Barriers to Balcony Solar and Plug-In Distributed Energy Resources in the United States

Abstract

Plug-in distributed energy resources (DERs), such as balcony solar, backfeed power to the home through a standard plug. These systems may represent the future of residential solar and storage, particularly as recent net metering policies have reduced the economic appeal of rooftop solar. While plug-in DERs have seen widespread success in Europe, their U.S. market is stagnant. This paper reviews the technical, interconnection, and regulatory barriers hindering the adoption of plug-in DERs. We first discuss the technical barriers, which include touch safety, breaker masking, and bidirectional ground-fault circuit interrupters. We then examine utility perspectives on plug-in DERs and strategies for navigating interconnection challenges. Finally, we discuss regulatory hurdles related to UL standards and the National Electrical Code.

1. Introduction

1.1. Background and Motivation for Plug-In Distributed Energy Resources

For decades, U.S. homeowners have benefited from residential-scale distributed energy resources (DERs). Rooftop solar can reduce electricity bills, with payback periods as short as five years under optimal conditions [1,2]. Residential energy storage systems (ESSs) have also grown in popularity, enabling load shifting, grid services, and multi-day resilience when paired with solar. However, despite recent battery price declines [3], adding storage increases system costs and extends the payback period.

Solar adoption has surged in recent years, with many individuals generating their own electricity and exporting excess power during peak solar hours. To address challenges in balancing grid-side supply and demand, public utility commissions have introduced new policies and tariff structures. In 2023, the California Public Utilities Commission implemented Net Energy Metering (NEM) 3.0 [4], significantly reducing compensation for new solar producers’ energy exports. While NEM 3.0 helps utilities manage costs and incentivizes home energy storage, it greatly reduces the financial benefits of solar ownership. As a result, the payback period for rooftop solar in California has risen to as much as 15 years [1]. NEM 3.0 has reportedly led to a 77–85% decline in rooftop solar sales, dealing a severe blow to the industry [5].

The residential solar market primarily serves homeowners, with limited access for renters. Studies highlight the potential for rooftop solar to simultaneously lower renters’ total expenses and increase landlords’ rental income [6,7]. However, these benefits remain largely unknown to both parties [6]. Renters often struggle to assess whether reduced electricity bills would offset higher rent, while landlords view solar as an investment that only benefits tenants. As a result, renter participation in the solar market remains low.

Over the past decade, material costs have dropped as photovoltaic (PV) modules and batteries have become commodities. Today, residential rooftop solar [8] and storage [9] are largely driven by soft costs, including engineering, permitting, procurement, overhead, and profit. Rooftop installations also include the costs of labor, truck rolls, and electrical balance-of-system (BOS) equipment. Together, these costs can double the capital cost of smaller systems. The U.S. Department of Energy (DOE) is actively exploring ways to reduce installation, BOS, and soft costs. One promising approach is a transition to plug-in DERs, which could eliminate these costs by transforming residential solar into a simple cord-and-plug solution.

We define plug-in DERs by their ability to backfeed power into the home through an AC plug. They include emerging technologies like balcony solar, plug-in solar carports, bidirectional UPS batteries, and bidirectional appliances with integrated batteries. Figure 1 illustrates a common setup for connecting balcony solar and storage to a building. Plug-in DERs are classified as cord-and-plug devices, and sold as retail hardware. While bidirectional EV chargers could eventually qualify as plug-in DERs, they currently require a hardwired connection.

Plug-in DERs, such as balcony solar, have gained popularity across Europe, primarily due to their potential for upfront cost savings. By eliminating BOS, installation, and soft costs, these systems offer a more affordable alternative. Most knowledge on balcony solar’s payback period has been compiled using the European Photovoltaic Geographical Information System (PVGIS) toolset [10]. A continent-wide study [11] found payback periods as short as three years in the Netherlands. Payback periods vary significantly based on the electricity rate, with estimates of 3.5 years in Germany and 5.5 years in France. In Slovenia, the payback period is estimated at 6.7 years, aligning with one of the few independent studies on this topic [12]. One of the only studies on the U.S. market estimates a 5-year payback period, based on 2015 costs and solar irradiance models for upper Michigan [13]. Given these figures, we would expect today’s payback periods to be even shorter in most of the U.S.

Beyond upfront cost savings, plug-in DERs offer several key advantages. They provide renters with a unique option to participate in the solar and storage markets that can be easily moved between homes and apartments. Their portability enables flexible resilience, allowing users to relocate units as needed. Additionally, they serve as a viable solution for homes with heavily loaded electrical panels, which would otherwise require costly upgrades to support solar or storage integration.

