Exploring practical synergies between MLPE and UL 3741 rapid shutdown tech

Solar panels in the desert sun (courtesy: Bureau of Land Management)

Contributed by John Lerch, Senior Director of Global Marketing at Tigo Energy

Rapid shutdown for solar energy systems has gone from an interesting safety feature to being mandated by building codes, standards bodies, and even insurance companies, as well as finding strong advocacy from firefighting organizations–all around the world.

Module-Level Power Electronics (MLPE) and microinverters were among the first technologies to offer a way to quickly de-energize a solar system at the module level, via a manual switch or automatically. The reasoning for requiring rapid shutdown is simple: protect the safety of first responders (e.g., firefighters), who may need to access the area in and around the array.

MLPE devices have been central to helping installers meet rapid shutdown requirements, and in 2020 the National Electric Code (NEC) added the mechanical protection scheme known as UL 3741 to incrementally enhance the protection rapid shutdown offers. Naturally, the solar trade show and continuing education circuits have been brimming with news, rumors, products, approaches to, and training for UL 3741, and installers are busy evaluating the merits and viability of the same.

As a mechanical solution, UL 3741 has also gained attention for the potential it may hold to reduce up-front capital expenditures (Capex), particularly for commercial and industrial (C&I) rooftop PV systems, because the specification does not explicitly require MLPE devices. As with so many of the twists and turns of the solar coaster, a little bit of context is required to get a more holistic view.

Zooming in, the very specific installation requirements indicated to comply with UL 3741 can be expensive; more expensive than using MLPE, in fact. While code compliance and Capex are important, if not vital, considerations for getting systems into the ground, the levelized cost of energy (LCOE) over time is no less important for the viability of solar in general. Taking the holistic view of how to deliver the most safety through rapid shutdown, however, requires an understanding of which system components are de-energized, to what extent they are de-energized, and how long they take to de-energize. MLPE and UL 3741 differ in this aspect, as well.

As with pretty much all solar projects, the best possible outcome means choosing the right approach and technology for a site based on a thorough understanding of the goals the asset owner has for the installation. Factors such as consistency of energy production, tolerance for operations and maintenance (O&M) costs, and system design flexibility, to name just a few, are no less important to ‘making solar sustainable’ over time. For example, focusing only on the Capex involved in a solar project can mean ignoring the post-commissioning life of a solar installation.

With O&M costs of a typical C&I solar system at approximately $18/kW per year and $28/kW per year for residential systems, the long view matters. To these ends, this article will identify and evaluate the value propositions of MLPE and UL 3741 to help guide installers towards the option that makes the most sense for their customers and uncover the value of thinking about MLPE and UL 3741 less in terms of ‘either/or,’ but rather as complementary.

Performance issues and fault conditions

The ability to understand the performance of a system and the behavior and health of the components therein has become a powerful tool for system owners. Instead of rolling a truck and conducting diagnostic work on-site – which can cost anywhere between $150-$500 – module-level monitoring (which is enabled by MLPE) can significantly reduce the effort and cost of identifying why performance is not as expected or locating a faulty component.

For example, a single failed diode in a module will reduce its production by at least 1/3^rd (if the system has MLPE optimizers) and could lead to ~$1000 worth of lost energy production over its lifetime at the site. However, monitoring only at the string or inverter level, this underperformance is exceedingly difficult to identify. Even when system underperformance is detected, pinpointing the exact issue is usually the result of a prolonged fault-finding process and increased time spent on site.

Studies have shown that module-level monitoring can reduce fault-finding time by up to 40%, which makes sense if the monitoring system allows the diagnostic work to start without ever putting a truck in motion. Without granular monitoring, this dynamic quickly compounds when multiple issues inevitably force multiple site visits. By knowing the precise location and nature of each issue, site visits can be maximized because technicians can be equipped with the necessary tools and materials for swift resolution.

Scaling solar portfolio management

Beyond the benefits for individual sites and system owners, MLPE can provide solar installers, O&M providers, and asset owners with a powerful set of management tools to service entire portfolios of systems. For example, operators can identify underperforming systems within a subset of locations from a single customer. Service teams can also optimize resource allocation and streamline maintenance operations by dispatching a technician to multiple problem sites on an optimized route. This is true for both unscheduled events as well as regular maintenance, such as module cleaning. Beyond service scheduling, advanced solar portfolio management systems make it possible to use a remote monitoring dashboard to filter sites by technology (e.g., module brand, inverter, etc.), system size, or by geographic location, which is particularly useful in the aftermath of a hailstorm, for example.

Tigo has modeled the operational benefits of having deep insight into sites across a variety of geographies, system sizes, and use cases and found that the efficiencies provided by module-level monitoring can reduce O&M costs by up to 23%. In addition, Tigo has provided a simple-to-use, brand-agnostic calculator to quantify the benefits of module-level optimization. Below are some of the relevant publicly available resources:

Resource title	Type	Link
Reduce truck rolls and ensure performance with module-level monitoring	Webinar	[Watch on-demand]
Case studies from solar installations around the world	Webinar	[Watch on-demand]
Solar optimizer benefits calculator	Online calculator	LINK
Reclaimed Energy statistics – gathered from tens of thousands of installations	Flyer	LINK

Maximizing power production

Unlike rapid shutdown with UL 3741, MLPE technology offers a simple way to also maximize energy production through module-level optimization. Photovoltaic systems are subject to energy losses from module mismatch, in which a disparity in performance among individual PV modules within a system can drag down the performance of the entire system. Factors such as shading, soiling, module degradation, and manufacturing discrepancies all contribute to reducing output through mismatch.

