The electric vehicle as a home energy storage device: How bidirectional charging works

The idea of supplying your home with electricity from the battery of an electric car seems extremely innovative. But while some aspects have already been successfully implemented, others still face challenges.

Electricity storage could increase independence from energy suppliers
So far only a few vehicles and charging stations are suitable for this
Standards and regulations still need to be developed by legislators and industry
The electric vehicle as an energy buffer

Many people want to be more independent of the electricity supplier. Anyone who has already installed a solar system on their roof is already one step further. However, it is difficult to cover more than 30 percent of your own energy needs with solar energy alone, as the weather and your own electricity consumption vary greatly from day to day.

In order to increase this share, the demand for large battery storage systems is becoming increasingly important. However, they are not cost-effective: you currently have to budget between 750 and 1200 francs per kilowatt hour of storage space. In general, the larger the capacity, the lower the average price per kWh.

So why buy a separate power storage system when the electric car already has a battery? The electric car could store excess electricity, for example from the solar system, during the day and release it again when needed - be it to an electrical device, the house network or the power grid.

Many experts see this option, bidirectional charging, as a significant step towards greater energy security: car batteries that can not only absorb electricity but also deliver it again could help balance the fluctuating energy supply from renewable sources.

Bidirectional charging: an overview of how it works

With bidirectional charging, electricity flows in two directions: first from the grid into a storage facility and then back from the storage facility into the grid. In principle, many devices already have this function. Every battery stores and delivers electricity, for example laptops and power banks supply mobile devices.

However, bidirectional charging with an electric car battery requires an additional step. Electric vehicles use direct current (DC), while households use alternating current (AC). Therefore, the alternating current must be converted into direct current during charging. This is made possible by a rectifier, either in the vehicle's on-board charger or in a DC wallbox. An inverter is needed to feed the electricity back into the grid.

Conversely, the direct current from the car battery must be converted back into alternating current if the electricity is to be returned to the house or power grid. This also requires technical adjustment.

V2L, V2H, V2G: Three variants

There are three versions of bidirectional charging. The simplest variant, which is already available in some models, is that the electric vehicle has a conventional Schuko socket to which electrical devices can be connected while on the move. This function is called Vehicle-to-Load (V2L) or Vehicle-to-Device (V2D). Vehicles such as the Hyundai Ioniq, the Kia Niro or models from MG offer this option. Although this is not a revolutionary innovation, it is still very practical, especially for campers or people who need power on the go, such as tradesmen who need to run or charge tools.

The second variant is that the electric car connected to the wallbox releases energy into the house network. This is called Vehicle-to-Home (V2H). As with charging, the electric vehicle is connected to the in-house wallbox, and if necessary - for example if the solar system on the roof does not provide electricity - the previously charged electricity is used for internal consumption in the house.

The most demanding option is to feed the electricity stored in the battery not only into the home, but also into the entire power grid: Vehicle-to-Grid (V2G). The vision behind it is to create a "virtual power plant" by connecting many thousands of electric vehicles to help stabilize energy supplies, especially during morning and evening peak times when many households turn on electrical devices at the same time.

These vehicles support bi-directional charging:

There are many car manufacturers who say that their cars can charge bidirectionally. In fact, only Nissan can do it with the Leaf and the NV200. In 2024 the Volvo EX 90 will also be able to do it - the rest is hot air.
The standard for bi-directional charging is in place. The technology for bi-directional charging is already available. So that it works smoothly and as far as possible

Now the manufacturers of wallboxes, electric vehicles and energy management systems have to bring standard-compliant and mutually compatible products onto the market at economically justifiable prices.

Load management for the entire power grid Intelligent load management will be crucial if the electricity is not only to be used in your own household (Vehicle-to-Home, V2H), but also to be fed into the general power grid (Vehicle-to-Grid, V2G). Charging and discharging, potentially of large numbers of EV batteries, then needs to be synchronized with supply and demand.

Another challenge is regulating billing with the electricity supplier for the electricity fed in. Solutions similar to those for feeding in solar power may need to be developed here, which are adapted to the requirements of electromobility.

There are also significant technical differences between the various electric car models, from battery size to on-board electrical system voltage. House connections and wallboxes must be able to deal with these differences in order to avoid malfunctions, short circuits or damage to the battery.

It is also uncertain how car manufacturers will deal with the guarantee for the vehicle battery if it is also approved for bidirectional charging. After all, the battery would then be charged and discharged more frequently than during normal use for driving alone. The question of liability for possible damage to the installation in the event of malfunctions is also open.

New laws are needed The legislature faces several challenges. From a legal perspective, electric cars are currently only classified as passenger cars and not as battery storage, for which stricter legal requirements sometimes apply. It must also be clarified how the electricity fed in is treated for tax purposes. This is a complex question