Central Inverters or String Inverters?

There is much confusion and debate in the Solar PV sector industry regarding String and Central inverters. When I started to research for this article I became aware of this confusion, to some extent caused by the fact that almost everyone who is knowledgeable in this topic has a motivation to promote their own side of this debate, and so the facts become muddy.

So in this article I sought to provide some clarity and use empirical data as much as possible to find the real differences between string and central inverters in terms of equipment costs, installation costs, and performance.

In order to do this, I spoke to multiple manufacturers of string inverters, and multiple manufacturers of central inverters, and compared their data with independent case studies and BOQs published by research institutes and universities, and also multiple articles and reports written about this topic, while trying to evaluate the bias of the authors for greater clarity. In this article I try to present this information in a fair way and present my conclusions after going through this exercise.

I have entered this with an open mind and I conducted research as objectively as possible, but it is still necessary in the interest of honesty to divulge that I have more personal experience with string inverters than central inverters.

In this article I will describe the history of central inverters and string inverters, noting how the two technologies have evolved over time and how we have reached the present day situation, in part because this technology changes so rapidly and I found that many of the arguments on both sides are using outdated information about the other technology. There are new features in the string inverters that are often unaccounted for in the literature, and the same is true for central inverters. Articles written 2 or 3 years ago often have key points that are no longer valid due to advances in product features for string and central inverters. I have tried to use the latest product information provided by the product manufacturers, and I have tried to highlight where some of the latest advances in the technology nullify some of the arguments that may have existed as little as 2 years ago.

I will highlight some key points that will help you to compare and understand data sheets and equipment specifications between different central inverters and string inverters. And we will examine the performance characteristics, and cost implications of a utility scale solar plant that uses central inverters versus string inverters.

The last point of order before we begin is that I must acknowledge the wide variety of opinions on this subject. I do expect that some people out there may disagree with my conclusions, and I welcome any open and honest discourse about any of the data or opinions in this article. In my research I found that there are many sources out there who state opinions without any supporting data. When I came across conflicting opinions, I relied on data to inform my understanding of the issue.

Lets start by defining the terms String Inverter and Central Inverter.

What are string and central inverters?

A String inverter has a much smaller capacity than a central inverter. String inverters are designed to be modular and scalable. This means they are made to connect with one another to work together to achieve a cumulative high output of AC power. It is called a string inverter because it is somewhat similar to the array of panels connected in a series that we commonly call a string of panels. The inverters are also connected in a similar way, hence the name string inverter. This is a distributed architecture of a solar plant because the inverters can be distributed throughout the PV array.

Central inverters have a much larger capacity than a single string inverter. They took the name Central inverters because they tend to centralize the long PV strings, rather than the distributed architecture seen with string inverters.

Ironically, the trend in capacity of each type is toward the middle: string inverters have been increasing in size and are now and can vary from 1kw each up to around 300kw or 350kw each, and central inverters have been shrinking in size and now range in size from as big 4.5MW to as small as 700kw

Brief history

In the 1980s inverters were developed to support the nascent solar industry. These inverters, however, were based on technology for the drive system industry, and they were not optimized for use in solar PV applications. These inverters were central inverters that were only a few kilowatts in size. In the 1990s there was much research done into the drawbacks of this type of inverter, such as the 1000 Roof Program in Germany. Some of the challenges that were noted at that time were

? Mismatching losses by using Maximum Power Point (MPP) control for a large group of PV modules;

? Losses and risk of electrical arc in DC wiring; and

? Poor expandability and adaptability to customers’ requirements, due to little or no design flexibility.

In order to combat these challenges a modular type of system was introduced. The benefits that were sought after with this module type of design include

? Cost reductions and reliability improvements by using similar components;

? Simpler system design and installation by combining standard units; and

? Simpler fault finding.

