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Selecting Residential Grid-Tied Inverters

April 22, 2017

When we think of solar photovoltaic (PV) systems, it is easy to focus most of our attention on the most visible components of the system - the solar modules. While there are some key considerations when selecting modules, most reputable modules will last 30-40 years. During that time, the solar inverter is likely going to need to replacement - perhaps just once if you use the right one. The inverter converts the direct current (DC) power generated by the solar array to alternating current (AC) used in our home and provided by the electrical utility. There are several common approaches to accomplish this in contemporary residential solar photovoltaic (PV) systems:

  • String inverter: a string inverter is the longest-used method for integrating a grid-tied PV system. The approach commonly uses a single inverter. PV modules are connected directly in series to form "strings." The system will include one or more strings that are wired to the DC input side of the inverter. In addition to converting this DC current to AC, a string inverter provides maximum power point tracking (MMPT). MPPT varies resistance within the string to maximize the output of the solar array. Some string inverters have a single MPPT channel, which means that the entire solar array is tracked together. Newer models may contain two or three MPPT channels, which allows the array to be tracked in smaller pieces. This is advantageous when part of the array faces a different direction or is shaded differently.
  • Microinverters: An approach that gained some traction about a decade ago was the use of microinverters. Rather than inverting at a system level, microinverters invert at the module level. The most common approach is to install a microinverter for each module. To reduce installation labor, some manufacturers integrated the microinverter directly into their module, forming what is commonly referred to as an "AC module". With microinverter systems, the modules are wired to their microinverter, and the microinverters are wired to one another. With microinverters, the inversion from DC to AC and MPPT occurs at each microinverter.
  • Optimized string inverter. With string inverters and microinverters, inversion and MPPT happens in the same device. In DC-optimized systems, a string inverter converts from DC to AC and DC optimizers installed at solar array provide MPPT. In many systems, an optimizer is installed for each module. Some systems - typically larger, commercial ones - utilize an optimizer for every two panels. Similarly to microinverters, the modules are wired to the optimizers and the optimizers are wired to one another.

Performance

One of the arguments for microinverters and optimizers - also known as module-level power electronics (MLPEs) - is that the output of each module is fairly independent of other modules in the array. For instance, if one module is shaded in a string with a standard string inverter, it can have an impact on the performance of other modules in the string. With MLPEs, the shaded module is impacted, but other modules in the array are minimally impacted, if at all. MLPEs also address the impact of module mismatch. Modules are commonly sorted by the manufacturer at 5 or 10W increments. As a result a set of 290W modules may vary in rated output from 290-295W. When wired in series without MLPEs, the output of a string will be determined by the lowest rated module. For systems that are constantly in full sun, there isn't a huge difference between these approaches as it relates to system output. When there is partial shading, the difference in performance depends significantly on the model of string inverter used. Older models are not nearly as shade tolerant as current models.

Life-Cycle Cost

Grid-tied PV systems have typically been considered low maintenance. The modules are fairly simple in construction and are expected to have a service life of 30-40 years. Inverters are expected to need replacement at least once during the lifespan of a PV module. With standard string inverters, inverter replacement is rather simple. The inverter is typically installed in an accessible location, which leads to an efficient process for removal and replacement. With internet monitoring standard with most grid-tied systems, a failed inverter can be identified and replaced in a single truck roll. The use of MLPE creates a more challenging situation, especially for roof-mounted systems. MLPEs are installed between the modules and the roof, requiring removal of modules for access. These electronics are also in the hottest portion of the system, increasing wear and tear. MLPEs have become rather popular as a result of code changes that require rapid shutdown of electricity in the DC wiring. The purpose of this requirement is to protect firefighters from electrocution in facilities where PV systems are present. Many MLPEs inherently provide rapid shutdown protection close to the array. With a few exceptions (one of which is our Solar Shed) string inverters commonly require a dedicated rapid shutdown device.

Our general approach

Our goal is to provide a high-performing, durable system that provides the strongest return on our clients' investments. We are also dedicated to doing the extra work needed to ensure that we are providing the solution that best fits our clients' needs. From a perspective of durability, we seek to minimize the number of electronic components in our system design. We have witnessed failure of MLPEs, such as Enphase microinverters and SolarEdge optimizers, on a scale that significantly exceeds their published failure rate. Given this track record in the relatively short time that these devices have been available, it only makes sense to utilize such equipment if it is necessary or impacts system production significantly enough to outweigh the additional liability. As a result, a majority of our systems utilize high-efficiency string inverters, such as the SMA Sunny Boy US series. These inverters feature up to three MPPT channels, high tolerance for shading, and are assembled in Colorado by a manufacturer with a strong reputation for standing behind their products. When combined with the REC Twin Peak module, which has a unique design that also mitigates the effects of shading, we are able to provide a system that generates electricity at a rate equivalent to those that utilize MLPEs while significantly reducing likely points of failure. For example, an average roof-mounted residential system with an SMA inverter has two electronic devices - the inverter and a single rapid shutdown device. The rapid shutdown device is the only electronic component installed between the array and the roof. An equivalently sized system with microinverters would have roughly 20 electronic components on the roof and a system with optimizers would have 20 components on the roof in addition to a string inverter.

Custom solutions

Occasionally, we encounter a unique design that requires a more customized approach. For example, if we are installing a system on two roofs where a portion of the array on the lower roof will be shaded for a significant portion of the day by the upper roof, we can strategically utilize Tigo optimizers with an SMA Sunny Boy inverter. This approach allows us to minimize points of failure while optimizing only those modules that will benefit. For systems composed of several small arrays that do not meet the minimum requirements for string inverters (DC-optimized or not), microinverters provide access to solar that might not otherwise be available.

Why not SolarEdge?

Our Maine solar clients occasionally ask us why we don't offer SolarEdge as an option. The SolarEdge system is the most commonly used approach in Maine. In the past we did offer SolarEdge - in fact, we installed over 50 SolarEdge systems in 2015 and 2016. This was one of the few options available at the time, as Maine had adopted rapid shutdown requirements and SMA had delays getting a viable rapid shutdown solution into the marketplace. Our experience with SolarEdge has not been a rewarding one. Of the 50-plus systems we installed, 60% required intervention due to failed optimizers, failed inverters, and firmware issues that caused systems to shutdown erroneously. Even when faced with a failure rate in the first months of system operation that far exceeded their published numbers, the company failed to adequately stand behind their product. Our reputation is important to us, and we spent a great deal of our own time and money resolving these problems. We offer an industry-leading 10-year labor warranty on the systems we install, which puts a greater liability on us to select the right product. Without adequate support from the manufacturer and amidst concerns about how these systems will survive 25 years in Maine's environment, we ended our business relationship with SolarEdge in late 2016.

Expertise you can trust

Based firmly in our commitment to traditional Maine values, we have worked to align our economic interests with those of our clients. Our 10-year warranty is one part of this equation. Insource Renewables has been built by word-of-mouth. We recognize that word-of-mouth either spells success when done right or can undermine a business that is not responsible and responsive. With this approach, we are able to provide Maine solar customers with a piece of mind - the piece of mind that you are receiving the best long-term solution for your particular application, and the piece of mind that should anything arise with the system that we will be there to address it.  

Contact Us For A Free Consultation

(207) 659-1054