V2G (vehicle-to-grid) is the bidirectional exchange of energy between an electric vehicle and the power grid, in which a parked, plugged-in car can discharge its battery back into the grid as well as charge from it. In charging infrastructure, V2G turns the vehicle into a distributed storage resource that can support grid balancing, reduce peak demand and earn revenue for its owner.
V2G goes a step beyond smart charging, which only shifts or modulates charging power. It requires a bidirectional charger, a vehicle that permits discharging, a communication protocol that negotiates energy flows and a commercial framework that pays for the service.
How vehicle-to-grid works
In a V2G session, the charger and the vehicle agree on a charging schedule that includes discharge periods. An aggregator or energy management system sends setpoints based on grid signals or prices, and the charger converts power in both directions. The driver sets constraints, typically a minimum state of charge and a departure time, so the car is always ready to drive.
Where the power conversion takes place matters. In DC bidirectional charging, the inverter sits in the wall charger and the car exchanges direct current with it. In AC bidirectional charging, the vehicle’s onboard charger acts as an inverter and feeds alternating current to the grid, which makes the charger cheaper but places grid-code compliance on the vehicle.
Every V2G cycle carries conversion losses. Energy passes through the charger inverter, the battery and back again, so the round-trip efficiency of a complete charge and discharge is clearly below 100 percent and is commonly estimated in the range of roughly 80 to 90 percent, depending on power level and hardware. These losses must be covered by the price difference or the service payment, otherwise discharging the car makes no economic sense.
The table summarizes the main building blocks of a V2G system.
| Element | Function | Typical requirement |
|---|---|---|
| Bidirectional EVSE | Charges and discharges the vehicle | Grid-code certified inverter |
| V2G-capable vehicle | Allows controlled discharge | Software and warranty support |
| Communication protocol | Negotiates power and schedules | ISO 15118-20, CHAdeMO |
| Charge point backend | Controls chargers remotely | OCPP 2.0.1 or later |
| Aggregator | Bundles many vehicles for markets | Market access and forecasting |
V2G and related bidirectional modes
Vehicle-to-grid belongs to a family of bidirectional charging modes, often abbreviated V2X. They differ in where the energy goes and whether the grid operator is involved, which strongly affects technical and regulatory complexity.
| Mode | Energy destination | Grid export | Complexity |
|---|---|---|---|
| V2L (vehicle-to-load) | Devices plugged into the car | No | Low |
| V2H (vehicle-to-home) | Household behind the meter | No, islanded or zero export | Medium |
| V2B (vehicle-to-building) | Commercial building load | Usually no | Medium |
| V2G (vehicle-to-grid) | Public distribution grid | Yes | High |
Most early deployments are V2H systems that cover household consumption from the car in the evening, because exporting to the grid requires metering, grid connection approval and a market for the energy. V2G is the most valuable mode for the power system but also the hardest to deploy at scale.
Grid services and value streams
A single car offers little to the grid, but thousands of aggregated vehicles can act like a virtual power plant. Typical passenger EV batteries hold roughly 40 to 100 kWh, and bidirectional home chargers usually discharge at about 7 to 11 kW, while commercial DC units can be larger.
| Service | What the vehicle does | Time scale |
|---|---|---|
| Frequency regulation | Rapidly adjusts charging or discharging | Seconds to minutes |
| Peak shaving | Discharges during demand peaks | Hours |
| Energy arbitrage | Charges when cheap, discharges when expensive | Hours |
| Renewable firming | Absorbs solar surplus, returns it in the evening | Hours to a day |
| Local congestion relief | Supports a constrained distribution feeder | Minutes to hours |
Frequency regulation is attractive because it needs high power but little energy, so it barely affects the battery state of charge. Arbitrage and peak shaving cycle more energy and therefore depend more on battery wear and on the price spread between cheap and expensive hours.
Revenue therefore depends less on the individual car and more on the aggregator’s forecasting quality: it must predict which vehicles will be plugged in, for how long and with how much spare energy, and it must bid that flexible capacity into the right market at the right time.
Standards and technical requirements
The Japanese CHAdeMO standard supported bidirectional DC charging early and enabled the first V2G pilots. For the Combined Charging System, ISO 15118-20 defines bidirectional power transfer between vehicle and charger, while OCPP 2.0.1 and its successors let the backend control charging stations. Inverters must also meet national grid codes for anti-islanding protection, voltage and frequency behavior.
- Vehicle approval: the carmaker must enable discharging in software and define warranty conditions for V2G use.
- Grid connection: the distribution system operator usually has to approve an exporting inverter at the site.
- Metering: energy flows in both directions must be measured correctly for billing and taxation.
- Interoperability: vehicle, charger, backend and aggregator must speak compatible protocol versions.
Battery wear, economics and limitations
The most common concern is battery degradation. Extra cycling adds wear, but shallow cycles in the middle of the state-of-charge window are relatively gentle for lithium-ion cells, and services such as frequency regulation cycle little energy. The real impact depends on cell chemistry, temperature, depth of discharge and how the aggregator manages the battery.
| Barrier | Effect | Mitigation |
|---|---|---|
| Bidirectional charger cost | Long payback for households | AC bidirectional designs, larger fleets |
| Battery wear concerns | Owner reluctance, warranty limits | Shallow cycling, clear warranty terms |
| Regulatory treatment | Double grid fees or taxes | Storage-friendly tariff rules |
| Low availability | Car away when needed | Fleets with predictable parking |
V2G works best where vehicles are parked for long, predictable periods, such as school buses during holidays, depot fleets overnight or commuter cars parked at work during midday solar peaks.
Put simply
An electric car spends most of its day parked, with a large battery full of energy. V2G lets that parked car lend energy back to the power grid when the grid needs it most.
The car charges when electricity is cheap or plentiful, for example on a sunny afternoon. It gives some energy back in the evening, when everyone switches on their appliances. You set how much charge must remain, so the car is ready when you leave. In return, the owner can earn money or save on bills.
In practice
A municipal school district runs a fleet of electric school buses with large batteries that sit idle between the morning and afternoon routes and for weeks during summer vacation. The district installs bidirectional DC chargers at its depot and signs up with an aggregator.
On hot summer evenings, when the local grid faces peak air-conditioning demand, the aggregator discharges the buses at a controlled rate while keeping each battery above an agreed minimum. During the school year it schedules discharge only between routes. The district receives payments for capacity during demand peaks, the utility avoids running an expensive peaking plant, and the buses still start every route fully ready. The outcome is a second revenue stream from assets that would otherwise stand idle.