A solar micro-inverter, or simply micro inverter, converts direct current (DC) electricity from a single solar panel to alternating current (AC). The output from several micro-inverters is combined and often fed to the electrical grid. Micro-inverters contrast with conventional string or central inverter devices, which are connected to multiple solar panels.
Micro-inverters have several advantages over conventional central inverters. The main advantage being small amounts of shading, debris or snow lines on any one solar panel, or even a complete panel failure, does not disproportionately reduce the output of the entire array. Each micro-inverter harvests optimum power by performing maximum power point tracking for its connected panel. They are also simple to design and stock, as there is normally only a single model of inverter that can be used with any size array and a wide variety of panels.
Their primary disadvantages include a higher initial equipment cost per peak watt than the equivalent power of a central inverter, and increased installation time since each inverter needs to be installed adjacent to a panel (usually on a roof). This also makes them harder to maintain. Some manufactures have addressed these issues with panels with built-in micro-inverters.
Solar panels produce direct current at a voltage that depends on module design and lighting conditions. Modern panels using 6-inch cells typically contain 60 cells and produce a nominal 30 volts. For conversion into AC, panels are connected in series to produce an array that is effectively a single large panel with a nominal rating of 300 to 600 VDC. The power then runs to an inverter, which converts it into standard AC voltage, typically 240VAC/60 Hz for the North American market, or 230VAC/50 Hz in Europe.
The main problem with the "string inverter" approach is the string of panels acts as if it were a single larger panel with a max current rating equivalent to the poorest performer in the string. For example, if one panel in a string has 5% higher resistance due to a minor manufacturing defect, the entire string suffers a 5% performance loss. This situation is dynamic. If a panel is shaded its output drops dramatically, affecting the output of the string, even if the other panels are not shaded. Even slight changes in orientation can cause output loss in this fashion. In the industry, this is known as the "Christmas-lights effect", referring to the way an entirely string of series-strung Christmas tree lights will fail if a single bulb fails.
Additionally, the efficiency of a panel's output is strongly affected by the load the inverter places on it. To maximize production, inverters use a technique called maximum power point tracking (MPPT) to ensure optimal energy harvest by adjusting the applied load. However, the same issues that cause output to vary from panel to panel, affect the proper load that the MPPT system should apply. If a single panel operates at a different point, a string inverter can only see the overall change, and moves the MPPT point to match. This results in not just losses from the shadowed panel, but the other panels too. Shading of as little as 9% of the surface of an array can, in some circumstances, reduce system-wide power as much as 54%.
Another issue, though minor, is that string inverters are available in a limited selection of power ratings. This means that a given array normally upsizes the inverter to the next-largest model over the rating of the panel array. For instance, a 10-panel array of 2300 W might have to use a 2500 or even 3000 W inverter, paying for conversion capability it cannot use. This same issue makes it difficult to change array size over time, adding power when funds are available. If the customer originally purchased a 2500 W inverter for their 2300 W of panels, they cannot add even a single panel without over-driving the inverter.
Other challenges associated with centralized inverters include the space required to locate the device, as well as heat dissipation requirements. Large central inverters are typically actively cooled. Cooling fans make noise, so location of the inverter relative to offices and occupied areas must be considered.
Micro-inverters are small inverters rated to handle the output of a single panel. Modern grid-tie panels are normally rated between 220 and 245W, but rarely produce this in practice, so micro-inverters are typically rated between 190 and 220 W. Because it is operated at this lower power point, many design issues inherent to larger designs simply go away; the need for a large transformer is generally eliminated, large electrolytic capacitors can be replaced by more reliable thin-film capacitors, and cooling loads are reduced so no fans are needed. Mean time between failures (MTBF) are quoted in hundreds of years.