Their progress resulted in the development of module integrated micro inverters. These are a small inverter that can be integrated into a single solar module, or added externally to a solar module. The concept is one micro inverter per module. This solved some of the inefficiencies of load matching because each solar module achieves its maximum power point through each micro inverter. However the low power ratings of these micro inverters proved to be quite inefficient and expensive, and replacements and maintenance proved to be costly and difficult.

With the pressure to increase system and manufacturing efficiencies the technology expanded in two directions. One direction was towards larger and larger central inverters to maximize on economies of scale in the manufacturing process, and reduce internal inefficiencies in the electrical components.. This direction led to the innovations which became the modern central inverters. And the other direction was towards a smaller, scalable, modular type of inverter that became the modern string inverters.

The first string inverters were introduced on the market in the mid 1990s. These string inverters were designed for one string of panels, and they were seen as a compromise between the module integrated micro-inverters and the larger central inverters.

In the year 2000 there was a major breakthrough in inverter technology that paved the way from the modern solar inverter industry. This breakthrough was the ability of an inverter to detect a failure in the grid supply and in the event of a grid failure, to automatically shut off its own power production. Before this breakthrough utility companies had been reluctant to connect solar inverters to the grid because of the high risk of safety hazards and damage to their equipment. The problem was that if a utility company needed to shut down a power distribution line for maintenance or for some other reason, the line must be completely de-energized for safety. Before this breakthrough feature of solar inverters, the inverter could not detect if a grid supply line had been shut down and it would continue to send the solar power back onto the line, compromising the safety of utility workers and equipment.

This breakthrough feature is now known as “anti-islanding” protection and it made the modern solar industry possible.

As new possibilities of applications of solar inverters opened up it became realized that with the manufacturing technologies available at that time that these units could be more cost effective if they were made even larger, mostly above 1MW per each. And so the first utility scale central inverters were produced. But these large sizes were not suitable for small and medium sized applications, so string inverters continued to be developed as well.

The early central inverters were relatively expensive, and performed with relatively low efficiency, but they proved to be quite sturdy. Generally the experience with these early central inverters is that they continued to perform through many obstacles and weather conditions. Over time inverter strides were made to decrease the cost of central inverters and increase their performance as well.

At the same time, string inverters were also becoming popular and were increasingly being used in small solar projects. String inverters were primarily used for residential applications around 1kw, and then slowly up to 5kw and 10kw, and then later into increasingly larger projects and then finally into utility scale projects. And over time the price reduced.

String inverters were soon developed with a transformer-less topology, which means there is no need for a transformer within the inverter. The internal change in voltage is achieved through power electronic components. This greatly improved the efficiency since there are no transformer losses within the inverter. It also allowed for a reduction in size and weight of the inverters. The negative side of the transformer-less inverter is that it requires some additional safety features, especially surge protection devices (SPD) Modern central and string inverters have these features built in, although external SPDs are often still used.

The next breakthrough was with the development of MPPT technology. The MPPT is a multi power point tracker. This dramatically increased the efficiency of inverters.?

Here you can see a graph of the IV curve and the power curve of a solar module. The vertical axis on the left is the current, and the horizontal axis on the bottom is the voltage. The redline shows how the current and voltage are related. At the maximum voltage, the current is zero, and at the maximum current, the voltage is zero. The maximum power point is the compromise point between voltage and current that results in the most power being produced. The blue curve shows the voltage across the bottom and the watts with the fight vertical axis, and the power that is produced at the maximum power point.

Every panel has a unique power curve at any time of the day which means each module has a unique maximum power point at any given time.

In a solar installation, the panels are connected in long strings with many panels connected in a series All of the panels in a string operate at a single power point. This means that there is a loss of efficiency because the weakest panel in a string will restrict the power point, and therefore restrict the production from all of the other panels. There are many reasons that a panel might be restricting the other panels in the string such as shadows, cloud cover, bird droppings, dirt cover, temperature differences, hidden cracks and more. Essentially, having more MPPTs and shorter strings will result in greater efficiency, and it will also give more flexibility for plant design using different module orientations, different elevations, different hours of sunlight and different shadow profiles without sacrificing production.

