Bidirectional charging: V2G for all

Using EV batteries to store renewable energy is profitable for the grid and users. The only thing missing is the standards needed to make the technology universally available.

By Michael Neißendorfer

It’s a real win-win situation: electric cars can use their battery not only to power their journeys but also to feed electricity back into the grid. A practical way to improve the use of renewable power and mitigate local load peaks. And an enticing prospect for car owners, who stand to earn money for providing this grid service.

In France, this promise has already become reality. The first end customer product for vehicle-to-grid (V2G) in Europe has been on the market here for a few months, with buyers of a Renault 5 able to participate in the electricity market using a dedicated wallbox from the carmaker. Customers receive eleven cents for every hour that their EV is connected to the grid and available as a temporary storage buffer. The solution relies on this specific hardware pairing and a special electricity tariff. The Mobility House, a Munich-based company, is involved in implementing the project. Managing Director Marcus Fendt emphasizes the pioneering nature of the partnership with Renault. “If their vehicle is connected to the grid for 13 hours a day, customers in France receive enough free electricity for around 10,000 kilometers of driving.”

Why doesn’t something like this exist in Germany yet? Ylber Azemi, Project Manager VDE ETG / Mobility, explains that common standards will be needed to roll V2G out more widely: “Interoperability is the keyword,” he says: “V2G must be safe and reliable to implement in Germany, but we are developing standards that can be applied globally, because our solutions should be usable worldwide. We haven’t got that far in terms of standardization yet, as we have to take certain fixed processes and deadlines into account.” Bidirectional charging is already possible, meaning that electric cars can use vehicle-to-load (V2L) to supply electricity directly from the vehicle battery to appliances of up to 3.6 kW, such as an electric barbecue, a hammer drill or a hedge trimmer.

Schematic representation of bidirectional charging

Bidirectional charging with electric cars, solar cells, and energy storage systems in a residential building.

| Polestar

Electric cars are already being used as intermediate storage units in the home (vehicle-to-home; V2H), allowing users to boost their consumption from home PV systems. These, too, are manufacturer-specific systems offered from the likes of Volkswagen and Ford with E3/DC. V2G pilot projects have shown that bidirectional charging also works at grid level. They include BDL Next, led by Bayernwerk, the Grids & Benefits project managed by the former Federal Ministry for Economic Affairs and Climate Protection (BMWK), and dozens of others worldwide (the Japanese car manufacturer and V2G pioneer Nissan alone has more than 40).

V2G-capable wallboxes have been in development for a while, and some are already available. Most of these are DC charging stations that convert direct current into alternating current. Unlike with AC charging stations, this means that no additional components are required in the car itself. For V2G to work with AC wallboxes, the electric car must have a bidirectional on-board charger to feed electricity back into the grid. Ultimately, both variants will be launched on the market. The standard for communication between wallbox and electric car – ISO 15118-20 – has already been developed for DC and is about to be published for AC.

“Completing the standardization process to ISO and IEC specifications takes several years,” says Azemi explaining why V2G has not yet progressed beyond the pilot stage. However, the adoption of the necessary system standards is on the home stretch. For AC, completion is scheduled for the end of 2026, while the current standardization roadmaps predict that work on DC will be completed by the end of 2027. Azemi expects the first products for end customers to be launched on the market in Germany almost immediately – from 2027 for AC and from 2028 for DC.

But there are still a few challenges to be addressed first. A major hurdle in Germany, which France has already overcome, is the high taxes and duties payable for electric cars as market participants on the energy exchange. In fact, these have to be paid twice: first when supplying the battery or charging the electric car, and second when discharging back into the grid. This is a major disadvantage compared to stationary battery storage systems, which are exempt from these grid fees. Many companies and associations are in favor of equal tax treatment for stationary and rolling storage systems (i.e. electric cars), including the German Automobile Club (ADAC), the German Association of Energy and Water Industries (BDEW), the National Centre for Charging Infrastructure (NOW) and the German Association of the Automotive Industry (VDA). “The new German government should work with the Federal Network Agency to push for changes that completely eliminate the problem of double taxation,” says VDA spokesperson Lena Anzenhofer.

Screenshots of an app for controlling bidirectional charging

The charging time and desired charge level can be managed via an app. Electric car manufacturers such as Polestar advertise that users can reduce their electricity costs by up to 30 percent thanks to bidirectional charging.

| Polestar

Who can drive the technology forward? “The car manufacturers don’t get much out of it,” says Azemi. “The topic is not a top priority for energy suppliers and grid operators either,” he adds, although they could save grid expansion costs in some places thanks to V2G, as electric cars would absorb local load peaks. For now, smart charging is a more attractive avenue for grid operators and customers. This synchronizes EV charging times with electricity generation levels, for example, so that excess renewable generation does not go to waste.

Another issue is that today’s grid systems only take into account the current electricity price, not the origin of the electricity. Azemi thinks there is still work to do here – and sees a great opportunity: “Distributing renewable energy more systematically, for example by channeling it into vehicles, would enormously improve the use of green electricity and the carbon footprint of electric cars,” he says.

The potential for end customers is difficult to estimate, Azemi continues. One factor here is the high purchase cost for the hardware – wallbox manufacturers currently expect a V2G-capable DC wallbox to be priced at around 3,000 euros, while AC systems will be significantly cheaper. This investment must first pay for itself. Is this possible? Azemi predicts that V2G will bring in around 200 to 300 euros per year. A recent study by the Fraunhofer Institutes for Solar Energy Systems (ISE) and Systems and Innovation Research (ISI) is much more optimistic. In the best-case scenario, it reckons with as much as 1,000 to 1,500 euros – albeit before duties and taxes. End customers stand to receive around half this amount. The more cautious Azemi believes that very low or even negative electricity prices – caused when renewable energy floods the grid – may be less common in future if more systematic use can be made of this power. This would in turn reduce the potential revenue from V2G.