While the European market is thriving, plug-in DERs face significant barriers to sales and adoption in the U.S. This paper examines the technical, regulatory, and utility interconnection challenges hindering their adoption, drawing insights from stakeholder interviews and independent standards research.

1.2. Adoption of Plug-In DERs

The most successful examples of plug-in DER adoption have been seen in European markets. By the end of 2024, over 1 million units had been sold in Germany [14,15], with no safety incidents reported, aside from cases of tampering. According to our German industry consultant interviewee, Germany’s success took over half a decade to achieve.

The effort began in 2017, when the German certification body Verband der Elektrotechnik (VDE) released the testing standard VDE V 0100-551-1 [16]. This standard addressed the safety requirements and integration of plug-in PV systems, such as balcony solar, into low-voltage electrical installations. In 2018, VDE released VDE AR N 4105 [17], which further outlined the technical requirements for connecting power-generating devices. This regulation established a 600 W limit for connection without an electrician’s signature, and was based on a 2016 Fraunhofer report [18] showing that 600 W of backfeed was safe even in worst-case scenarios. The limit was later raised to 800 W to align with the lower limit of regulation under the European Network Code [19]. Today, Germans can purchase, register, and plug in up to 800 W of balcony solar.

German utilities initially had mixed reactions, with concerns focused on liability, safety, fire risks, and potential damage to their public image. Much of the dialogue to alleviate liability concerns involved technical explanations and reasoning. Surprisingly, profit was of secondary concern, and many German utilities even expressed interest in using balcony solar for public relations purposes. The creation in 2019 of a universal online registration form for plug-in PV was a key turning point in these discussions and played a crucial role in gaining utility acceptance of the technology.

The German political climate also played a significant role in the widespread adoption of balcony solar. Since 2018, activists, media, and eventually politicians advocated for broad acceptance. While legislation like the 2022 German Easter Package initially created unfavorable conditions for balcony solar, these provisions were eventually removed following active lobbying and media pressure. Ultimately, the Russo-Ukrainian War in 2022 and Germany’s subsequent move away from Russian gas became the most influential political factor driving the adoption of balcony solar.

Recent technical reports highlight the market growth of balcony solar and outline the remaining regulatory barriers across European Union member states [20]. As of today, plug-in solar is permitted with proper registration or notification in several European countries, including Germany, the Netherlands, Belgium, Spain, Portugal, Switzerland, Austria, Italy, Slovenia, Slovakia, Poland, and Lithuania [11]. France allows balcony solar but prohibits power export. In Great Britain and Greece, balcony solar must comply with building codes, while Croatia and Bulgaria apply rooftop PV regulations to balcony systems—often to their detriment. Plug-in solar is currently banned in Denmark, Sweden, and Hungary. Other European countries do not have specific regulations governing plug-in DERs. Among the limited research on the U.S. regulatory environment, a pioneering 2016 study examines how plug-in DERs align with the National Electrical Code (NEC) [21]. It also proposes an early concept for a device registration form to streamline interconnection agreements. Since that study, the U.S. has seen little progress toward achieving regulatory compliance.

Many parties in the U.S. are now aware of the success plug-in DERs have achieved in Europe, and many smaller vendors are eager to enter the U.S. market. However, as previously mentioned, the technology faces technical, regulatory, and utility interconnection barriers. The recent Utah bill HB340 [22] may help remove some of these barriers, particularly regarding utility interconnection within the state. It permits up to 1200 W of backfeed, though it requires devices to be tested and certified by a recognized lab such as Underwriter Laboratories (UL). Despite this state law, technical and regulatory challenges will still need to be addressed. Nonetheless, HB340 received a unanimous vote by Utah’s state legislation, highlighting a bipartisan excitement for legalizing plug-in DERs [23].

Beyond Utah, few other jurisdictions have officially approved plug-in DERs, and none at the state level. We are only aware of two other utilities that have approved interconnection agreements with plug-in DERs: Lacreek Electric at the Oglala Lakota Reservation in South Dakota (October 2024), and NV Energy in Nevada (December 2024).

Despite these barriers, several companies and organizations have found foothold markets for installation in the U.S. Our nonprofit installer interviewee has developed a subscription-based business model, allowing occupants to rent plug-in DERs at a fraction of the expected energy-cost offset. The rental also includes technical support and installation. Some systems require basic electrical skills, such as opening a breaker panel or replacing an outlet. The plug-in industry may well create an “electrician-lite” job class for low-cost basic electrical work (also referred to as a qualified electrical worker).