Ideally, every module would be able to operate at its own maximum power point, but this would require PV optimizers or micro inverters, and this has proven to be too costly on utility scale plant. The compromise is to have more MPPTs built into the inverters.

String inverters started having more and more MPPTs, which means more and more efficient power production, but also a relative increase in cost. Central inverters typically do not have many MPPTs, many central inverters only have 1 or 2 MPPTs per inverter, while string inverters can have as many as 10MPPTs per inverter.

As early as 2010 there was a significant increase in string inverters for utility scale plants, and a serious discussion about which technology is more effective at utility scale. Since then, string inverters have claimed more and more of the utility scale market share until around 2019 when the market share was almost evenly split, and since then string has surpassed central inverters, but the growth trend has slowed and remained stable at approximately 50% string and 50% central inverters.

Other developments in both string and central inverter technology include built in safety features,, advanced remote monitoring, advanced grid scheduling features, advanced fault detection and more. These will be discussed below.

When comparing the cost of central and string inverters, the central inverters are significantly less expensive. Sometimes the cost of string inverters can be almost twice as high as the cost of equivalent capacity of central inverters. However, inverters typically only represent around 5-7% of the total cost of the PV plant, so this increased inverter cost is often able to be absorbed.

Here is an example of a pie chart showing an approximate breakdown of different costs in a PV plant.

The performance of string inverters is significantly higher than the performance of central inverters in most scenarios. Estimates range from 4-8% more production, depending on the environment and other factors. And here we have the crux of the debate between central and string inverters for utility scale solar plants.

How much more expensive are the string inverters? What about the associated installation costs and accessories? Which is more expensive overall and by how much? What is the difference in performance and production? What are the O&M costs of string and central inverters?

All of these questions are crucial to understanding if string or central inverters might be right for your utility scale solar project, and what follows is my attempt to answer these questions in an unbiased and fact based manner, despite my warning earlier that there is much conflicting data out there and usually those publishing information only publish what supports their own business model.

Central Inverters and centralized plant architecture

Central inverters are a more mature technology than utility scale string inverters; they have been used in utility scale plants for years before the string inverters were used in utility scale plants. Therefore there is a more established pattern of performance over time and longevity.

Many developers and consultants choose central inverters because they feel safer in the knowledge that this technology is proven to be quite reliable and has been in use for more years than string inverters. There are many more examples of central inverters operating for more than 10 years than there are examples of string inverters operating for more than 10 years because string inverters are a newer technology, and therefore the perceived risk of central inverters is lower than with string inverters. This does not mean that string inverters are unreliable, the data shows that string inverters can have very low failure rates and can be used successfully in harsh weather conditions. There are hundreds of Gigawatts of central and string inverters both working well. Some Central inverter enthusiasts have claimed that central inverters are better suited for harsh weather conditions, however there is data to show that string inverters are perfectly capable of working in harsh weather conditions as well.

The architecture of a PV plant using central inverters is “centralized”, meaning the inverters are often in a central location with many PV strings terminating at these central locations. Often on large PV plants have many central inverters in more than one location around the PV array.?

This centralized architecture requires slightly more land area than a decentralized architecture because of the centralized inverter housing which requires space for itself, but also attention must be used to so that the centralized inverter housing will not cast shadows on the nearby PV panels.

Central inverters will typically require a concrete foundation to be laid in order to carry the weight of the central inverters, and also a crane is typically required to place the central inverters on the concrete foundation. This can add to the installation costs of a central inverter system, and also requires some analysis of the soil and bedrock for a reliable concrete foundation that will not settle over time.?

Currently, many central inverter manufacturers will offer a full “turn-key” solution that can include transformers, and switchgears, AC and DC disconnects, and other accessories that are pre configured and connected, which can make this aspect of the installation cheaper and faster than with using string inverters or non-packaged central inverter solutions.

