Integrator Perpectives on Module-Level Power Electronics

Integrators weigh in on equipment advances, market variables and NEC requirements that impact the sale and deployment of module-level power electronics systems in the US.

Seven years have elapsed since Enphase Energy released its first-generation microinverter system in July 2008, a launch that for practical purposes created the modern module-level power electronics (MLPE) product class. In the ensuing years, microinverter products from vendors including ABB, APS, Darfon, SMA America and SolarBridge Technologies (acquired by SunPower) have entered the US market, as well as module-level dc optimizer products that include systems from SolarEdge Technologies and Tigo Energy.

As the MLPE class has evolved, string inverter manufacturers have been hard at work refining and enhancing products with new features such as dual independent MPP trackers and wide voltage input windows. These upgrades make string inverters and their centralized, off-the-rooftop power conditioning and optimization more flexible in terms of array layout, and more competitive with some of the benefits MLPE has historically offered, such as shade mitigation and the ability to configure source circuits (ac or dc) with different numbers of modules.

Integrators have experienced the highs and the lows associated with the development and deployment of MLPE. I reached out to several integrators across the US to get their perspectives on the past, present and future of module-level systems.
 

How would you characterize your experience with MLPE systems?

Over the last 7 years, Sun Light & Power has dabbled in several MLPE products and deployed others in earnest. We had an early interest in microinverters, especially for residential projects, but we experienced a relatively high failure rate in the field. DC optimizers seem more reliable, with the inverting taking place off the roof and in a centralized device.

We have designed hundreds of systems with Ampt, Enphase Energy, HiQ Solar, Maxim, SolarBridge Technologies, SolarEdge Technologies and Tigo Energy module-level products. The majority of our recent experience has been with Tigo and HiQ, with about 100 systems built. We have become more interested in string-level MPPT devices such as Ampt String Optimizers and HiQ TrueString inverters for commercial systems and have been moving away from module-level devices for projects other than residential rooftops.

On my house I have Tigo optimizers on a 3 kW system that overproduces compared to modeled performance. It has two additional monitoring systems; as an engineer, I like to have all of the data. The PV system powering the Sun Light & Power headquarters uses the HiQ 208V Mini Inverter and HiQ Balancers. Since the HiQ Balancers provide MPPT for each module, we built the system from a medley of surplus modules without worrying about matching their electrical characteristics. Access to granular data from our rooftop array is very useful for testing.

The largest MLPE system we have designed and built is about 2,000 modules, just over 500 kW. In retrospect, the amount of data available is more overwhelming than useful.

—Blake Gleason, PE, director of engineering, Sun Light & Power, Berkeley, California

Technicians For Sustainability has installed more than 30 microinverter systems, the first in December 2008. Our experience is limited to Enphase and Power-One (ABB). We generally install microinverters in residential systems with less than 12 modules.

—Kevin Koch, president, Technicians For Sustainability, Tucson, Arizona

We have been installing systems using MLPE for about the last 5 years. We have projects with Enphase microinverters and SolarEdge optimizers and inverters, as well as LG and SunPower ac modules. We have roughly 400–500 MLPE systems installed, the majority of which are Enphase, although SolarEdge is quickly catching up. Our largest MLPE system is approximately 20 kW.

—Fortunat Mueller, co-founder, ReVision Energy, Portland, Maine

At my previous employer, we installed the entire San Diego Unified School District project—31 different schools—exclusively with Enphase microinverters. The sites ranged from 60 kW to 200 kW, for a total of approximately 3.2 MW.

With Sunsense, I primarily use optimizers and inverters from SolarEdge. I’ve also worked with SunPower ac modules and ABB’s microinverter system. Sunsense began using SolarEdge products in early 2014, when Colorado started to require rapid-shutdown functionality. Previously, the majority of our residential systems used SunPower-branded string inverters or Enphase microinverters, which Sunsense has installed for the last 5 or 6 years. We typically use MLPE on residential and small commercial systems.

The largest system that we’ve installed using dc optimizers is a 15.6 kW residential array in Colorado’s Roaring Fork Valley. We have three additional 25 kW–50 kW and 11 residential MLPE projects on our schedule. These projects will utilize SolarEdge optimizers and inverters. We also have one scheduled Enphase project and one ABB microinverter project. I would not be surprised to see MLPE system numbers double by the end of this year.

