Microinverters vs. optimizers

Module-level electronics: The challenge of increasing the yield of a typical solar module has been taken up with aplomb by both microinverter and power optimizer suppliers over the past few years. Yet with both technologies claiming the upper hand in terms of power performance, pv magazine examines both sides of what looks increasingly like the same coin.

Issue 10-2014

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Is the module-level power electronics (MLPE) market on the cusp of delivering disruptive change to the solar industry? A report in October last year by Navigant Research suggested that the sector will grow from the 1,158 MW installed in 2013 to 12,844 MW installations annually by 2020. Worth an estimated $308 million last year, the MLPE market could top $2 billion within a decade, the analysts forecast. Currently, according to GTM Research, the sector accounts for just 2.5% of the global PV market, but is rising fast. In 2013 alone, the industry attracted more than $600 million in venture capital (VC) funding.
In September, IHS Technology published their own report suggesting the market will enjoy a compound annual growth rate (CAGR) of 27% to reach $1.1 billion by 2018 as demand for MLPE takes off in both established and emerging markets. “The market has grown to more than $300 million in size, despite continued price pressure due to new entrants into the business and decreasing PV system prices,” said IHS senior analyst for solar inverters, Cormac Gilligan.
However, solar commentator and former SunEdison CEO Jigar Shah recently questioned the industry’s sustained appetite for MLPE technologies. In a Greentech Media podcast in July, Shah asked whether the leading proponents of MLPE – namely DC power optimizer and AC microinverter technologies – had yet fulfilled their potential, and queried whether the solar industry was in fact too conservative to embrace technologies that are, in the main, progressive.
As the centralized inverter market continues to effectively salve the wounds it suffered in the immediate aftermath of the global solar downturn, Shah’s concerns should be taken seriously. The continued pre-eminence of traditional inverter giants SMA and Power-One (the latter’s ABB acquisition notwithstanding) suggests such conservatism is rife: long-dominant, bankable market players extending their reach in the industry based on reliable reputations and cast iron supply chains abound. “Many traditional inverter suppliers have been cautious to date in entering the microinverter market,” said Gilligan. The analyst added, however, that an increasing number of inverter manufacturers have begun exploring microinverter acquisitions and even in-house creation of their own products.
Navigant Research evidently expects a disruption of this stranglehold to emanate from within the MLPE market, but Shah’s concerns over solar’s innate reticence are well-founded. With both power optimizers and microinverters vying for their own slice of the solar pie, there is no clear winner, with experts divided over which technology is best placed to lead this anticipated MLPE surge. And there is nothing the solar industry deploresmore than shades of gray. So the question remains: will it be power optimizers or microinverters powering MLPE’s drive for disruption over the next decade?

No clear leader

The debate over optimizers versus microinverters has been tackled on only a handful of occasions in the past.
Navigant Research expects 52.7 GW of MLPE technologies to be installed between now and 2020, but shies away from predicting which technology will dominate. A February report by GTM Research found that over half of all residential solar PV installations in the U.S. have a microinverter or DC optimizer installed. IHS forecast last month that power optimizer shipments will increase 160% in the U.S. in 2014, while in Japan the technology will achieve a 7% penetration rate in all PV installations by 2018. In reporting terms, the two technologies are grouped as one, but at an installation and marketing level the battle lines have long been drawn, with proponents from both camps arguing their cause passionately.
Lior Handelsman, SolarEdge’s VP for marketing and product strategy, and company founder, told pv magazine that SolarEdge’s “edge” over its chief rival in the MLPE space – microinverter specialist Enphase Energy – lies in the fact that power optimizers are more flexible, reliable, and cost-effective than microinverters.
“The topology of DC optimizers is better, for one. Microinverters are hampered by their limited geographic range,” said Handelsman, explaining that because grid costs and complexities differ across the globe, compliance with microinverters is often more difficult. “The elaborate nature of microinverter installations not only increases installation and maintenance costs, but actually limits the technology’s ability to grow into new markets. Because optimizers are connected to a central inverter, it is easier to move from market to market.” SolarEdge’s chief growth market is the U.S., but the company has also enjoyed recent success in France, U.K., the Netherlands, Austria, Switzerland, and Germany, and is branching out into Japan and other Asian markets. Handelsman cites cost, efficiency and peace of mind as the chief demand drivers for the technology’s growth. “Price-wise, microinverters are inherently more expensive. They are usually installed one per module, so the dollar per watt price is the same for a kilowatt system as it is for a megawatt system,” he said.
SolarEdge claims that its optimizers offer a cost of $0.40 – $0.50 per watt, as opposed to $0.50 – $0.60 per watt for microinverters. This pricing decreases as installation size rises, making optimizers a more scalable option. Furthermore, SolarEdge claims, a DC optimizer will also run at approximately 98% efficiency, typically losing just 2% of its performance through heat. The best microinverters on the market can typically only manage a 96/4 split.
“Based on a typical datasheet, optimizers do have, by design, higher efficiency,” said Zhi-Min Ling, CEO and Chairman of China-based microinverter manufacturer APS. “DC to DC is more efficient, so it is not surprising that optimizer suppliers claim these higher efficiencies. But this is not the real argument. The real issue is overall kilowatt hour production, so if you run the two systems side-by-side to assess the true energy production, I am confident that microinverters perform better.” Both industries promise potential power increases of 25% – claims that can rarely be sustained in real world conditions, experts argue. At the other end of the scale, power increases often come in between 2 – 4% on average for both technologies, tests have shown. Faced with such difficulty in assessing the true efficacy of either technology, the debate over which is most suitable is shunted on to other decisive factors, namely cost (ROI), ease of installation, and safety.