Here you can see a modern type of central inverter used on a utility scale plant. Each of those container shaped objects is an inverter housing with inverter capacity of more than 1 MW and accessories. You can refer to the SLD below as an example of how this plant might be constructed.

Invariably, central inverters rely on DC combiner boxes to aggregate the PV strings. This means that the PV strings are run to DC combiner boxes, and from the combiner boxes then into the central inverter. A central inverter may typically have around 10 DC inputs, and each of these would normally come from a DC combiner. A DC combiner can have as many as 20 DC inputs, and 1 DC output. So, long strings of PV panels are combined at the DC combiner and then taken to the central inverter.?

Here you can see a SLD for a central inverter plant design. The inverter in this SLD is 2.2MVA, so a utility scale plant could have 10 or more of this block in place.

The PV strings on a central inverter design can be between 10kw to 20kw each. With around 8 to 15 DC combiners for each central inverter, and multiple strings in each combiner, the desired input range is reached.

This centralized plant architecture usually requires longer runs of DC cable, and often a larger diameter of DC cable than is commonly required for a plant design using string inverters. Typically the cost of DC cables is around 33% higher with Central Inverters.

This also means there will be extra power losses through the DC cables with Central Inverters. Typically the power losses through DC cables are around 1% more than the DC cable losses with string inverters.

Also, working with DC power is more dangerous than working with AC power. This means there are greater risks and greater costs during installation and maintenance when there is increased DC cable runs. In some countries like in the USA, a certified DC electrician is paid significantly more than a certified AC electrician. This can raise the costs of a central inverter installation.

String Inverters plant architecture

String inverters use a “de-centralized architecture” which means the inverters are located throughout the PV arrays. There is no inverter housing required, the inverters are typically installed on the panel mounting structure beneath the panels as shown below. Modern string inverters are water proof and dust proof so no inverter housing is required. This means you can save some of the installation costs required for laying a foundation for a central inverter station. However, the time to make the wired connections to dozens or hundreds of string inverters is greater than the time to connect to pre-packaged central inverters, and this can affect the installation costs.

Here you can see a utility scale solar plant using string inverters that are distributed throughout the PV array. This is distributed architecture of a solar plant. The inverters are connected in series similar to the way modules are connected in strings, hence the name “string inverters”. You can refer to the SLD below as an example of how this plant might be constructed

Each modern string inverter can be around 200-350kw, with as many as 18 or 20 string inputs directly on the inverter. This eliminates the need for DC combiners as a cost on the BOQ because the strings can terminate directly into the string inverters. This also means that the DC cables required are typically less, and typically smaller in diameter than those required for a central inverter installation. And this means the DC cable losses are typically less with a string inverter installation.

AC combiners are not required in modern string installations because often there is an AC panel built into the transformer station. However, the AC cables required in a string inverter installation are greater. This is because in a central inverter installation the output of AC power can be placed near the transformer and grid connection, while in a string inverter installation each inverter has an AC output, and the inverters are located throughout the array, so long lengths of AC cables are required from each inverter to the transformer and grid connection point. This means that the AC losses with string inverters are typically greater than the AC losses with central inverters. The AC losses in a string inverter installation are around 0.5% greater than with central inverters.

MPPT Density

Most central inverters have a single MPPT (or sometimes two MPPTs). This means all of the strings from all of the combiners operate at a single Power Point. With a Central inverter it is common that around 2000 to 4500 panels or more could be operating on this single MPPT. This results in significant performance and efficiency losses due to module mismatch.

There is no disagreement that module mismatch is greatly reduced with string inverters. And module mismatch can increase over time with the degradation of solar panels; as each panel degrades at a different rate, the module mismatch accordingly increases over time. Hotter weather conditions also contribute to a higher module mismatch due to temperature differences on the modules and other reasons. This increases the difference in performance ratio and production between string and central inverters.

