May 30, 2017

Micro Inverters vs. Single Inverter

Some Solar companies always push for Micro Inverters as the superior system. Let’s take a closer look.

15 years ago almost all Residential Solar Inverters were single, one Input Inverters. The maximum power window was much smaller than today. The panels had lower output and you needed 40 panels in 4 strings parallel of 10 each to get 5 kW AC output. If in 2 strings 2 panels each were shaded during mid-day you lost almost 50% of your output caused by only 10% shading. That is because the Inverter would adjust to the 2 remaining full sunlight strings and the voltage would be too low in the 2 shaded strings to push any significant current into the inverter.

Then came the Micro Inverter. Each Solar Panel has now it’s own Inverter and all Inverter outputs are added together. So, when 4 panels out of 40 are shaded you now lose only 10%. That is a huge advantage in this specific situation. Also Micro Inverters allow you to install panels in all directions and angles if you want to, because no panel depends on the same output of another one.

Today, panels have about double the output and Single Inverters have 2 or 3 Inputs with a wide input range. Now most installations are wired in one string per input. If you lose let’s say 6 of 12 panels in that string to shade, your output of that string is still 50%. There is still a small advantage to Micro Inverters because if you lose 9 panels you would still have 3 panels operating vs. none because the voltage for the Single Inverter is now too low. Also if single panels are partially shaded Micro Inverters can produce some output.

On the other hand Micro Inverters have considerable disadvantages.

First, the price of a Micro Inverter System is $ 1000 to 3000 more in an average residential Installation. Also since the Micro Inverters are mounted under the panels they can experience very high temperatures in that location. The first series of Micro Inverters had high failure rates because the components would deteriorate fast in that environment. Now the components are built for extreme high temperatures and the failure rates are lower. Still, one failure of 20 Micro Inverters is much more likely than of a Single Inverter which could be mounted in a cool garage.  By Murphy’s Law the Micro inverter which will fail could be in the middle of an array and several panels have to be removed to access that Inverter. Since hardly any Solar Panels fail anymore there is no reason to ever go on the roof if a Single Inverter installed at ground level is used. The efficiencies of both systems are very close with .5 to 1% advantage for the Single Inverter. Also we do not recommend Micro Inverter systems which have electronic under each panel AND a central Inverter. Troubleshooting can become very difficult in those systems.

Conclusion

Micro Inverters still have some advantage if there are lots of wandering shades effecting the array during peak production and if the roof has different angles and directions. Also, if the array is mounted not flush to the roof but in angled rows with walking space in between, Micro Inverters make sense. The operate cooler and are easy accessible.

On the other hand if the installation is a flush mount with no shading other than early and late in the day we recommend the lower cost Single Inverter.