Commercial and safety issues

SolarEdge’s Handelsman argues that optimizers come up trumps in terms of cost, installation, and flexibility. “You can have two modules to one DC optimizer, which lowers costs and is great for commercial-scale installations,” he said. “Indeed, you rarely see commercial-scale solar installations with microinverters, and they are unheard of at a megawatt scale.” This alleged rarity of microinverters in the commercial rooftop space is disputed by Ling. Although the APS CEO agrees that microinverter technology is not particularly well suited for ground-mounted installations, large commercial rooftops have begun to see the benefits of installing microinverters. “The technology has a lot of advantages for factories, shopping malls, government buildings, schools, and public areas, with the key demand driver being safety,” said Ling, who revealed that an increased awareness about safety issues such as arcing and DC voltages among the general public has helped swell demand for microinverters.
“When we talk to the customer, safety is always our – and their – number one priority. Since the introduction of the NEC 2014 (National Electrical Code adopted in the U.S.) and a wider awareness of DC safety levels for rooftops in China (below 120 DC), discussions about safety have increased,” Ling said. A microinverter limits the amount of DC current on a rooftop, converting the electricity to AC almost immediately – a situation that is potentially favorable for some types of rooftop installation, particularly in the U.S. where wooden houses and rooftops present a greater fire risk.
In Japan, microinverter suppliers could soon gain Japan Electrical Safety & Environment Technology Laboratories (JET) certification, says IHS, which may open up the country’s huge residential and commercial market. “Even if they gain JET certification,” warns Gilligan, “there will still be some challenges for microinverter suppliers in Japan. There is a huge preference for local brands in that market, and inverters or microinverters would normally be sold in kits to residential owners via the module manufacturer.”
Compliance with safety regulations, allied to invaluable adherence to customers’ peace of mind, is equally paramount for the optimizer sector. “Power optimizers are no less safe than microinverters, because whenever the AC to the system goes down, the product is designed to automatically shutdown the modules, meaning there is no dangerous DC voltage if the inverter stops working,” said Handelsman.

Assessing the benefits

Ostensibly complicated, the purpose of MLPE is simple: to overcome the shortcomings of traditional, centralized inverters. Optimizer proponents will argue that it is inefficient and expensive to add a microinverter to every module if DC power optimizers can offer improved design flexibility, enhanced safety, maximized power, monitoring benefits, lower costs and greater reliability. But is either technology worth the unverifiable and erratic power output increases?
“A typical DC optimizer will eliminate power mismatches effectively, delivering better performance than microinverters under soiled or shaded conditions for a better power return all year round,” claims Handelsman, who also argues that the design flexibility afforded by DC optimizers is another plus. “With a fixed string voltage, cabling is reduced and combiner costs are cut in half.” Ling stresses that eliminating complexity at installation is a big attraction to customers and installers, and believes that APS’s three-phase microinverter makes the product more attractive than ever for residential and commercial installations, even up to 100 kW in size. “The three-phase microinverter is ideal for larger rooftops. While the installation may seem complex, installers do not have to worry about balancing, and the output AC side is three-phase based on a parallel input system. It is not only safe, but quicker to install.”
In debating installation and complexity, it is easy to overlook the bigger picture. Proponents of power optimizers argue that a smoother installation is favorable to cheaper and quicker. With fewer parts, no intricate wiring and a more simplified architecture on the rooftop, there is an argument to suggest that power optimizers are more durable than microinverters. With potentially greater scalability, optimizers hold plenty of cards. Then again, optimizers are reliant on the performance of one central inverter. If that goes, the system goes. With microinverters, a module or inverter failure affects just one module. Point, counter point. Optimizer suppliers claim, however, that a central point of failure is easy to identify and fix, whereas microinverters’ more complex system is not only harder to fix, but more costly to replace. In this great game of PV top trumps, both sides appear to hold a strong hand.
But the trump card – performance – remains face down on the table. Neither technology can confidently claim to offer better power output at lower cost than the other. Yet somewhere between these two opposing viewpoints lies an encouraging truth – in developing their technologies, microinverter and power optimizer suppliers are collectively moving MLPE forward. Outside the inner sanctum of either, the PV industry is gratefully grouping them together and – as evidenced by recent reports – appears confident that both will prove a decisive and disruptive addition to the solar PV family in the very near future. That, as Shah pointed out, is perhaps the only discussion worth having.

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To optimize or microinvert?

Pro-DC:
A DC power optimizer is designed to increase the power yield of solar modules. Attached to either every module in an array or two-to-one, the optimizers – not the modules – form a string that leads to a central inverter at the end. The benefit comes when some modules may be soiled, shaded, or degenerating faster than others. Rather than a poorly performing module dragging down the entire array’s performance, optimizers perform maximum power point tracking (MPPT) at module level before sending the optimized voltage to a central inverter to convert from DC to AC.

Pro-AC:

A microinverter is attached directly on to a solar module, converting DC electricity from each module into the AC power that all electrical devices and grids require. The benefit being that microinverters can produce more power than central inverters, which group all the power from multiple solar panels before converting it into AC. In a nutshell, a microinverter will deliver the optimum power available for each module, whereas a central inverter exposes the entire output to the weakest link, such as a solar panel that is shaded or soiled, or oriented slightly differently.
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