Other factors that increase string mismatch and therefore increase the advantage of string inverters are clouds, and bifacial panels. If the location has clouds at any point during the day at any time during the year, the module mismatch will be increased during that time, and so the difference in performance between string and central inverters will be greater during that time. Bifacial panels are panels that have the solar cells on the top and on the bottom of the solar panel, and it is known that this greatly increases module mismatch, and therefore string inverters with their multiple MPPTs will perform better with bifacial modules than central inverters with bifacial modules.

Each inverter in a string inverter in an installation can have as many as 10 MPPTs, and a string inverter installation can have 4 or 5 inverters for each MW. This means for 1MW of string inverters you can have around 45MPPTs, compared to 1 MPPT with a central inverter. This is called high MPPT density, the greater the number of MPPTs in an array the higher the MPPT density is, and the higher the efficiency will be.

We have already discussed how this will increase efficiency and overall production due to the reduction of module mismatch, but high MPPT density also offers increased design flexibility. The greater number of MPPTs means that there can be greater variation in the PV strings without sacrificing production; there can be different orientations, different elevations, and different string configurations.

This is one reason that in a utility scale installation that uses single axis trackers, there is a slight advantage for string inverters. In a utility scale application with single axis trackers spread over an area of hundreds of acres, the trackers often operate somewhat independently to give a more precise angle of tilt to get the greatest direct solar irradiation for the greatest power output. This is especially true on the outer edges of the PV array; a utility scale solar array is often hundreds of acres, and so on opposite edges of the giant array there may be different angles required for optimal solar tracking and maximum production. This can increase the module mismatch, and so the effect of multiple MPPTs will be greater.

The multiple MPPTs is also one of the reasons that in a terrain that is not perfectly flat, string inverters have a distinct advantage. The uneven terrain creates increased module mismatch, which creates a greater advantage for string inverters. But also, a central inverter housing can often create larger shadows on an uneven terrain, which will require greater attention and greater space around the housing to prevent shadows on the nearby panels. Generally speaking, if the terrain is uneven it is accepted that string inverters are the stronger choice for your inverters.

However, this does not necessarily mean that on a flat terrain that central inverters are the consensus best choice. There is not a disadvantage to string inverters if the terrain is flat rather than uneven.

Black Start Capabilities

A?black start?is the process of restoring an electric?power station?or a part of an?electric grid?to operation without relying on the external?electric power transmission network?to recover from a total or partial shutdown.

Normally, the electric power used within the plant is provided by the station's own generators. If all of the plant's main generators are shut down, station service power is provided by drawing power from the grid through the plant's transmission line. However, during a wide-area outage, off-site power from the grid is not available. In the absence of grid power, a so-called black start needs to be performed to?bootstrap?the power grid into operation.

String inverters normally do not have this black start capability, while modern central inverters do have this capability. If having black start capability is important to your project, central inverters might be the best choice for you. Another option would be to have lithium ion storage added to your solar pv project with string inverters, which will add significant costs.

Remote Monitoring

A key component of modern solar installations is remote monitoring. Remote monitoring technology has come a long way for both string inverters and central inverters, however it is generally not disputed that string inverters have stronger remote monitoring than central inverters. This is mainly due to the fact that the string inverters can show string level information, and the central inverters cannot show string level information. Also the panel strings that are used with string inverters are much shorter than the panel strings for central inverters, so the remote monitoring of string inverters can show much greater detail than for central inverters.

If we refer to the above line diagrams for central inverters and string inverters as an example, the central inverter could only show data per each combiner box, which has 338 panels. The string inverter scenario could show string level information which means it can show performance data for each group of 26 panels. This has a significant benefit on troubleshooting and O&M activities because you can easily identify and locate under performing strings and modules.

Operations and Maintenance

Operation and Maintenance costs are an important factor to consider when selecting between string and central inverters.

