When it’s time to choose an inverter for a PV system, the system owner and the installer have the choice to use a string inverter or micro inverters for the installation. It is important to realistically assess the viability of this technology.

Micro Inverter or the AC-Module Inverter: Truths vs. Myths

Contributed by - KACO New Energy Inc.

Filed Under - General Industry Articles - Technology Discussions

Kaco New Energy

KACO feels that some important issues have been overlooked and need to be discussed in depth before a decision on which inverter technology to buy takes place.

First let’s take a brief look at micro inverter history. Some folks overlook the fact that the micro inverter concept was first introduced in the 1970’s and that real world products were launched in the 1990’s. Many of the concepts used today in the design of the micro inverter are based on research done by Professor Werner Kleinkauf from the Institut für Solar Energie-versorgungstechnik (ISET). i His work in the development of micro inverters or “module integrated converters” prompted the manufacturing of AC products all over the world. Most companies like ZSW manufactured a range of micro inverters from 100 to 400 watts. Due to the small size of the PV market and a lack of financial backing however ZSW and companies like it were defunct or acquired before the PV market grew in the early 2000’s. Other companies like AES (United States), OKE-Services (Netherlands; recently acquired by SMA), and Ascension Technology gave the micro inverter a try in the 90’s. Let’s take a closer look at some of the issues that were present then and still today have not quite been answered.

Truth or Myth?

Residential performance monitoring per module for PV monitoring helps homeowners.

This is an interesting feature of the micro inverter options in the market today, and at first glance may seem desirable. Nonetheless, when we look closer it becomes clear that an inherent flaw in this feature has been overlooked. A system owner who carefully watches their PV system performance and does not understand the mechanics of a PV system is liable to over react to the multiple data points. For example, if one panel produces less than expected, this not so tech savvy homeowner is immediately alerted. The overall performance of the system is still at an acceptable level, however, this system owner will contact the installer to complain about their “under performing” system. It is also important for thorough training of the user as to how to use the monitoring capabilities of this system fully. Therefore, with increased maintenance calls, training costs and the upfront cost of the hardware and software the lifecycle costs associated with maintaining this owners’ system could then be much higher than with a string inverter. It is apparent that the ability to monitor the performance of each single module at all times can present both convenient and inconvenient scenarios for the system owner and the party responsible for maintenance.

At the other end of the spectrum are system owners who prefer the convenience of a built in inverter display. This system owner may not be as technically interested in the day to day functionality of their PV system and will want to check if their inverter works properly without additional accessories and monitoring software. String inverters, from most manufacturers are sold with a display that shows the most pertinent performance data. This is not possible with micro inverters. The micro inverter system requires the use of monitoring hardware and software to access the systems’ performance data. Not all system owners demand the granularity of module level monitoring or even want this level of performance data. High performance, accurate monitoring and easy maintenance are all important aspects of a well designed and properly installed solar system. However, it will become both physically and financially overwhelming over the course of 20 years to make certain that every panel in every 1.5kW system in the field performs to 100%.

Truth or Myth?

A micro inverter has an MTBF (Mean Time Between Failures) as high as 300 years – This means they are more reliable than string inverters.

Most current commercially available versions of the micro inverter concept have been on the market for no more than three years. The MTBF calculation is frequently given as a metric to prove the reliability of the micro inverter. However, this is a rather imperfect measurement of reliability, since the products are not “tested” except by using very basic theoretical mathematical analysis of the probability of component failures. It is true that many of the modern conveniences we enjoy today are tested by this same method. Imagine however that car crash safety rating was based solely on mathematical evaluations of impacts through modeling alone. This is convenient but not practical in the real world. This approach to validating the effectiveness and reliability of the micro inverter components has been used since the inception of the AC module concept. Thorough research was conducted by many organizations in the 1990’s about the longevity and reliability of micro inverters. One study published in 1997 mentioned that “It is expected that before the next century 50,000 – 100,000 AC modules [with micro inverters] will be installed. With these numbers the reliability of AC modules [with micro inverters] will be proven by the year 2000.” ii Why all the hype with no results? As with many other “revolutionary” ideas funded with vast sums of investment capital the arguments have been theoretical and based on scientific and mathematical calculations yet to be proven in the real world.