—Doni Nicoll, logistics manager and residential systems designer, Sunsense Solar, Carbondale, Colorado

I have been selling Enphase microinverters almost exclusively since late 2009. We have designed and installed hundreds of residential and commercial systems. At one site, we have four buildings, each with 27 kW and totaling 108 kW. The largest single-building Enphase system we have designed is 43 kW.

—Jason Szumlanski, principal solar designer, Florida Solar Design Group, Fort Myers, Florida

In terms of annual installed system capacity, approximately what percentage of your installations use MLPE? How has this changed over time?

About 20% of our installed systems in the last 3 years have used MLPE products. However, the MLPE projects tend to be residential, so the overall installed capacity is more like 5%–10%. For example, I looked at 63 of our projects with Tigo Energy MLPE. The average system size is 29 kW, and the median size is 8 kW. We had a ramp-up of MLPE projects starting in 2009 that peaked about 2 years ago. Since then we have focused more on string-level MPPT.

—Blake Gleason, PE, Sun Light & Power

Less than 3% of our system designs use MLPE. With the advent of dual MPPT string inverters, we expect that percentage to decrease.

—Kevin Koch, Technicians For Sustainability

Historically, we used MLPE only when there was a compelling, site-specific reason, such as substantial shading, multiple roof planes or a system that we expected to expand in small increments over time. These projects represented about 20%–25% of our residential volume in 2013. In 2014, with the introduction of small string inverters with wide input-voltage windows, better shade tolerance and dual MPPT inputs (SMA America’s Sunny Boy TL-US models in particular), the percentage of ReVision’s projects using MLPE fell to 10%–15%. However, in 2015, due to the adoption of NEC 2014 in one of our markets, the number of systems using MLPE increased rapidly to 30%–35% of our overall residential volume and close to 100% of our residential systems in New Hampshire and Massachusetts. Almost all of these systems use SolarEdge equipment.

—Fortunat Mueller, ReVision Energy

In just the last year, Sunsense has moved almost exclusively to SolarEdge products on the residential side. We also specify Enphase and ABB microinverter systems. In 2014, approximately 30% of our systems utilized MLPE. This year approximately 82% of our residential and small commercial systems will use some type of MLPE.

—Doni Nicoll, Sunsense Solar

Before adoption of NEC 2014 in Massachusetts in January 2014, MLPE systems were roughly 10% of our business. We mainly utilized MLPE as an upgrade for existing systems that had developed shading issues. Eighteen months later, MLPE systems make up more than 90% of our business.

—Brandon Stephens, director of Massachusetts operations, Sunlight Solar Energy

In 2009, the company I ran went from 100% string inverters to more than 95% microinverters overnight. A few customers still wanted string inverters for various reasons, and some modules we sold were not compatible with microinverters.

—Jason Szumlanski, Florida Solar Design Group

What version of the NEC is in effect in the regions your company serves? Have Code changes, such as Section 690.12, impacted your use of MLPE?

California generally adopted NEC 2011 in 2014. We have not yet been tempted to use MLPE for rapid shutdown because we do not yet have the rapid-shutdown requirement. However, we have been pushing toward rack-mounted string inverters for a variety of reasons, including looking forward to meeting rapid-shutdown requirements. I do not anticipate needing module-level shutdown capabilities. Occasionally we work with a local fire marshal who requires an unusual level of remote disconnecting means. In some cases, features such as PVSafe from Tigo Energy have been sufficient to satisfy those fire marshals.

—Blake Gleason, PE, Sun Light & Power

The jurisdictions we work in have not adopted NEC 2014, and we are not subject to rapid-shutdown requirements. This has allowed us to continue to select products based on quality, performance, reliability, manufacturer stability and strength of warranty.

—Kevin Koch, Technicians For Sustainability

We are on NEC 2014 in Maine, Massachusetts and New Hampshire. Massachusetts and New Hampshire are starting to enforce 690.12, which forced a move to MLPE. So far, Maine is not enforcing the rapid-shutdown requirements. The state is waiting for more products to come to market that are listed and work effectively with popular string inverters.

—Fortunat Mueller, ReVision Energy

In our installation area, we have at least 22 different AHJs. We deal primarily with NEC 2011 and NEC 2014. Prior to the adoption of NEC 2014, we were aware of some of the advantages of dc optimization but never had the right excuse to fully dive in.