Generally speaking string inverters do not require on going maintenance. However, when you increase the number of inverters at an installation this means that you will have a greater number of failed units, and so you will need a plan for operation of the many inverters. Typically developers and plant owners of string inverter installations will keep spare inverters on site for quick replacement in the case of a failure. Maintenance of string inverter is quite convenient because if a string inverter fails, it will be replaced by a new one and the replacement work will be done on the site of the solar power plant. And with the distributed architecture of string inverter plants, these few failures have an almost negligible effect on the overall production of the plant.

For example: on a 20MW site with central inverters using 20 pieces of 1MW inverter, if one inverter fails you are losing 5% of the production for that time. Often the repairs require a specialized technician, and it may require spare parts that are expensive and not easily available and so the time for repair can be extended.

In an equivalent plant using string inverters, 20MW using 100 pieces of 200kw inverter, even if 4 inverters fail at the same time you would only lose 4% during that time. And the standard practice is to have spare string inverters on site, so the faulty string inverter can be replaced quickly and easily without requiring specialized training. This problem can be solved in a matter of hours rather than days or weeks. The difference in lost revenue in these examples would be quite massive.

Central inverters are more complex pieces of equipment that typically have oil requirements, filters to be replaced, and cooling mechanisms that must be maintained. Central inverter plants typically require a small team of specialized technicians to remain on site and to perform ongoing checks and procedures to replace filters and check cooling mechanisms. While string inverter plants also often have technicians who remain on site, in most cases the number required is fewer than with central installations, and the tasks to be performed at a string installation site do not require the level of specialization and training that is required for the maintenance tasks at a central inverter site.

There are many sources who agree that O&M costs on a string inverter site will be much less than the O&M costs on a central inverter site. And there are a few sources that say the O&M on a central inverter site will be very close or even less than the O&M costs on a string inverter site. In my research the wealth of information shows that O&M on a central inverter site is more rigorous and more constant and requires more specialized technicians on site and uses consumable parts like filters, so I believe that the lower O&M costs belong to string inverter sites, even if it is not an enormous difference. Of course the specific O&M arrangement at each particular site can make a big difference on the O&M costs.

Modern string inverters have a great new feature to reduce O&M costs dramatically, and that is called IV Curve diagnostics. This is actually related to the Operation and Maintenance of the PV panels themselves. If a PV plant is not producing the expected amount of power, it is sometimes likely that there may be some problems with the PV panels. The old solution would be to send some technicians to the site to perform tests on each string using specialized equipment known as an IV Curve tracer. The technicians would be measuring the current and voltage of the strings to try to detect any abnormalities that might be hindering full power production. The technicians might spend 2 -3 days at the site, and while they are doing these measurements the DC power must be shut down for their safety which means significant loss of power. There are usually too many string and too many panels to test all of them, so they must use random sampling to hopefully identify what could be the problem. After 2 – 3 days at the site the technicians must analyze their data to produce a report that will hopefully identify the problem and solutions. This process of writing the report can take 1 – 2 weeks. All the while the PV plant is underperforming, and there was significant loss of power during the testing and you will have to hope that the random sampling will identify the problem! The new method using this IV Curve diagnostic feature is that the inverters themselves can perform this test. It can be completed in about 15 minutes, and it will complete the test on 100% of the strings, no random sampling required. This feature will also instantly generate a report that will show any abnormal IV curves, and suggest the possible cause of these abnormalities. The string level information will make it much easier to locate the problems, and when using string inverters the repairs can be done while shutting down a very small portion of the PV array which will significantly reduce the costs and production losses of this exercise.

I was not able to find any Central Inverters with a similar feature.

Power Line Communication

Previously the network communication of string inverters was seen as a challenge, and long lengths of communication cable were required to link all the string inverters, and these communication cables were often subject to interference, and damage requiring additional capex and O&M costs. However string inverters now often have a feature called Power Line Communication or PLC (not to be confused with Programmable Logic Controller). This feature uses the mega frequencies of the AC cables that are not in use for communication to carry data. The power is carried at frequencies around 50-60Hz, and this communication occurs at frequencies in the range of 10,000Hz. So in this way additional communication cables are no longer needed.