1.3. Scope, Contributions, and Overview of Paper

This paper provides an overview of the technical, regulatory, and utility-related barriers to the adoption of plug-in DERs in the U.S. Rather than presenting exhaustive simulations, experimental analysis, or detailed code development, our goal is to motivate future work in those areas. Specifically, this paper’s contributions are as follows:

• Identifying key technical barriers that could pose safety risks if left unaddressed;
• Outlining utility concerns, interconnection challenges, and potential mitigation strategies;
• Detailing regulatory hurdles, including relevant testing standards and restrictive building codes.

Our study was largely informed by twelve stakeholder interviews. Some interviewees wish to remain anonymous. In this paper, the interviewees will be referred to as follows:

• Plug-in solar company—Zoltux;
• Plug-in solar/storage company—Craftstrom;
• Plug-in solar carport company—GismoPower;
• Plug-in battery company—Mana Energy;
• Battery-integrated cooktop company—Impulse Labs;
• Non-export solar/storage company—Raya Power;
• Electrical safety officer—Lawrence Berkeley National Laboratory;
• German industry consultant—Empower Source;
• Nonprofit installer—Brightsaver;
• Eastern utility—Dominion Energy;
• Western utility—(anonymous);
• Underwriter Laboratories (UL).

The paper’s sections are organized by barrier type. Section 2 details the technical barriers, including touch safety, breaker masking, and bidirectional ground-fault circuit interrupters (GFCIs). Section 3 describes the utility interconnection barriers, including device registration and power export. Finally, Section 4 details regulatory barriers associated with UL standards and the National Electrical Code (NEC).

2. Technical Barriers to Adoption of Plug-In DERs

While plug-in technology has been proven in the European market, there are several technical challenges in transitioning to the U.S. market. Three concerns, in particular, have been repeatedly highlighted as potential fire or safety risks:

• Touch-safe plugs;
• Breaker masking;
• Bidirectional GFCIs.

2.1. Touch-Safe Plugs

Touch-safe devices prevent accidental contact with energized parts or circuits, thereby minimizing the risk of electric shock. The exposed prongs of the plug could cause a shock hazard if improperly energized. Two mitigation strategies were discussed in the interviews.

First, the inverter must rapidly cut power to the AC prongs when the detected voltage deviates from the nominal range, such as when unplugged. UL 1741-compliant inverters already meet this requirement through anti-islanding, a safety feature designed to protect linemen. While the specified clearing time of two seconds ensures linemen’s safety, it may be too long, allowing users to potentially touch the energized prongs after unplugging.

The U.S. standard NEMA 5-15 plugs and receptacles, shown in Figure 2, are not fully touch safe, even for loads. Figure 3 illustrates how the prongs can be energized while partially exposed, posing a shock hazard, especially for small fingers. In contrast, European Schuko receptacles, shown in Figure 2, are recessed, preventing contact with the prongs while energized. Regardless, U.S. society still considers NEMA 5-15 to be sufficiently touch-safe for household plug loads.

For NEMA 5-15 plug-in DERs to be considered touch-safe, standards bodies must define a time period after decoupling (shown in Figure 3) during which the inverter must shut off. Previous inverter testing indicates anti-islanding disconnection times typically range from 100 ms to 300 ms [24,25], though they can be as low as 40 ms [26]. The latter is more common in countries like Japan, where anti-islanding requirements are less than 0.2 s [27]. For comparison, UL 943 [28] defines trip-time requirements for ground-fault circuit interrupters (GFCIs) using the formula $T={(20/\mathrm{mA})}^{1.43}$. According to this, GFCIs must trip within 7 s for 5 mA of ground current (startling shock), 1 s for 20 mA (painful shock), and 270 ms for 50 mA (deadly shock).

The second mitigation strategy involves using a sheathed and touch-safe design for the inverter plug and receptacle interface. Interviewees proposed the Wieland, Betteri, and Neutrik plugs, shown in Figure 4. With these, users would need to replace their NEMA 5-15 outlets with a compatible receptacle, an easy task for an electrician-lite. An alternative is to develop a screw-on adapter faceplate, as shown in Figure 5. While faceplate replacement technically requires an electrician, it is simple enough for most customers to handle themselves. The final alternative is to create a spring-loaded NEMA 5-15 plug, where prong sleeves extend upon unplugging. Currently, such plugs are only available in Europe and are still quite rare.

2.2. Breaker Masking

Most U.S. plug-in DERs are designed for 120 V NEMA 5-15 receptacles, which are often connected in parallel on non-dedicated circuits. Backfeeding onto non-dedicated circuits can cause a technical issue known as breaker masking, where the current on a circuit can exceed the rating of the circuit’s breaker. Breakers are designed to trip above a rated current as seen at the panel. When only loads are present, the breaker current is the sum of the load currents. However, plug-in DERs can effectively cancel out load current as seen by the breaker. Figure 6 illustrates a worst-case scenario, where two 14 A hair dryers operate on a 20 A circuit. In this case, 28 A flows through a section of the circuit, rapidly overheating the wire. The breaker only detects 18 A and will not trip.