—Doni Nicoll, Sunsense Solar

Massachusetts adopted NEC 2014 in January 2014.  Initially, we used a contactor-based rapid-shutdown approach, as the industry was trying to catch up with 690.12 requirements. Only one manufacturer had the equipment to meet the new requirement, and it was expensive and difficult to procure. We chose to design an in-house shutdown system, based on dc contactors mounted in junction boxes. We installed one within 10 feet of the array and one near the inverter, as required. This approach, combined with an additional kill switch located outside the home, near the utility meter, was expensive, but it enabled us to pass our electrical inspections. Thankfully, by Q2 of 2014, SolarEdge’s line of inverters and optimizers offered a rapid-shutdown solution that enabled us to make an efficient, cost-saving transition.

In our experience, AHJs approve SolarEdge systems because they meet all section 690.12 requirements, make system shutdown easy and are not overly complex. We have passed 95% of our electrical inspections in the last year since we made the switch to full MLPE utilization.

—Brandon Stephens, Sunlight Solar Energy

Florida is currently on NEC 2008.

—Jason Szumlanski, Florida Solar Design Group

How do MLPE products impact the cost of systems? Is there an effect on your cost of doing business? For example, do you realize increased sales, system design, inventory, installation or O&M efficiencies?

MLPE products generally cause a significant increase in our installed cost and the end customer’s price. Many of these costs were not obvious at first, and we had to learn the hard way. Some of these costs have decreased over time as MLPE products have improved, and as we have become familiar with various products and developed our own best practices.

Certainly, there is an up-front equipment cost increase for MLPE. Often, there is additional design cost in figuring out details such as mounting (if MLPE is nonintegrated), grounding, custom cables and monitoring requirements. When an MLPE product is relatively new, there has been a beta-testing cost; we end up working with product manufacturers to smooth out the wrinkles in their product, processes and documentation. Sourcing and securing the product adds procurement costs, especially since normal procurement channels are still coming up to speed and since MLPE devices can evolve so quickly that last month’s standard product is now obsolete.

Installation costs include mounting the boxes, wiring, grounding, dealing with communications, and mapping modules and associated devices from their physical location to a paper layout. After installation, there are additional setup costs for the monitoring websites, including transferring the paper map to the website map of individual module locations.

On the flip side, we have occasionally realized some savings on O&M when the MLPE assists with troubleshooting array problems such as faults. However, it is much more common for us to incur additional O&M costs to fix, reboot or replace malfunctioning MLPE products or related communications devices.

The primary benefit of MLPE, besides increased production in some cases, has been on the sales end. In certain opportunities, offering the MLPE solution has increased our close rate and therefore reduced our cost of sales. If we were more comfortable with microinverters on residential projects (and if we sold more residential projects), we could increase sales and preliminary design efficiency because of reduced focus on string length requirements.

—Blake Gleason, PE, Sun Light & Power

Microinverter designs improve the cost equation for very small systems, and this is our primary application of MLPE. For installations below about 2.5 kW, or less than nine or 10 modules, the installed cost of a microinverter system is lower than that of a string inverter system. We do not see an installed cost savings for dc optimizer–based systems.

On the flip side, our O&M costs are significantly higher for microinverter systems, as we have had multiple failures requiring repeated deployments. The jury is still out as to whether dc optimizer systems such as those from SolarEdge and Tigo Energy will have higher or lower O&M and long-term performance costs compared to string and microinverter systems.

—Kevin Koch, Technicians For Sustainability

Using MLPE definitely increases the installation cost (both labor and hardware), but the difference isn’t huge. I don’t think we’ve seen any savings related to increased sales, design or inventory savings. Frankly, our O&M experience with MLPE systems has not been great. First-generation Enphase inverters failed at an intolerably high rate in our coastal climate. Our experience with the first-generation SunPower ac modules was equally bad, though we installed significantly fewer of those. The cost and customer disruption for microinverter replacements are why we have been less than enthusiastic adopters of MLPE, and why we’ve moved back into that realm only due to regulatory pressure. So far, our experience with the newer generation of products from Enphase and SolarEdge has been substantially better. We are cautiously optimistic that the MLPE vendors have resolved some of the early design and manufacturing issues.

—Fortunat Mueller, ReVision Energy

The additional equipment cost associated with MLPE systems has not slowed sales. We’re on track to sell more residential systems than we did in the previous year. One improvement to MLPE is that system monitoring has become easier to configure. From an O&M standpoint, these products allow us to dive into a system and find an issue, but most likely, we’ll get an email before we even know about the issue. Designing and permitting systems has become more streamlined. MLPE offers a wide range of string options and allows mismatched strings with different numbers of modules. From a design and installation point of view, this has been extremely beneficial.