Micro inverters have gained a significant amount of combined field operation and therefore it seems folly to use the MTBF as a proof for reliability. This is questionable for at least two obvious reasons: First, it is obvious that in depth, long term testing of the products currently for sale in the market has not been possible within the time frame of the current micro inverter manufacturer’s first successful launch until today; Second, MTBF can not realistically predict the reliability of the individual components that inverter manufacturers use for inverter production. Don’t forget that bad batches of components make it through to final production even for the most trusted and experienced manufacturers and even passive devices, such as resistors, if compromised can halt the proper function of a PV inverter.

Truth or Myth?

Folks need micro inverters because they allow the use of mismatched PV module brands.

This other alleged advantage of micro inverters when compared to the use of string inverters is important to consider. KACO believes it is important to realistically consider how many systems under 10kW will benefit from this type of advantage to begin with. Yes, it gives the owner the flexibility to add modules to the system over time and makes it possible to exchange modules with another brand after the system has been installed. But is this a real concern for most residential installers?

Also, it is important to remember that most micro inverters are not compatible with positive grounded modules. Many string inverters are versatile in accepting both positive and negative

Truth or Myth?

Micro inverters do not represent a single point of failure, and this is a good thing for PV performance and reliability.

On the surface this argument is valid when looking at one string inverter vs. many micro inverters in one system. However, consider that a micro inverter is a small device consisting of well over 100 electronic components. Each component just as in a string inverter is a possible point of failure. Therefore you are effectively increasing the potential points of failure exponentially by utilizing micro inverters vs a string inverter. The micro inverters must be located in harsh environments and high heat locations that will quickly decrease the life of the components regardless of the temperature ratings of the components. Most evidence presented to convince users that high heat degradation is an invalid concern is based on sometimes vague and undefined references to “test” sites. iii Frequently no reference is made to the duration of the test, only the size, site location and model number of the micro inverter and/or reference to a competitive string inverter. Study after study researched for this essay made no mention as to how long temperatures were measured on testing sites iv , no mention of the orientation or azimuth is usually referenced and no mention as to whether or not competing arrays were identical to the test array – nonetheless time and time again the outcome claims that the micro inverter site exists and data has been gathered and extrapolated confirms the micro inverter should last up to 50 years in the field and produces 3% more than comparable string inverters. Very large thermal deviations over time will certainly affect the electro- mechanical connections of the components inside the inverter via expansion and contraction.

Incidentally the location of the micro inverter installation will subject the product to more frequent and probable surges from nearby lightning strikes. Most micro inverters provide very limited circuit protection (small surges up to 6kV seem standard, the peak current uses is relatively low compared to the significantly higher real world surges), as per UL 1741.

A string inverter on the other hand, has the following protective components already integrated:

  • Heavy duty EMC filters to protect against high-frequency line interference and transient spikes on both the PV input as well as the AC output
  • Semiconductors which have higher power ratings designed to withstand longer term and higher energy transient conditions
  • Large overvoltage varistors and spark gaps to protect power semiconductors from high energy transients found on the grid side
  • DC overvoltage varistors to protect against transients and surge pulses

The micro inverter is more susceptible to damage from energy transients and surges from near lightning strikes and from grid transients. The micro inverters basically represent a single point of failure as all of the inverters will inevitably fail as would any other product in catastrophic situations. When this happens, all micro inverters will need to be replaced at a huge cost that will include downtime, as well as re-installation costs whereas a string inverter represents a single point of replacement.

It is important to note that most micro inverters can not be repaired. String inverters can be repaired, recycled and sometimes re-used. When a micro inverter fails they will likely be thrown in the trash. The units are set in potting compound which is a type of epoxy resin which can not be removed without harsh chemicals. Harsh chemicals must be used to loosen the epoxy to recycle the electronics.

Truth or Myth?

Micro inverters minimize the impact that module shading has on cumulative power yield.

In a system that uses a string inverter the poorest performing module will determine the system’s overall harvested power. With a micro inverter system, each module is an independent, optimized power producer. Regardless, it is unwise to mount solar modules where there is significant chance of shade. It won’t matter which inverter the installer chooses as no amount of harvesting will replace absent irradiation. This argument makes sense for only a very small portion of the residential PV market.