More importantly, a micro-inverter attached to a single panel allows it to isolate and tune the output of that panel. A dual micro-inverter does this for two panels. For example, in the same 10-panel array used as an example above, with micro-inverters any panel that is under-performing has no effect on panels around it. In that case, the array as a whole produces as much as 5% more power than it would with a string inverter. When shadowing is factored in, if present, these gains can become considerable, with manufacturers generally claiming 5% better output at a minimum, and up to 25% better in some cases. Furthermore, as a single model can be used with a wide variety of panels, new panels can be added to an array at any time, and do not have to have the same rating as existing panels.
Micro-inverters produce grid-matching power directly at the back of the panel. Arrays of panels are connected in parallel to each other, and then to the grid. This has the major advantage that a single failing panel or inverter cannot take the entire string offline. Combined with the lower power and heat loads, and improved MTBF, some suggest that overall array reliability of a micro-inverter-based system is significantly greater than a string inverter-based one. This assertion is supported by longer warranties, typically 15 to 25 years, compared with 5 or 10 year warranties that are more typical for string inverters. Additionally, when faults occur, they are identifiable to a single point, as opposed to an entire string. This not only makes fault isolation easier, but unmasks minor problems that might not otherwise become visible – a single underperforming panel may not affect a long string's output enough to be noticed.
The main disadvantage of the micro-inverter concept has, until recently, been cost. Because each panel has to duplicate much of the complexity of a string inverter, costs are marginally greater. This offsets any advantage in terms of simplification of individual components. As of October 2010, a central inverter costs approximately $0.40 per watt, whereas a micro-inverter costs approximately $0.52 per watt. Like string inverters, economic considerations force manufacturers to limit the number of models they produce. Most produce a single model that may be over or under-size when matched with a specific panel. With steadily decreasing prices, the introduction of dual micro-inverters that accept DC input from two solar panels, and the advent of wider model selections to match PV module output more closely, cost is less of an obstacle so micro-inverters may now spread more widely. In 2011, the introduction of dual micro inverters, that accept DC input from two solar modules rather than one, reduced equipment costs to the extent that PV systems based on this kind of micro-inverter are comparable in cost with those using string inverters.
Micro-inverters have become common where array sizes are small and maximizing performance from every panel is a concern. In these cases, differential in price-per-watt is minimized due to the small number of panels, and has little effect on overall system cost. The improvement in energy harvest given a fixed size array can offset this difference in cost. For this reason, micro-inverters have been most successful in the residential market, where limited space for panels constrains array size, and shading from nearby trees or other objects is often an issue. Micro-inverter manufacturers list many installations, some as small as a single panel and the majority under 50.
Micro-inverters that accept DC input from two solar panels, rather than one, are a recent development. For small items like micros, the cost of packaging can be a considerable fraction of the overall cost of the input parts. By placing two inverters in a single case, the parts cost is reduced per inverter, especially as a function of cost per watt. Some systems simply place two entire micros in a single box, while others duplicate only the MPPT section of the system and use a single DC-to-AC stage for further cost reductions. Some have suggested that this approach will make micro-inverters comparable in cost with those using string inverters.
The microinverter concept has been in the solar industry since its inception. However, flat costs in manufacturing, like the cost of the transformer or enclosure, scaled favorably with size, and meant that larger devices were inherently less expensive in terms of price per watt. Small inverters were available from companies like ExcelTech and others, but these were simply small versions of larger designs with poor price performance, and were aimed at niche markets.
In 1991 the US company Ascension Technology started work on what was essentially a shrunken version of a traditional inverter, intended to be mounted on a panel to form an AC panel. This design was based on the conventional linear regulator, which is not particularly efficient and dissipates considerable heat. In 1994 they sent an example to Sandia Labs for testing. In 1997, Ascension partnered with US panel company ASE Americas to introduce the 300 W SunSine panel.
Design of, what would today be recognized as a "true" microinverter, traces its history to late 1980s work by Werner Kleinkauf at the Institut für Solare Energieversorgungstechnik (ISET). These designs were based on modern high-frequency switching power supply technology, which is much more efficient. His work on "module integrated converters" was highly influential, especially in Europe.