Not all circuit configurations are problematic. As shown in Figure 7, plug-in DERs at the end of a circuit may be safe. However, we cannot expect occupants to have knowledge of their home’s internal wiring.

The previously mentioned Fraunhofer study [18] suggests that overloading a circuit by 2.6 A is acceptable, setting a 600 W limit for European 230 V plug-in DERs. German experts argue that an 800 W limit is still acceptable on a 16 A residential circuit. Here, the rare worst-case scenario of a 3.5 A (i.e., 20%) overload might cause slightly faster wire degradation but would never start a fire. However, these limits are less applicable to U.S. 120 V circuits, where the same 3.5 A buffer would allow only 420 W of plug-in DERs per household.

The breaker masking issue is similar to the 120% rule for rooftop solar, outlined in NEC 705.12(B) [29]. This rule limits the sum of solar and load to 120% of the breaker panel’s busbar rating. In other words, the worst-case scenario would only overload the busbar by 20%. Many argue that proper positioning of the solar breaker can prevent masking of the main breaker, safely allowing for more solar capacity. However, code-making officials established the 120% rule as a catch-all: even electricians are not expected to understand breaker masking when adding solar circuits.

The interviewees mentioned several potential technical solutions for breaker masking. The plug-in solar/storage company developed a smart outlet that disconnects attached DERs upon detecting a breaker-masking event. It precisely measures wall voltage and can detect voltage droop due to downstream loading. The German industry consultant mentioned two other solutions currently being explored in Germany. One uses a smart metering circuit breaker that communicates with the plug-in DERs. The DERs will curtail power if the sum of the metered and generated power exceeds the circuit’s rating. This approach could allow up to 1800 W of generation on a lightly used 120 V, 15 A circuit. The other solution employs time-domain reflectometry to measure the load on a circuit. It sends a pulse down the line and calculates load based on the amplitude and delay of the reflection. While all three solutions could potentially resolve the breaker masking issue, they would likely require certification before gaining widespread acceptance in the U.S.

2.3. Bidirectional GFCIs

Ground-fault circuit interrupters (GFCIs) are breakers or outlets that disconnect a downstream circuit when they detect a ground fault. They typically use a Rogowski coil to sense the differential current between the hot and neutral wires. GFCIs have specific markings indicating how the line (upstream) and load (downstream) terminals should be wired. As explained in Section 4.2, the NEC has strict requirements regarding GFCI markings.

None of today’s GFCIs are officially rated for bidirectional power flow, nor is there an official UL testing procedure for bidirectional GFCIs. Our UL interviewee recently conducted a series of informal backfeed tests. While most of the GFCIs functioned properly, a few samples were reportedly damaged due to backfeed. There are no other reports on the effects of backfeeding a GFCI. Future research may uncover the technical causes of the reported damage and inform GFCI designs on how to better handle backfeed. Nevertheless, the potential for GFCI damage presents a significant technical barrier to adoption in the U.S., as damaged GFCIs would fail to protect the occupant’s plug-in DER and could pose a shock hazard to future occupants.

Currently, there are no officially marked bidirectional GFCIs on the market, which may require the development of a new product for the plug-in power industry to progress. An electrician-lite could easily replace a home’s existing GFCI with a bidirectional one.

3. Utility Interconnection Barriers to Adoption

Utility and interconnection barriers relate to rules or regulations that may impede the installation of plug-in DERs. Ratepayers sign an electrical service agreement outlining the conditions for receiving electrical service from the grid. This agreement allows utilities to regulate what occupants connect behind the meter, leveraging the possibility of disconnecting electrical service. While debates continue over whether utilities should have authority behind the meter, most U.S. public utilities currently have such authority. Many agreements technically require notifying the utility of any changes in connected load or DERs. In practice, occupants rarely comply, and utilities typically do not enforce it. However, many utilities reserve the right to investigate significant changes in a customer’s load profile.

Ratepayers with the ability to export power must sign an interconnection agreement, which can be a lengthy and complex process, typically requiring an electrical permit. Figure 8 shows how the permitting process invokes the NEC, and Section 4.2 explains how the 2023 NEC may be unfavorable to plug-in DERs. Additionally, obtaining a permit requires the landlord’s permission, adding another barrier for renters. Regardless of city approval, utilities can deny interconnection agreements that may cause technical issues. Depending on the state’s public utilities commission, violating the NEC could qualify as a technical issue.