—Doni Nicoll, Sunsense Solar

We appreciate the flexibility in module selection for both the 60- and 72-cell SolarEdge variants, and we’ve found that SolarEdge systems are reliable and simple to install. Module-level monitoring is beneficial for both our clients and our installers. The increased efficiency in monitoring allows us to remotely troubleshoot the rare system malfunction and limit costly service calls.

—Brandon Stephens, Sunlight Solar Energy

Sticking with one primary product greatly reduces procurement complexity, design work and stale inventory. While microinverters cost more on a dollar-per-watt basis, the advantages of streamlined operations nearly outweigh the additional costs. The advantages in terms of sales features easily overcome the price premium. Additionally, the peace of mind that module-level monitoring provides is valuable to system owners.

—Jason Szumlanski, Florida Solar Design Group

What are some primary sales, design or installation advantages of MLPE systems?

On the sales side, MLPE allows us to “ignore” or, in reality, mitigate shade. For presentation purposes, many MLPE products allow us to showcase flashy granular monitoring. For design and installation, we can vary the number of modules per string or per ac circuit, which allows more flexibility. We also can eliminate jumpers between arrays. Finally, in many cases we can use longer and therefore fewer strings.

—Blake Gleason, PE, Sun Light & Power

Microinverters reduce the inverter and associated labor cost for small systems and provide increased design flexibility. We have used microinverters to facilitate cost-effective system expansion. However, we generally feel that these advantages are not worth the long-term O&M risk.

—Kevin Koch, Technicians For Sustainability

The biggest advantage is that MLPE opens up areas of the roof that would historically have been unusable, such as areas partially behind a chimney or a dormer adjoining the main roof. Being able to utilize more of the rooftop helps us meet our customers’ growing desire to meet more of their energy needs with a PV system. MLPE simplifies design somewhat, but, considering the current generation of string inverters with dual MPPT inputs and wide voltage windows, residential system design is already incredibly flexible and straightforward.

—Fortunat Mueller, ReVision Energy

Anyone who designs SunPower systems knows that working with its high-voltage modules limits MLPE equipment options, which can be challenging at times. SolarEdge offers a good solution for these systems.

—Doni Nicoll, Sunsense Solar

We have installed more than 30 residential systems in 2015 to date, ranging from 2.5 kW to 15 kW, and we have another 30 systems scheduled for installation in the next three months. In a highly competitive market such as Massachusetts, technological advances in system components must be customer friendly. Key selling points of MLPE systems are shade mitigation, design flexibility, module-level performance monitoring and overall increased system output. The SolarEdge system offers all of these features, plus a 30% equipment savings over microinverter systems and a small 2%–3% cost increase over current string inverters. This has led to higher customer satisfaction and an increase in sales.

—Brandon Stephens, Sunlight Solar Energy

Module-level monitoring, diagnostics and repair are clear selling points. MLPE greatly simplifies design. For example, a single-line diagram can describe a wide range of microinverter system sizes when the contractor enters just a few parameters. This approach expedites the AHJ’s plan review, as the personnel reviewing drawings become comfortable with the standardized electrical schematics. Installation is somewhat streamlined. Simplifying the training of new employees is a key advantage.

—Jason Szumlanski, Florida Solar Design Group

What are some primary sales, design or installation limitations of MLPE systems?

The primary limitations are in two very important areas: long-term O&M costs and the availability of compatible replacement products over the 25-year design life of a PV system. Inverters will need replacing at least once during that time. For example, a 30-module string inverter system would require at least one inverter replacement. With an MLPE system, the inverters may require servicing 30 times or even more to maintain the operation of that system.

Of more concern, however, is the possibility that the MLPE product used will no longer be available. If a string inverter manufacturer goes out of business, we feel confident that we can substitute another brand of string inverter. This may require limited reworking of conduit runs, but otherwise it would be a straightforward process. In an MLPE system, if the manufacturer goes out of business, the trunk cable will create compatibility issues if we need to substitute replacement products.

MLPE replacement is significantly more labor intensive than string inverter replacement. This labor and technology risk is something consumers should be aware of. In our cost-benefit analysis, the gain in flexibility or performance that MLPE systems offer over the current string-level inverter technology does not justify that risk. It is imperative for the long-term viability of the distributed generation solar market that we think through and prepare customers for the O&M issues that will come up over time.

—Kevin Koch, Technicians For Sustainability

Because our MLPE systems are currently mostly SolarEdge, we are limited somewhat by the choice of inverter sizes and voltages. In particular, there are not great options for smaller projects connecting to 3-phase power systems, but that is an issue with both microinverters and string inverters.