In 1993 Mastervolt introduced their first grid-tie inverter, the Sunmaster 130S, based on a collaborative effort between Shell Solar, Ecofys and ECN. The 130 was designed to mount directly to the back of the panel, connecting both AC and DC lines with compression fittings. In 2000, the 130 was replaced by the Soladin 120, a micro-inverter in the form of an AC adapter that allows panels to be connected simply by plugging them into any wall socket.
In 1995, OKE-Services designed a new high-frequency version with improved efficiency, which was introduced commercially as the OK4-100 in 1995 by NKF Kabel, and re-branded for US sales as the Trace Microsine. A new version, the OK4All, improved efficiency and had wider operating ranges.
In spite of this promising start, by 2003 most of these projects had ended. Ascension Technology was purchased by Applied Power Corporation, a large integrator. APC was in turn purchased by Schott in 2002, and SunSine production was canceled in favor of Schott's existing designs. NKF ended production of the OK4 series in 2003 when a subsidy program ended. Mastervolt has moved on to a line of "mini-inverters" combining the ease-of-use of the 120 in a system designed to support up to 600 W of panels.
In the aftermath of the 2001 Telecoms crash, Martin Fornage of Cerent Corporation was looking for new projects. When he saw the low performance of the string inverter for the solar array on his ranch, he found the project he was looking for. In 2006 he formed Enphase Energy with another Cerent engineer, Raghu Belur, and they spent the next year applying their telecommunications design expertise to the inverter problem.
Released in 2008, the Enphase M175 model was the first commercially successful microinverter. A successor, the M190, was introduced in 2009, and the latest model, the M215, in 2011. Backed by $100 million in private equity, Enphase quickly grew to 13% marketshare by mid-2010, aiming for 20% by year-end. They shipped their 500,000th inverter in early 2011, and their 1,000,000th in September of the same year. In early 2011, they announced that re-branded versions of the new design will be sold by Siemens directly to electrical contractors for widespread distribution.
Enphase's success did not go unnoticed, and since 2010 a host of competitors have appeared. Many of these are identical to the M190 in specs, and even in the casing and mounting details. Some differentiate by competing head-to-head with Enphase in terms of price or performance, while others are attacking niche markets. A major competitor outside Enphase's core North American markets is Enecsys, who started in the UK and now sells worldwide.
Larger firms have also stepped into the field; OKE-Services updated OK4-All product was recently bought by SMA and released as the SunnyBoy 240, while Power-One has introduced the AURORA 300. Other major players include Enecsys, SolarBridge and SolarEdge. Since 2009, several companies from Europe to China, including major central inverter manufacturers, have launched micro-inverters—validating the micro-inverter as an established technology and one of the biggest technology shifts in the PV industry in recent years.
Another approach is the power optimizer, which is essentially a DC-to-DC version of the microinverter - everything except the actual "inverter". The power optimizer has all the benefits of the micro inverter in terms of MPPT performance, module isolation, monitoring etc., but is even smaller and simpler than the microinverter. The arrays are then connected to a string inverter as normal. This approach minimizes the distributed costs, and theoretically produces a lower-cost system overall. However, the long runs of high-voltage DC wiring feeding the inverters remain, along with the need to group panels into acceptable groups to get those voltage levels.
Two companies dominate the optimizer approach. SolarEdge uses a complete MPPT and DC-to-DC buck/boost converter on the panel, and uses a simplified inverter, lacking these features, at the end of the strings. The inverter also centralized communications between the optimizers and external networks, notably the internet. In 2010, the company shipped an estimated 250,000 power optimizers and 12,000 inverters, amounting to a total generation of 50 megawatts.
In North America, the market leader As of 2012[update] is Tigo Energy of California. Tigo systems do not provide MPPT on a module basis, but instead use a system of impedance matching to reduce the negative impact of under-performing panels. This greatly reduces the complexity and weight of the on-panel system, to the point that they clip onto the panel rather than having to be mounted to the panel racking systems. The output from the optimizers is, in effect, simply a "corrected" version of the normal DC output the panel would produce, and the panels are then strung in series and connected to any conventional string inverter.
