The X in V2X Matters: Energization versus Interconnection of Bidirectional Charging Systems

The Opportunity for Bidirectional Charging

Bidirectional charging has potential to transform how consumers view and use their electric vehicles (EVs). Bidirectional charging allows EVs to become a flexible resource for power systems that act as both a flexible load and an energy resource. The ability to act as both a load and an energy resource creates new revenue and grid services value streams for customers and utilities alike.

Grid-tied, bidirectional-capable EVs can support peak shaving, store renewable generation, provide ancillary services (such as voltage support, ramping support, and distribution congestion), and act as resilience assets. The scale at which bidirectional-capable EVs can participate in these grid services is a significant benefit for grid operators. With a sales-weighted average battery size of 60 kilowatt-hours (kWh) for light-duty EVs, the United States’ (U.S.) 2.1 million battery electric vehicles (BEVs) represent approximately 126 Gigawatt-hours (GWh) in storage capacity. This amount of battery storage is equivalent to five times the amount of stationary battery storage currently on the grid (25 GWh in 2023). While few of these vehicles currently have bidirectional charging capabilities, this amount of storage provides a largely untapped resource for power systems.

The bidirectional charging industry is in the early stages of transitioning to a commercial product ready for mass-market adoption. At this time, challenges and barriers to implementing bidirectional charging at scale remain. The Smart Electric Power Alliance’s (SEPA) report “The State of Bidirectional Charging in 2023,” includes an overview of the bidirectional charging industry, highlights perspectives from industry stakeholders, explores existing bidirectional charging deployments, and explains the opportunities and barriers to wide-scale adoption of bidirectional charging technologies.

Figure 1. Illustration of a Bidirectional Charging System.

Types of Bidirectional Charging

Bidirectional charging requires a bidirectional vehicle and a bidirectional electric vehicle supply equipment (EVSE) to discharge (Figure 1). Bidirectional charging systems also require software to communicate among these devices and to interact with utilities and aggregator providers. The bidirectional charging configuration is an important aspect to the types of use cases the system can support.

Bidirectional charging encompasses different types of use cases, usually under the term vehicle-to-everything (V2X). V2X includes subcategories of unique use cases:

1. Vehicle-to-Grid (V2G): V2G involves any grid-tied discharge from an EV battery in parallel to grid operations, whether to a building, home, or microgrid, or directly to the grid. Grid-tied discharge requires utility approval and interconnection studies and agreements. V2G supports grid services, including providing grid capacity, energy arbitrage, localized voltage support, frequency regulation, and other ancillary services. Individual vehicles can be aggregated and used in virtual power plants (VPPs) to provide these grid services.
2. Vehicle-to-Microgrid (V2M): V2M is a resilience application of bidirectional charging that focuses on supplying backup power to a microgrid (for example, a utility substation, a college campus, a military base, a community center, or an evacuation center). V2M applications typically need a microgrid controller and involve sending power to more than one building.
3. Vehicle-to-Home/Building (V2H/V2B): V2H/V2B uses an EV to provide supplementary power to a building while connected to or disconnected from the grid. V2H/V2B is a behind-the-meter application that saves customers money by reducing their peak demand from the utility and/or providing backup power to a building during blackouts. V2H/V2B is non-exporting (does not send power back to the grid) and in some use cases only operates when isolated from the grid (such as in the case of backup power). While V2H/V2B use cases are non-exporting, many utilities will require customers to have an interconnection agreement for the system.
4. Vehicle-to-Load (V2L): V2L uses the EV battery to charge or power small, external or auxiliary loads, including construction tools, home appliances, or remote locations such as campsites.
5. Vehicle-to-Vehicle (V2V): V2V uses one EV’s battery to charge another EV.

Figure 2. Overlay of V2X Configurations.

Energization versus Interconnection

The X in V2X matters, especially as it relates to whether a bidirectional charging system is considered to be energized or interconnected. Increasingly, interconnected is used to describe EVSE installation; however, there are distinct differences between energizing and interconnecting EVSE. Different bidirectional charging types (V2B, V2H, V2M, V2G, etc.) have implications for the charging system’s configuration and ultimately the system’s interaction with the grid. Utilities often characterize grid-tied assets as either “energized” or “interconnected.” Energized assets are those that only receive electricity from the grid (such as unidirectional EV charging), whereas, interconnected assets are those that have the ability to discharge to the grid (such as rooftop solar and stationary battery storage). Resources that discharge to the grid must undergo an interconnection process and obtain an interconnection agreement from the local utility.

Some bidirectional charging systems will be considered energized and others will be considered interconnected,depending on the configuration (Table 3). From a utility standpoint, V2L, V2V, V2M, and islanded V2H/V2B are all considered energization because they are not sending electricity to grid-connected loads. These types of systems have the same impact on the grid as unidirectional (also known as V1G) charging systems.

V2G sends electricity directly to the grid and always needs an interconnection agreement. Grid-tied V2H/V2B will also need an interconnection agreement because these systems send electricity to a home or building, which are interconnected to the grid. In this case, while the bidirectional charging system is not designed to send electricity to the grid, there is still a utility safety concern because there is a bidirectional power flow occurring, which can have impacts on the utility system.

Lessons Learned from Utility Interconnection Processes

During SEPA’s interviews with utilities, vehicle OEMs, software providers, charger manufacturers, project implementation partners, and other industry stakeholders, interconnection was commonly cited as one of the most significant challenges in deploying bidirectional charging systems. Many utilities are either in the process of designing their bidirectional charging interconnection process or have not yet begun and do not have an interconnection process available to customers.

In utility territories that already have stationary storage interconnection tariffs, the customer portals, smart inverter requirements, and customer support teams that are already in place can be used (with some modification) for V2G and other grid-tied EV charging applications. In these jurisdictions, there is less utility effort needed to adapt interconnection processes to include bidirectional charging systems (See National Grid and Revel case studies). In contrast, utility territories that do not have a behind-the-meter stationary storage program may have to design an entirely new process or use their rooftop solar interconnection process as a starting point. Interconnection rules are different in every utility jurisdiction, making it difficult for utilities to directly copy other utility interconnection processes. In the early stages of adoption, companies and utilities will need to work closely to interconnect bidirectional charging systems.

Industry Collaboration on Interconnection

As utilities develop their own bidirectional charging interconnection processes, they can work together to address and share the most relevant challenges and solutions related to different V2X use cases. Customer and utility learnings can be shared through industry reports, ongoing cross-stakeholder roundtables, and focused workshops. SEPA and Clean Power Research are supporting a utility workshop in late October 2023 to facilitate utility conversations on streamlining bidirectional and unidirectional EVSE energization and interconnection. Reach out to the author Brittany Blair at [email protected] or the project manager Garrett Fitzgerald at [email protected] for more details on the upcoming workshop.

1. IEA (2023).
2. IEA (2023)