Truth or Myth?

Costs for a micro inverter are higher but simplified installation makes up the difference..

Based on industry research on current MSRP pricing at leading distributors of PV equipment this argument can not be considered to be entirely true. KACO staff completed a caparison of the costs of two 5 kW PV systems – one with a string inverter and the other using a micro inverter.

The result: a micro inverter system plus monitoring is nearly 2 times the cost of a comparable KACO string inverter with a watchDOG monitoring solution. In the research conducted by KACO this comparison played out with multiple manufacturers in several scenarios.

When the same comparison was done for a 100 kW system using these two inverter technologies the difference is even more significant. The cost of using micro inverters for a system of this size is almost 3 times higher than using a string inverter.

Estimated shipping costs were considered in this analysis. An often overlooked fact for smaller systems is that the weight of a micro inverter shipment is usually double that of a string inverter. The weight of the inverters necessary to complete the analysis of our 5 kW reference system is more than twice as high as the weight of one string inverter. Therefore on top of higher upfront product cost there will be higher shipping costs.

Conclusion?

In today’s dynamic solar industry it seems that the micro inverter technology appears to be most commonly considered for residential systems because of the apparent shading mitigation advantage and module level granularity. However, residential system costs must continue to drop to be attractive to the home owner whose own home value has dropped significantly over the last two years. There are definitive theoretical benefits to the concept of the micro inverter. However, it is vital to realistically consider the pros and the cons of all technologies. It should be at the forefront of this discussion that the pressure to reduce the cost of the micro inverters will eventually demand that compromises be made which will inevitably affect their reliability.

Also important is the underlying question: Is it necessary for the homeowner who operates a 3.5 kW system to see every module and be alerted every time debris reduces the production of their system by 4%? We think not. The benefit is not great enough to the homeowners and business owners that want solar to be forced to pay for the additional benefits. These are currently being sold yet again to the industry at large as necessary evolution of the industry. More cost effective solutions have been developed to solve the issues brought up in this paper. KACO will ensure that these are available with our inverters and welcome a healthy discussion about this topic. These are some of KACO’s thoughts about micro inverters. What are yours?

i - H. Oldenkamp, I.J. de Jong, AC modules: past, present and future; Workshop Installing the Solar Solution, 22‐23 January 1998, Hatfield, UK
http://www.oke‐services.nl/downloads/199807_acmodules_paper_wcpec02.pdf

ii - H. Oldenkamp, I.J. de Jong, AC modules: past, present and future; Workshop Installing the solar solution, 22‐23 January 1998, Hatfield, UK; section: The future: a growing market and improving designs
http://www.oke‐services.nl/downloads/199807_acmodules_paper_wcpec02.pdf  

iii - H. Oldenkamp, I.J. de Jong, The return of the AC‐Module Inverter, 2009, The Netherlands, section 5
http://www.oke‐services.nl/downloads/200909_acmodules_paper_24eupvsec.doc

iv - H. Oldenkamp, I.J. de Jong, The return of the AC‐Module Inverter, 2009, The Netherlands, section 5
http://www.oke‐services.nl/downloads/200909_acmodules_paper_24eupvsec.doc

Comments (2)

Posted by D Thomas on 12/23/10, 07:11 AM

I can tell you that micro inverters make a huge improvement in harvest if there is any shading, or as we have in the northeast, snow on panels. Snow on even one panel, kills a string output, when other panels may be clear. I know this from first hand when living in upstate NY where snow is a fact of life. Without individual panel monitoring you don't know what you are loosing, but trust me it may be a lot. Also, you don't have to subscribe to online monitoring if you don't wish and still have the same basic data available as a central inverter system at no cost. I would not consider a string/central system anymore. Enphase seems to be the leader in micros and I have been very satisfied with them. Check out real systems data output on there website and see for yourself.

Posted by Bruce Sturgis on 05/20/10, 12:22 PM

Truth or Myth: Micro Inverters allow easier connection to a regular AC household circuit and does not require running separate DC wiring and circuits. Thus simplifying installation a bit... Also simplifying adding more PV modules to current installations.

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