Our interviewees described several concerns utilities have regarding plug-in DERs. According to our German consultant interviewee, European utilities mainly worried about fire, safety, and liability for accidents behind the meter. Our U.S. utility interviewees expressed two very different concerns.

The interviewee from the eastern utility described their primary concern as economic. Previously, under NEM 2.0, rooftop solar could eliminate a home’s electric bill, while the utility remained responsible for maintaining its service. Even with policies like NEM 3.0, the eastern utility remained uneasy about the economic impact of widespread offsetting of electrical load, particularly if plug-in solar gains popularity. Plug-in storage could help alleviate this concern. Interestingly, the western utility interviewee expressed little concern over the economic effects of plug-in DERs, especially as states transition to policies that resemble NEM 3.0.

The western utility’s primary concern centers on the mobility of plug-in generation and storage. They are already facing challenges in planning for the integration of bidirectional vehicles. Their concerns include operational visibility, infrastructure planning, and the unknown impacts of mobile assets on the grid. Many large utilities grew through the acquisition of smaller municipal utilities, each with varying development practices, equipment, and voltage levels. As a result, these larger utilities tend to take a more conservative approach when planning significant changes to their networks.

It is also worth noting that neither utility interviewee was outright dismissive of plug-in DERs; both acknowledged that plug-in DERs could offer an opportunity to reduce or defer future infrastructure upgrade costs. Plug-in solar has a similar grid impact to utility-scale solar but offers the advantage of being closer to the point of use. Several studies have simulated this impact and found that widespread plug-in solar can be managed effectively with storage and smart load shifting [30,31]. Our German industry consultant noted that the marketing departments of early-adopter German utilities were excited about the business prospects and incentives for on-site generation. If plug-in batteries become widespread, utilities may see tremendous benefit through incentivized grid services and dynamic pricing.

Our non-utility interviewees collectively determined several solutions to reduce or eliminate today’s interconnection barriers for plug-in DERs:

• Zero export;
• Device registration;
• State legislation;
• Interstate regulation.

3.1. Zero Export

Inadvertent export occurs when a building or home without an interconnection agreement exports power to the grid for more than 1 s. Rules vary by utility—some allow up to two seconds, while others permit occasional occurrences without penalty. Anecdotes from the interviews suggest that penalties also differ; some utilities are indifferent, others initiate an interconnection agreement, and some have even disconnected power without warning. Without utility approval, plug-in DERs must likely avoid export entirely.

Zero-export solutions offer a discreet workaround, enabling customers to generate and use power without exporting it. Among our interviewees, the plug-in solar/storage and battery-integrated cooktop companies described their zero-export power meters, installed behind the panel with CTs that clamp to the main feeder. The meter wirelessly communicates with plug-in DERs, typically using a long-range peer-to-peer protocol. If the feeder current drops below a set threshold, the DERs stop backfeeding within milliseconds—well under the 1 s rule. They also cease backfeeding if communication with the meter is lost.

3.2. Device Registration

The western utility raised particular concerns about mobile generation assets. Lineman safety is no longer a concern: today’s anti-islanding requirements guarantee lines are not energized during an outage. Instead, the interviewee was concerned about the challenge of understanding and modeling how mobile generation may impact their networks.

In Germany, device registration was key to utility acceptance of plug-in DERs. Today, Germany’s Federal Network Agency offers a simple online form [32,33], allowing users to register devices to a specific address. This process limits legal balcony PV to 800 W, preventing the breaker masking issues in Section 2.2 and providing valuable locational data for utility network modeling. Adopting a similar system in the U.S. would likely increase utility confidence in plug-in DERs, but in return, registration should guarantee customers a free and expedited interconnection agreement.

3.3. State Legislation

Utah’s HB340 [22] highlights state legislation as a potential pathway for plug-in DER adoption. As shown in Figure 8, state laws could enable customers to bypass standard utility regulations. Alternatively, state public utility commissions could rule on the inclusion of plug-in DERs, though legislation provides a stronger foundation. HB340 specifically requires applicable plug-in DERs to be UL-listed (or equivalent), making this approach dependent on a UL standard for plug-in DERs.

3.4. Interstate Regulation

The U.S. Federal Energy Regulatory Commission (FERC) regulates interstate electricity transmission and may have some authority over utilities operating across multiple states. Recent FERC orders have been favorable to small-scale DERs. FERC Order 2222 [34] allows small-scale DERs to aggregate and meet minimum requirements for participation in wholesale energy markets [35]. It also highlights several critical issues, one of which is the lack of systems and standards for two-way power flow [36]. FERC Order 2023-A [37] streamlines interconnection procedures and reduces administrative burdens for integrating DERs into grid operations [38]. Both orders aim to expand market access and participation opportunities. Although their direct impact on plug-in DERs remains uncertain, FERC orders represent a promising pathway worthy of continued investigation.