Even in a new array, a small amount of mismatch between modules can drag the performance of an entire string down to the level of its weakest module. The collective impact of these mismatch sources typically represents a 4-7% energy loss in a new array. Beyond inherent mismatch, according to the US National Renewable Energy Laboratory (NREL), “Partial shading can lead to annual performance losses of 10%–20% or more in residential installations.” Also, according to NREL, “Module-level power electronics such as microinverters or DC power optimizers have been shown to reduce mismatch in systems, recovering 30%–40% of the power lost due to partial shading.”

By keeping each module working at its individual peak power point, module-level optimization can increase the energy output of any solar array. An analysis by Tigo of tens of thousands of sites with optimizers installed showed that optimizers improved solar production by an average of 6.6%. On a 10-kilowatt (kW) system, optimization would produce 1,156 kWh more electricity and save the system owner more than $6,000 over the 25-year life of the project (assuming electricity rates at $0.18/kWh, growing at 3% per year).

Newer commercial systems, which are more likely to use bifacial modules due to their increased energy yield, are especially susceptible to mismatch losses caused by non-uniform albedo and rear-side irradiance. When Tigo compared the energy production from two otherwise identical strings of bifacial modules, the company found that the optimized string produced 12% more power than the string without MLPE. Solar developers typically use historical data to estimate mismatch losses due to soiling between 1%-4%.

Design flexibility

Power optimizers are an excellent solution for more complicated roof layouts, with modules facing different directions or tilting at varying degrees to azimuth. Even if some panels face east and others face south, module-level power optimizers will allow each module to perform at its maximum even with disparate amounts of irradiance. The ability to install panels at different orientations means the ability to maximize the size of the system, bring down the fixed costs per watt, and generate better ROI for system owners.

While there are always exceptions, UL3741 requires keeping the inverter DC input conductors inside the inner array boundary, for example, adding significant complexity compared to rapid shutdown with MLPE. These considerations will likely keep the mechanical approach to rapid shutdown in the commercial and industrial solar space and eliminate it for most residential rooftop projects.

The cost factor

Even when code compliance is the only consideration around rapid shutdown, the cost of using MLPE versus mechanical solutions is not entirely straightforward. Because a full cost analysis is beyond the scope of this document, the interested reader is encouraged to review a whitepaper published by SolarEdge entitled, UL 3741 Cost Comparison Whitepaper: DC-Optimized vs Non-DC-Optimized Solutions, which provides concrete examples of the cost disparity between MLPE and UL 3741 systems. While the motivations of the authors of this whitepaper might appear a little self-serving, the findings therein were vetted by an independent engineer.

Shutting down and safety

Beyond cost and feature disparities, the core rapid shutdown safety functions of MLPE and UL 3741 also differ. While UL 3741 prioritizes firefighter safety during emergencies by keeping the inner boundary voltage lower, the approach leaves 600V or 1000V worth of potential in the modules and conductors within the inside boundary. These higher voltages pose an increased risk to O&M teams, on-site technicians troubleshooting the system, or other contractors, and necessitates the use of higher-rated protective equipment. With a different set of safety risks, mechanical approaches to rapid shutdown will also call for updated safety documentation as well as technician retraining. From a holistic perspective, systems built around UL 3741 that include MLPE for rapid shutdown are likely to yield the best overall results.

A job well done

Regardless of the approach to rapid shutdown, no solar system survives an encounter with poor quality. That goes for everything from components, system design, technician training, installation, and service. To achieve Total Quality Solar, the entire value chain must work together, communicate, and make the details matter. Solar systems have become immensely complex agglomerations of technology in which the specification and use of compatible connectors matter just as much as joining them properly. Both MLPE and UL 3741 systems require careful design, strict compliance with equipment use and safety guidelines, and proper installation practices by well-trained installers who adhere to relevant standards. To this end, implementing robust Quality Assurance (QA) procedures and regular inspections will help identify and address potential issues before they escalate into real problems.

When combined with high-quality MLPE devices, a UL 3741 system can help deliver an incremental and differentiated way to achieving rapid shutdown compliance. Project-specific factors, safety considerations, and long-term benefits beyond compliance should be carefully evaluated when choosing the most suitable combination of these technologies. With the vast level of variability from one site to the next, a one-size-fits-all approach does not work. The ideal balance of code compliance, rapid shutdown protection, and remote monitoring capabilities should drive the technology selection process. In the end, informed decision-making based on project and customer requirements requires a nuanced understanding of rapid shutdown compliance options, and there is no shortage of good solutions on the market.

About the author

John Lerch is the Senior Director of Global Marketing at Tigo Energy, where he leads critical marketing initiatives, effectively communicating the value of complex products and services in the solar industry.

With extensive experience in sales, marketing, and business development, John specializes in strategic analysis, financial modeling, and value proposition creation within the solar sector. John’s ability to craft and execute successful sales strategies has been instrumental in building lasting partnerships with customers worldwide.