—Fortunat Mueller, ReVision Energy

Most MLPE manufacturers are less than 10 years old and several are already out of business. Manufacturing defects or bad batches are also an issue. In our experience, MLPE vendors offer outstanding customer service, back their products and replace faulty equipment in a timely manner. On the installation side, many MLPE systems require an Internet connection for commissioning, which can be problematic.

—Doni Nicoll, Sunsense Solar

The additional cost of microinverters puts installers at a slight competitive disadvantage compared to installers using string inverters. The competitive arguments against microinverters, including increased potential points of failure, can be a disadvantage at times. Consumers who are strictly price shoppers may not see the value in MLPE.

—Jason Szumlanski, Florida Solar Design Group

What has been your experience with the reliability of MLPE components? How does it compare to non-MLPE components or systems? Has your experience with MLPE reliability changed over time?

Our experience with MLPE reliability has generally been poor, although it has improved somewhat over time. The arguments MLPE manufacturers have put forth—that many small points of failure are better than one or several large points of failure, and that you can defer component replacement until you perform regularly scheduled maintenance—have not held water. The reality is that most customers do not want faulty equipment languishing, so we end up with more truck rolls. A handful of MLPE failures versus a central or string inverter failure once in 10 years leaves customers wondering about the overall reliability of the system they purchased.

Specifically, the weak points of MLPE appear to be communications, component failures or obsolescence, and compatibility with AFCI devices for dc optimization products. We have been through a few painful cycles of MLPE beta testing and experiencing the “bleeding edge” along with our customers. Overall, the reliability and maturity of MLPE is increasing.

—Blake Gleason, PE, Sun Light & Power

Our experience with first-generation microinverters was not positive, and that has made us fairly conservative about adopting this technology. As a result, string inverters tend to be our default choice unless a compelling reason, such as site conditions or Code requirements, pushes us to MLPE. In the last few years, we have had better experiences, although we still have more issues overall with MLPE systems than with string inverter systems.

—Fortunat Mueller, ReVision Energy

The Enphase M190 microinverter admittedly had a very high failure rate. Some of our systems required 100% replacements with either the same model or upgrades to the M215. Enphase has been proactive and generous in supporting its warranty on early models. The failure rate went to near zero with the introduction of the M215, with most failures occurring at or soon after installation. We have had essentially no failures of the Enphase M250. 

String-inverter failure rates were also high for our clients, with some brands outperforming others. One brand in particular had a near 100% failure rate in less than 10 years. Other brands have been very solid, with near-zero failure rates. I would consider the new generation of microinverters to be as reliable as the best brands of string inverters, although only time will tell.

—Jason Szumlanski, Florida Solar Design Group

What advancements in MLPE components or systems would increase your use of these technologies?

True integration of the module-level components with the modules themselves is a necessary first step. Several manufacturers have accomplished this. The MLPE warranty must be covered for the full term of the module warranty and must be covered by the module manufacturer. Additionally, dc optimizers need full compatibility with AFCI and other string inverter features.

—Blake Gleason, PE, Sun Light & Power

Universal connections and standardization for compatibility between products would be a major development, as would ownership by stable businesses capable of supporting the service requirements over the system design life. Ultimately, advancements that minimize long-term O&M costs will benefit the solar industry and contribute to the increased adoption of MLPE technologies.

—Kevin Koch, Technicians For Sustainability

Most important, we need increased reliability and decreased cost. A distant third would be continued improvements in the robustness of communications and ease of setup. However, MLPE communications are now much more reliable and user friendly than they were just a few years ago.

—Fortunat Mueller, ReVision Energy

More MLPE options for high-voltage modules are needed. MLPE cost needs to continue decreasing while manufacturers develop products that can last as long as the module. Most inverters have warranties of only 5–10 years, whereas most modules have 25-year warranties. That’s a tough act to follow.

—Doni Nicoll, Sunsense Solar

Integrating monitoring into the MLPE would be a good idea. The monitoring gateway is a point of failure with a shorter warranty period that adds complexity to the system. Enabling microinverters to connect directly to existing Internet routers would be ideal. Alternatively, a lower-cost and simpler gateway device would be a good improvement.

—Jason Szumlanski, Florida Solar Design Group

 

CONTACT:

Joe Schwartz / SolarPro / Ashland, OR / solarprofessional.com

Article Discussion

Related Articles