4. Regulatory Barriers to Adoption for Plug-In DERs

Codes and standards determine whether electrical devices and practices are approved by local authorities having jurisdiction (AHJs). Since AHJs can interpret them differently, it is crucial that codes and standards are written with precision. The most relevant codes and standards are the following:

• Underwriter Laboratories (UL) Standards;
• National Electrical Code (NEC);
• International Residential Code (IRC).

While AHJs regulate everything behind the outlet, they have little authority over the plug loads consumers purchase. The primary regulation for plug-in DERs comes from Nationally Recognized Testing Laboratories (NRTLs) like UL, which certify hardware through official safety listings. NRTLs test devices based on UL standards, many of which are derived from the NEC and IRC. Therefore, all three regulatory components must be considered in a barrier assessment.

4.1. UL Standards

Based on our interviews, we found several reasons companies would pursue an NRTL listing. Electricians and many non-electrician customers often look for the NRTL label on the hardware’s plastic enclosure, as it indicates successful third-party safety testing by a reputable lab. This label is considered more credible than the CE marking, which only reflects manufacturer self-testing. In addition to boosting customer confidence, the NRTL label simplifies obtaining liability insurance for a product. Finally, emerging state legislation such as HB340 [22] may only protect NRTL-listed products. Despite these benefits, some emerging plug-in DER companies are successfully selling their products without a listing, though they assume greater risk in doing so. The NRTL testing process can cost tens of thousands of dollars and may significantly delay products’ time to market [39].

UL consists of three entities: the NRTL, UL Solutions; the research group, UL Research Institutes; and the body responsible for creating safety testing standards followed by many NRTLs, UL Standards and Engagement. UL Solutions offers several pathways for developing safety requirements for new technology. Their New and Innovative (N&I) process enables early certification and listing of emerging technologies, creating an Outline of Investigation (OOI), which provides interim technical requirements for products lacking an established standard and can evolve into a full standard over time. UL Standards and Engagement will publish those UL standards based on the UL Solutions OOI. As a product family matures, UL Solutions can issue a Certification Requirement Decision (CRD) to clarify or supplement existing standards relevant to that family of products. A CRD cannot change consensus requirements in a UL Standards and Engagement standard, but it can add requirements associated with the N&I process or clarifications of existing requirements. All CRDs will be simultaneously added into the consensus process to be incorporated into an existing standard or developed into a new standalone standard, both of which require a consensus vote by a standards committee. UL standards can obtain supplements, which clarify or expand specific aspects of the standard, ensuring that products are evaluated according to the most current and relevant requirements.

Based on our interviews, we have identified several UL standards that may be broadly relevant to plug-in DERs:

• UL 1741—Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources. This standard outlines requirements for all inverters, including those integrated into plug-in DERs. It includes rapid shut-off requirements in Supplement B, and upcoming requirements for movable inverters and bidirectional vehicle chargers in Supplement C. UL 1741 might need to be updated with provisions for flexible cord and plug connections.
• UL 943—Ground-Fault Circuit-Interrupters. This standard outlines the technical and testing requirements for GFCIs, and will need to be updated with guidelines for testing and marking bidirectional GFCI breakers and outlets.
• UL 3141—Power Control Systems. This standard is relevant to plug-in DERs that involve meters and utility export control. Zero-export systems can be listed under this standard.
• UL 3741—ANSI/CAN/UL Photovoltaic Hazard Control. This standard outlines rapid shutdown requirements for the DC side of a PV inverter. While rapid shutdown is typically required for rooftop solar to ensure firefighter safety, its necessity for balcony solar is yet to be determined.
• UL 9540—Energy Storage Systems (ESS) and Equipment. This system-level listing applies only to stationary energy storage systems like the Powerwall. However, the IRC and NEC currently classify any battery-containing device as an ESS, imposing strict usage restrictions. Thus, plug-in DERs with storage must avoid UL 9540 as their system-level listing.
• UL 1973—Batteries for Use in Stationary and Motive Auxiliary Power Applications. This is an alternative system-level standard for stationary battery products that would not classify them as an ESS.

We also list several UL standards that were highly relevant to several of our interviewees:

• UL 858—Household Electric Ranges. This system-level standard defines requirements and testing procedures for stoves, ovens, and ranges. The proposed Supplement C would permit these appliances to include a battery and allow their classification as appliances rather than ESSs under the NEC and IRC.
• UL 2595—General Requirements for Battery-Powered Appliances. This standard will be referenced in the proposed UL 858 SC, and is completely detached from UL 9540.
• UL 9741—Electric Vehicle Power Export Equipment (EVPE). This system-level listing is necessary for bidirectional EV chargers. It currently requires a hardwired connection, thus disqualifying the bidirectional charging of a plug-in DER.
• UL 414—Meter Sockets. A standard for meter sockets, which is one of the first to allow plug-in DERs.

