Microinverters, String Inverters & Optimizers

(Originally posted on Heatspring, NorCal Solar board member Sean White discusses the latest in inverter technology. 

Student 1: I am enjoying the course and I like the way you explain things by citing real world examples. I just want to get your opinion on microinverters. These provide great solutions for power optimization and rapid shutdown, not to mention the flexibility of combining and running the wires.

My concern, however, is how well they can withstand adverse weather conditions on the roof compared to better conditions where the string inverters are usually located? My other concern is the microinverter maintenance and replacement– someone has to climb on the roof to find and replace it.

Thank you!

Sean White: Hi Student 1. The benefits of microinverters are obvious, and if you talked to microinverter people, you might wonder why anyone would install anything else. I will give you the string inverter position that many string inverter people get emotional about.

This is what SMA says:

The Enphase microinverter came out in 2008 (8 years ago), they have a 25 year warranty, but nobody knows what will happen in 25 years. Electronics tend to go by what they call the “bathtub curve”… or, failure at the beginning of the lifespan or at the end. Typically, the middle is safe.

If you have 30 modules on a roof and you install 30 microinverters and each microinverter lasts an average of 30 years, then you could have 30 instances in which you have to deal with broken inverters over 30 years. If you have 30 modules and a single string inverter and if the string inverter lasted 15 years on average, then you would have to replace inverters 1 or 2 times. String inverters are significantly less expensive, so you could buy a few extra modules to make up for the decrease in performance.

String inverters are not subject to the christmas light effect. If you have a string and a module or 2 or 3 is shaded, then the bypass diodes will kick in and bypass the shaded section of the module. The modules are divided in thirds, so a shadow can take as little as 1/3 of a modules performance off of the string.

With a microinverter, instead of the bypass diode kicking in, the module can work as if it were in the shade, which is a better than being totally bypassed. A module in the shade may produce 10% of production, so it is not a lot to worry about. Most string inverters now have 2 channel MPPT, so shading with one string or different string lengths is not a problem. Also SMA has a Secure Power Supply, which is really cool.

Now, let’s look at why optimizers are often said to be the best… as a Solar Edge representative might say:

Optimizers only have the DC under the module and those microinverters will break because the AC components can’t handle the rooftop and the test of time. Optimizers let you maximize your “string” length, because the electronics can control the voltage and current, unlike a normal PV source circuit.

Another thing that optimizers can do is feed the inverter the exact voltage which will make it work most efficiently.

690.12 Rapid Shutdown will be enforced in most places where solar is installed in 2017, so there is still 11 months where many systems will be installed without rapid shutdown (half of the solar in the United States is installed in California, and many other states also are on a similar code cycle).

An argument against optimizers: you have to install equipment under the PV and a regular inverter.

All being said, microinverters are easy to install by any electrician, they are safe, the monitoring is cool and the production is great. I install all of the options and try not to take sides. I hope I appeared neutral here.