all shading is equal. SMA uses their patented Global Peak OptiTrac to mitigate the effects of shaded modules. Other manufacturers, like Schneider Electric, employ similar technologies as well. Heavily shaded arrays, of course, will significantly lower inverter output - but not even a micro inverter can mitigate that much shade. Heavily shaded arrays should be a red flag during the site visit anyway.

Also, the author indicates a big safety concern using central inverters citing 700-1600 volts in the PV array. The National Electric Code limits maximum PV voltages to 600V unless the installation is a behind-the-fence application. In Germany, where 1000V PV array voltages are common place even in residential applications, 1600 volts from a PV array is really high.

Lastly, the article states an apparent, but incorrect, weakness of centrals in their inability to oversize the PV array for fear of having to buy another central to take the extra power. You cannot damage an inverter by putting too much power into it. For example, you can put 10kW into a 2Kw inverter and nothing would happen! The inverter will only generate 2kW of power and the other 8kW would be a gross oversizing and subsequent loss of money on modules that cannot be utilized.

There were a few other micro inverter inconsistencies in the article but SMA, like SolarTown, wishes to educate people on the solar industry in an appropriate forum. Please do not misinterpret my comments as heavily biased coming from a string/central inverter manufacturer- SMA will be launching our own micro inverter soon. The positive attributes micro inverters bring to this industry are real and the potential they have to dominate their niche in this market is undeniable. But it also hurts the entire industry when disparaging one technology over another especially when the information is not accurate.