This list is not exhaustive but generally provides information on what may form the basis of an eventual plug-in DER standard.

4.2. National Electrical Code

The NEC is Code 70 of the National Fire Protection Agency (NFPA) and governs the rules and regulations for power distribution in homes and buildings [29]. It serves as the foundation for each state’s electrical codes, which electricians and inspectors refer to when designing and inspecting electrical work. The NEC is updated every three years and includes a process where an appointed code-making panel accepts or rejects public input. Electrical codes play a critical role in fire and shock safety, thus changes tend to be slow and conservative.

While the NEC generally does not regulate plug loads, its rules can significantly impact the ability of plug-in DERs to source or sink power. As shown in Figure 8, interconnection agreements often require permits and inspections, during which the NEC is referenced. UL standards also frequently reference the NEC. We have listed several 2023 NEC articles that are highly relevant to the plug-in DER industry. Many NEC barriers arise from vague wording or informal notes, which AHJs may interpret at their discretion. While UL standards almost never contradict the NEC, they can definitely help clarify vague or ambiguous language.

NEC Articles 690 and 705 govern the connection of an inverter’s AC side to a home. These articles are largely similar, and our solar carport interviewee described several instances of rejection due to their application [40]. Article 690’s scope broadly covers “solar PV systems”, while Article 705 applies broadly to grid-following inverter systems. Though these articles are intended for rooftop solar installations, AHJs will quite likely apply them to plug-in DERs. For instance, our electrical safety officer interviewee highlighted NEC 705.12, which governs load-side source connections, as lacking any provisions for connection via plug and receptacle. In general, the lack of explicit provisions often leads to rejection by default. Both articles define acceptable means for disconnecting AC power (690.13 and 705.20); unplugging is not mentioned, giving grounds for denial. Additionally, both articles require installation by “qualified persons” (690.4 and 705.8). Although Article 100 does not explicitly define “qualified persons” as licensed electricians, our interviewee’s experience indicates it is often interpreted this way.

Article 406 covers receptacles, cord connectors, and attachment plugs, with a focus on receptacles. Our electrical safety officer interviewee mentioned that NEC 406.7(B) prohibits receptacles that require an energized plug as their source of power. While this provision should not apply to plug-in DERs, AHJs may interpret it as prohibiting energized plugs altogether.

Article 400 of the NEC governs the use of flexible cords and cables and is one of the few sections that directly relates to plug loads. Flexible cord or cable is generally defined as having a stranded wire core. NEC 400.10(A) outlines permissible uses, which includes appliances. However, whether plug-in DERs qualify as appliances is subject to the AHJ. NEC 400.10(B) requires appliances to be energized via an outlet or a cord connector body, the latter of which might apply to plug-in DERs. However, NEC 400.12 prohibits flexible cords from running through windows, walls, being attached to the building, or routed in areas prone to physical damage. These prohibitions may cause interpretation challenges.

It is also important to note that NEC 110.3(B) generally requires equipment to be used in accordance with its labeling and instructions. Our solar carport interviewee pointed out that this could pose a problem for plug-in DERs utilizing off-the-shelf inverters, as these inverters often lack explicit instructions permitting use with backfeeding plugs.

Article 210.8 introduces GFCIs and specifies where GFCI protection is required. In general, GFCI breakers or outlets must protect accessible circuits in wet locations, where ground faults are particularly dangerous. Plug-in DERs may use outdoor, kitchen, or garage outlets, all of which require GFCI protection per Article 210. Therefore, the lack of bidirectional GFCIs is a significant barrier to code compliance and the safety of plug-in DERs. Article 625.54 states that cord-and-plug electric vehicle service equipment (EVSE) must be supplied through a GFCI-protected outlet. Without bidirectional GFCIs, cord-and-plug bidirectional vehicle chargers may never be possible.

Article 422.5 specifies which appliances require GFCI protection, including both hard-wired and cord-and-plug types. Several types of GFCIs are allowed, including breakers, outlets, and even device-integrated models. While 422.5 does not require GFCIs for cooking appliances, 210.8(D) does. The NEC correlating committee is currently investigating this contradiction and will determine which of the lists in 210.8 or 422.5 will take priority.

Our solar carport interviewee proposed bidirectional EVSE as a potential pathway for plug-in DERs, backfeeding via an automotive plug. We have identified two NEC barriers to this strategy. First, the NEC defines vehicles in Article 100 explicitly as automotive vehicles and includes an informal note stating that other vehicles, such as forklifts, are excluded from the guidelines for bidirectional vehicles. Ultimately, it is up to UL to decide whether “non-vehicles” can backfeed through “vehicle” plug formats.

