Do Electrolytic Capacitors Cripple Microinverter Reliability?

Part 1 of an interview with Enphase VP of Quality & Reliability: Ciaran Fox

In my experience solar installers either really love microinverters, or are incredibly wary of them. There seems to be very little middle ground.

Those who love them talk about their ease of installation, increased power output, flexibility of panel layout, panel level monitoring and shade performance.

Those who are wary of them generally worry about their reliability.

A microinverter is a box of sophisticated electronics that sits on your roof (under each solar panel). In Australia your roof gets hot! And if a microinverter fails, replacing it means climbing on the roof and removing the panel. So installers’ concerns are understandable.

A microinverter. You generally use one per solar panel instead of a single large central inverter. They are about as big as an iPad mini

To get some authoritative answers, I asked installers that I know to send me their toughest questions about microinverter reliability.

I then put those questions to Ciaran Fox who is the Vice President of Quality & Reliability at the world’s biggest microinverter manufacturer, Enphase.

The installers had one concern above all others. They believed that electronics with electrolytic capacitors are inherently unreliable at high temperatures.

Microinverters generally contain multiple electrolytic capacitors. 

Electrolytic capacitors are small electronic components that have a reputation for being fragile. They look like this:

Electrolytic Capacitors on a circuit board

Here’s what Ciaran had to say about the issue:

Finn: A microinverter contains a lot of electronics including electrolytic capacitors and sit on a hot roof. Doesn’t this logically mean that they will be less reliable than a central inverter?

Ciaran: Let me start with electrolytic capacitors. We are confident in the reliability of electrolytic capacitors based on the design, the testing, our supplier relationships and our precise knowledge of the field usage conditions. In terms of design, one of the things we do is we minimize the stress on all components, but specifically the electrolytic capacitors. There are very specific de-rating requirements we have – otherwise the product doesn’t ship.

And that builds in a significant margin in terms of safe and reliable usage.

Secondly – let’s say for the sake of discussion that there was a failure of an electrolytic capacitor in an Enphase microinverter (there won’t be! But let’s imagine there was). There is a built in redundancy such that the component failure would not fail the unit. And that’s part of our design process. We design to a very high MPPT efficiency, and to get that efficiency, you need that redundancy.

And then there’s the product development process itself. We extensively test at the component level, and these environmental stress tests can run more than a year. Then we do the unit level testing. Between the two tests, we literally test thousands of units under high environmental stress conditions (extreme humidity and temperature etc.) before the product is launched.

To date we have not seen an electrolytic capacitor fail in those tests.

This may sound a little strange to you, but when we use electrolytic capacitors in the design we can get a higher efficiency, which means you get less temperature stress on all the other components. So the net reliability of an Enphase microinverter with electrolytic capacitors is higher than if we used an alternative component.

And our field data reflects that. We’ve now shipped millions and millions of units containing multiple electrolytic capacitors (approximately 9 million units and 36 million capacitors). And the number of electrolytic capacitors that have failed in the field are less than my number of fingers. The electrolytic capacitors we deploy are extremely reliable and we are very proud of that.

Note from Finn: we were speaking on the phone so I couldn’t confirm how many fingers Ciaran had, however I have now got firm photographic proof that Ciaran has a full complement of 10 fingers. So less than 10 capacitor failures from 9 million micro inverter shipments.

Ciaran Fox, VP of Quality & Reliability at Enphase.

Finn: Where do you think this myth that electrolytic capacitors are incompatible with reliable power electronics comes from?

Ciaran: It is a very common myth and it actually goes back to some early experiences with PCs. Their problems were driven by some variations in the supplier’s quality control, which had a poisoning effect on the component quality. That together with how the capacitors were being stressed in the finished PC created this issue of lots of failures. I think this all happened more than 10 years ago. This caused a significant issue in the PC industry and the electrolytic capacitor’s reputation was seriously hurt.

Electrolytic capacitors are used in many different products, including airbags. These things have to be reliable! But you have to choose your suppliers very carefully. Our suppliers are the best in the world in my opinion, and if you were ever to visit their factories, you’d see they were state of the art.

To get a reliable microinverter you need to know the stresses that the units are getting out in the field. Because our microinverters communicate their performance back to us by default through the Enlighten platform, we know the stresses placed on millions of microinverters around the world. In particular we have a very precise understanding of the temperatures they are subject to.

This understanding feeds back into our product design cycle to improve the reliability with every new generation of microinverter. For example the reliability of our 4^th generation microinverter (Enphase M250) is currently 6 times better than our third generation microinverter (Enphase M215) which was a very reliable product to start with.

My thoughts:

So there you have it. Less than 10 capacitor failures from 36 million capacitors on hot roofs. Electrolytic capacitors can very well be part of an inverter that operates reliably at high temperatures. Yes, low quality capacitors in badly designed circuits will fail, and God knows we have no shortage of el cheapo inverters in Australia where this happens regularly. But a high quality electrolytic capacitor in a well-designed circuit will not be the weakest link in an inverter, even at high temperatures.

So – if you are considering microinverters here are two takeaways:

• Get the latest generation inverter, as the reliability is increasing substantially with each iteration
• Buy a good quality microinverter. In my opinion your only choice here is Enphase, SMA or SolarBridge. Unfortunately since SolarBridge was bought out by Sunpower, SolarBridge micros are not for sale any more, unless you want it on a Sunpower panel.

Next time I’ll post Part 2 of our interview where I lob this curvy one at Ciaran:

“If you use microinverters for a 5kW system, you have 20 separate inverters. Doesn’t this mean that to be as reliable as a central inverter system, the microinverters have to be 20x more reliable than a central inverter?”