The period between 2009 and 2012 included unprecedented downward price movement in the PV market. At the beginning of this period, panels were generally around $2.00 to $2.50/W, and inverters around 50 to 65 cents/W. By the end of 2012, panels were widely available in wholesale at 65 to 70 cents, and string inverters around 30 to 35 cents/W. In comparison, micro-inverters have proven relatively immune to these same sorts of price declines, moving from about 65 cents/W to 50 to 55 once cabling is factored in. This has led to widening losses as the suppliers attempt to remain competitive.
- Where Microinverter and Panel Manufacturer Meet Up Zipp, Kathleen “Solar Power World”, US, 24 October 2011.
- Market and Technology Competition Increases as Solar Inverter Demand Peaks Greentech Media Staff from GTM Research. Greentech Media, USrs, 26 May 2009. Retrieved on 4 April 2012.
- SolarWorld's SW 245 is a typical modern panel, using 6" cells in a 6 by 10 arrangement and a of 30.8 V
- SMA's SunnyBoy series is available in US and European versions, and the recommended input range is 500 to 600 VDC.
- "Productive", Enphase
- Muenster, R. 2009-02-02 “Shade Happens” Renewable Energy World.com. Retrieved on 2009-03-09.
- "Increase Power Production", eIQ Energy
- "Enphase Microinverter M190", Enphase Energy
- Kerry Dolan, "Enphase's Rooftop Solar Revolution", Forbes, 8 November 2010
- Micro inverter model ranges stepping up roughly in 10 Watt or 20 Watt increments. Ecopen.homelinux.net. Retrieved on 2012-12-07.
- "All systems", the very first entry on 25-March-2011 was a single-panel system
- SolarBridge and PV Microinverter Reliability, Wesoff, Eric.Greentech Media, US, 2 June 2011. Retrieved on 4 April 2012.
- Katz, p. 3
- Katz, p. 4
- "Appreciation Prof. Dr. Werner Kleinkauf", EUROSOLAR
- "Connect to the Sun", Mastervolt, p. 7
- "Utility Line Tie Power", Trace Engineering, p. 3
- "OK4All", OK-Services
- "GreenRay Solar, History of the Technology". Greenraysolar.com. Retrieved on 2012-12-07.
- Katz, p. 7
- "Connect to the Sun", Mastervolt, p. 9
- "Enphase Energy surpasses 500,000 solar PV inverter units shipped"
- "Journey to the 1,000,000th Microinverter"
- Yuliya Chernova, "Will Solar Become A Standard Offering In Construction?", Wall Street Journal, 2 February 2011
- See this product for instance, or this one, and compare to photos of the M190
- SPARQ's design uses a single high-power digital signal contoller with few supporting components
- Like Island Technology's system aimed at thin-film modules which have different voltage ranges than conventional cells
- "Power-One launches 300W Micro-Inverter and DC/DC Power Optimizer", Power-One press release, 4 May 2011
- "A roundup of new players", Greentechmedia
- Eric Wessof, "$25M for SolarEdge’s Solar Panel Power Optimizers ", greentechsolar, 4 October 2010
- Impedance Matching", Tigo Energy, 20 February 2013
- Galen Barbose, Naïm Darghouth, Ryan Wiser, "Tracking the Sun V", Lawrence Berkeley Lab, 2012
- Eric Wesoff, "Enphase Update: Stock Price Slammed After PV Microinverter Firm Loses CFO, Losses Widen", Greentech Media, 8 August 2012
- David Katz, "Micro-Inverters and AC Modules",
- Model based control of photovoltaic inverter Simulation, description and working VisSim source code diagram
- Micro-inverters vs. Central Inverters: Is There a Clear Winner?, podcast debating the ups and downs of the microinverter approach.
- Grid-Connected Solar Microinverter Reference Design Very detailed article for micro-inverter electronics design.