Advanced Grid Features

Some central inverter enthusiasts have claimed that central inverters have much more built-in intelligence than the string inverters, which allows them to have greater flexibility of production of reactive power, or grid scheduling and other advanced grid features. This may have been true several years ago, but now the latest string inverters also have reactive power control, and grid scheduling features that are on par with the features of central inverters.

Previously, only central inverters offered flexible grid management options like low voltage ride-through, remote-controlled power reduction, frequency stabilization through active power control, voltage stabilization through reactive power control and reactive power at night. But now these features are available in string inverters as well.

Operating Voltage Range

Another source of extra production from string inverters is the extra production every morning and evening when the sun is low in the sky and the voltage on the panels is low. Despite recent advances in expanding the DC input range of central inverters, modern string inverters typically have a wider operating range in terms of the DC input voltage. Every night when the sun is gone the voltage on the PV modules is 0 or almost 0, and as the sun rises in the sky the voltage on the PV modules slowly increases with the increasing solar irradiation. Because string inverters typically have a wider operating range in terms of DV input voltage, this means the string inverters will switch on and begin producing power earlier every morning. And the same is true in the evening, as the sun goes lower and lower in the sky the voltage on the PV arrays slowly decreases, and the central inverters will typically shut down earlier in the evening before the string inverters will. This is easily verifiable on the data sheets of the inverters. If you look for the DC input section, and then look for Operating range, you can compare to see which inverter will operate for longer hours every day, and typically the string inverters have a wider DC input voltage operating range and therefore longer hours of operation every day. Perhaps the difference is only 15 minutes every morning and 15 minutes every evening, this may not seem significant, but this results in 30 minutes every day, and the expected life of a solar plant is close to 20 years, so this ends up being quite significant.

Here you can see a solar irradiation curve across the hours of the day, and how a wider voltage range on the inverter can add extra hours of operation in the mornings and evenings.

Conclusion

After studying this topic and detail, my conclusions are as follows:

It is probably wise to conduct a detailed projection of the yields and the costs for both central inverters and string inverters for your utility scale solar project. You should take into consideration the terrain, the clouds may affect production, the temperature, and other factors as well.

Based on all data known to me, I would expect that the String inverters will have a greater yield by around 2-5% per annum, and the central inverters will have an overall lower BOQ by around 2-4%.

With results like this, basically what we are looking at is a choice between a lower CAPEX, or a higher yield and higher ROI over time. Generally speaking, I expect string inverters to produce a lower LCOE, which means the money that you spend on CAPEX can be recovered over time in more efficient power production with string inverters.

Additionally, my preference is toward using single axis trackers and bifacial modules, both of which will increase the advantages of using central inverters. In a study published by the National Renewable Energy Laboratory (NREL) it was shown that bifacial modules with single axis trackers will produce the lowest LCOE of all types of solar installations on 90% of the surface of the globe. This means that in 90% of the land area of earth, you will have a lower LCOE if you invest in single axis trackers with bifacial modules.

In this article we have explained many differences between string and central inverters, including the fact that the advantages of string inverters with higher MPPT density will be even greater when bifacial modules are used, and also when single axis trackers are used. Therefore, it follows that the most efficient solar PV plants that will produce the lowest LCOE and the highest return on investment will be plants that are constructed and optimized for

• Single axis trackers (please refer to my article on solar trackers for more specific data and information on how to optimize for single axis trackers) and
• Bifacial modules (please refer to my article on bifacial modules for specific data and information on how to optimize for bifacial modules) and
• String inverters

In conclusion I believe that detailed yield projections and detailed BOQ analysis for each site is very useful and should be conducted. I believe that string inverters will give a lower LCOE in most scenarios, and that the available data shows that it is very likely that for your utility scale solar plant the best choice is to use string inverters with single axis trackers and bifacial modules.

I hope this article was at least interesting and informative on this topic.

If you have any questions or comments or points of clarification I would be very happy to hear them and engage in discussions. Thank you for reading.