Another EVSE barrier lies in the wording of Article 625.44 on EVSE connections. It states that “fixed in place” equipment must be hardwired, and Article 100 defines “fixed in place” as anything that requires a tool for removal. Many cord-and-plug EVSE models are mounted with a tool, as are most plug-in solar panels. The NEC allows plugs on equipment that uses (less-secure) hand fasteners, which it considers as “fastened-in-place”. Additionally, 625.44 explicitly specifies that only “non-locking” receptacles are permitted for fastened-in-place equipment. Such language technically prohibits connectors such as Wieland, Betteri, or Neutrik on the receptacle side of an EVSE.

4.3. International Residential Code

The International Residential Code (IRC) is a set of building codes for residential structures, including electrical systems [41]. The relationship between the NEC and the electrical chapters of the IRC can be confusing. Technically, inspectors are supposed to defer to the IRC in the event of an explicit discrepancy. More often than not, the IRC and NEC do not conflict, and inspectors follow the NEC due to its stricter guidelines. The electrical sections of the IRC are written by the NFPA but are often a decade behind the NEC. States often have their own electrical codes, which are based on a combination of the NEC and IRC.

The International Fire Code (IFC) [42] is a sister code to the IRC and International Building Code (IBC). State electrical codes often defer to the IFC in the same way they do the IRC. IFC 1207 is the only article that addresses energy storage in multifamily housing. According to our battery-integrated cooktop interviewee, no codes govern storage of <20 kWh per fire area. As a result, inspectors must refer to the guidelines in IFC 1207 for >20 kWh of battery storage, which in turn references NFPA 855’s rules and regulations on the installation and operation of an ESS. While these strict rules ensure safety for an ESS of any size, they make no distinction between an ESS and a regular battery. Batteries are being integrated into a wide variety of non-ESS products, and the regulations in NFPA 855 make installing these products prohibitively burdensome. In response, code officials are developing NFPA 800, which will regulate non-ESS devices with smaller batteries, such as battery-integrated induction stoves. Eventually, IFC 1207 could refer to NFPA 800 for <20 kWh of storage. Until then, deployment of non-ESS devices may need to rely on certain exceptions within NFPA 855, possibly using UL 9540 certification or limiting the energy capacity installed per defined fire area.

The IRC also presents a major challenge for battery-integrated plug loads, as IRC 330 prohibits ESSs in habitable spaces, including kitchens, where cooktops and ranges with integrated batteries would be installed. The IRC also fails to distinguish between a battery and an ESS. Most AHJs recognize that this rule is outdated; banning laptops from indoor spaces simply because they contain a battery would be unreasonable. As a result, many AHJs defer to the NFPA on how to regulate battery-integrated appliances. These products should ideally fall under the scopes of NFPA 800 and UL 1973 (for batteries), rather than NFPA 855 and UL 9540 (for ESSs). However, if the latter applies, there is a vague exception that allows devices in accordance with UL 9540 and marked “for use in residential dwelling units”. Since UL 9540 does not actually define such a marking, the exact meaning or intent of this exception remains unclear. In general, such ambiguities are less than ideal, as they encourage arbitrary interpretation by AHJs.

5. Conclusions and Future Work

While plug-in DERs have demonstrated significant economic benefits for European homeowners and renters, their adoption in the U.S. has faced several barriers. In this paper, we outline key technical challenges, including touch safety, breaker masking, and the lack of bidirectional GFCIs. We then discuss utility concerns and strategies companies have used to navigate interconnection. Finally, we examine regulatory pathways involving UL standards and the NEC.

We find plug-in DERs to have several viable market pathways today, including approaches that use zero-export technology to enable installation without an interconnection agreement. However, key technical issues remain that could pose fire and safety risks, highlighting the need for a new UL standard to test plug-in DERs. While UL standards cannot override the NEC, the most problematic NEC provisions appear to stem from interpretation rather than explicit prohibitions, suggesting that a new UL standard is feasible.

Future work should target the barriers outlined in this paper. Touch safety may be resolved through rigorous testing and a standardized inverter shut-off duration. Breaker masking solutions will require worst-case scenario testing and eventual inclusion in a UL standard. An eventual UL standard for bidirectional GFCIs will enable manufacturers to produce compliant devices. While it is possible to navigate interconnection barriers today, Germany has shown that persistent education and outreach can lead to widespread utility acceptance. Ultimately, a UL standard tailored to plug-in DERs will be essential to overcoming many of the remaining regulatory hurdles.