Bidirectional charging lets an EV's battery send energy back out: to a house, a building, or the grid. The idea is simple and attractive, since a parked car is a large battery that is idle most of the day. The implementation spans vehicles, chargers, communication standards, grid interconnection rules and commercial arrangements, and progress in each moves at a different speed. This guide separates what the terms mean, what the standards now support, and what an operator can realistically do.
The V2X family
| Term | Energy flows to | Typical use | Grid connection |
|---|---|---|---|
| V2L (vehicle-to-load) | Devices plugged into the car | Tools, camping, appliances | None; the car acts as a power outlet |
| V2H (vehicle-to-home) | A house | Backup power, self-consumption of solar | Usually islanded from or coordinated with the grid via a home energy system |
| V2B (vehicle-to-building) | A commercial building | Peak shaving, reducing demand charges | Behind the meter |
| V2G (vehicle-to-grid) | The distribution grid | Grid services, energy arbitrage | Exports to the grid; needs interconnection approval |
V2X is the umbrella term. The distinction that matters most is whether energy is exported to the grid (V2G) or used behind the meter (V2H, V2B). Exporting brings utility interconnection requirements, metering and market rules; staying behind the meter avoids most of that.
AC vs DC bidirectional
Bidirectional power needs an inverter somewhere to turn battery DC back into AC.
- DC bidirectional: the inverter is in the charger. The car exposes its battery over the DC connector and the charger handles conversion and grid compliance. This is the more established route, used in CHAdeMO-based V2G and in CCS systems with ISO 15118-20 support. Chargers are more expensive, but the grid-facing equipment is a stationary product that can be certified once.
- AC bidirectional: the car's onboard charger runs in reverse and exports AC through an AC connector. Chargers are cheaper, but the car itself becomes the grid-connected inverter, which raises the question of how a moving product meets grid codes that vary by country and utility.
The standards
Vehicle to charger: CHAdeMO and ISO 15118-20
CHAdeMO supported bidirectional power early, which is why many first-generation V2G and V2H projects used CHAdeMO vehicles. For CCS, bidirectional power transfer is defined in ISO 15118-20, published in 2022, which also introduced a revised security model and more flexible energy scheduling between car and charger. ISO 15118-2, the earlier edition most CCS vehicles support, does not define discharging. So a bidirectional CCS deployment needs both the car and the charger to implement ISO 15118-20, including its bidirectional services. Support for ISO 15118-20 in vehicles is growing but still uneven, so check specific models and firmware versions.
Charger to CSMS: OCPP 2.1
OCPP 1.6 and 2.0.1 were designed around charging. You can approximate some behaviour with vendor extensions, but there is no standard way to command discharge or report it properly. OCPP 2.1, released by the Open Charge Alliance in January 2025 and since published by IEC as IEC 63584-210, adds a functional block for bidirectional power transfer and another for DER (distributed energy resource) control, along with ISO 15118-20 support. OCPP 2.1 builds on 2.0.1, and application logic written for 2.0.1 is designed to remain compatible. See what's new in OCPP 2.1.
The conceptual change is that charging limits can go negative. A charging schedule can tell the station to discharge at a given power for a period, subject to what the vehicle has agreed to over ISO 15118-20. The CSMS also has to handle energy flowing both ways in meter values: imported and exported energy are separate measurands (Energy.Active.Import.Register and Energy.Active.Export.Register), and billing has to treat them separately.
CSMS to grid
For V2G, someone has to decide when to discharge. That signal usually comes from an aggregator, utility program or energy market interface, using protocols such as OpenADR or IEEE 2030.5, or a market-specific API. The CSMS converts those signals into per-vehicle schedules. See EV charging protocols explained.
What has to be true for V2G to work
- The vehicle supports bidirectional power on the connector you use, and the manufacturer permits it under the battery warranty for the use case.
- The charger supports bidirectional power and the relevant protocol (CHAdeMO, or ISO 15118-20 with bidirectional services).
- The installation has interconnection approval from the utility, using equipment certified to the local grid-interconnection standards.
- The CSMS can schedule discharge: OCPP 2.1 or a vendor extension, plus metering of exported energy.
- There is a commercial reason: a tariff, grid service program or building cost saving that pays the driver or site owner more than the cost of battery wear and hardware.
- The driver's needs are protected: minimum state of charge and departure time are respected. ISO 15118-20 lets the car communicate energy needs and limits, and the scheduler must honour them.
Every item on that list is achievable; the difficulty is that all six must be true at the same time, for the same vehicle, at the same site.
The practical status
- V2L is widely available on recent EVs and needs nothing from the charging network.
- V2H is commercially available in several markets through specific vehicle and home charger pairings, often bundled by the automaker or a partner.
- V2B and depot V2G are viable where a fleet operator controls the vehicles, chargers and building: school buses and other fleets with predictable schedules are common pilot candidates.
- Public V2G at scale, with arbitrary vehicles at public chargers, is still mostly at the pilot stage, held back by vehicle support, interconnection processes and the lack of mature compensation programs.
What to do now as an operator
- Build smart charging properly first. Unidirectional managed charging captures much of the value (peak shaving, demand charge reduction, time-of-use shifting) with none of the V2G complexity. See smart charging and load management and OCPP 2.0.1 smart charging.
- Keep the CSMS data model ready for bidirectional energy: signed power limits, import and export registers, and pricing that can credit exported energy.
- Prefer hardware with a credible OCPP 2.0.1 to 2.1 upgrade path and ISO 15118-20 support where you expect fleet or depot V2G.
- Pilot with a controlled fleet before anything public.
Key takeaways
- V2L and V2H are here; fleet and depot V2G is practical in the right conditions; public V2G is still early.
- ISO 15118-20 (or CHAdeMO) handles the car-charger side; OCPP 2.1 adds standard bidirectional and DER control between charger and CSMS.
- Managed unidirectional charging is the foundation and delivers value today.
HashTech builds smart charging and load management into the CSMS platforms we deliver.
