Where is the energy converted: in the car or in the charger?
The wallbox on the garage wall is larger than the household's old one. Inside it, a converter works in both directions.
The neighbour's car uses a different approach: its own onboard charger feeds the house, through a simple wallbox.
The questionWhere does direct current from the battery become household current?
- Batterydirect current
- DC cableto the wallbox
- Wallboxconverts to AC
- House230 and 400 V
- Batterydirect current
- Onboard chargerconverts to AC
- AC cableto the wallbox
- House230 and 400 V
Where the converter sits decides who has to meet the grid's technical rules: the wallbox or the car.
Two paths, two trade-offs
- DC bidirectional: the wallbox converts between direct and alternating current itself.
- AC bidirectional: the conversion takes place in the car, which needs a bidirectional for it.
Either way, the car and its , inside the car or in the wallbox, must meet the rules for generation units once they work in parallel with the grid. In Germany, DC solutions are further along in pilots and first commercial use than AC ones.
The 2026 amendment of ISO 15118-20 adds an AC DER service, a communication service for this AC case.
The same question as for charging
It is the mirror of normal charging: with AC, the car converts; with DC, the charger converts. The charging side of this is explained in Charging hardware, stop 01.
Check your understanding
3 questions. Answer all of them to complete this stop. Each answer explains itself.
Where is the energy converted with DC bidirectional charging?
The DC wallbox contains a converter that works in both directions.
What is the main advantage of AC bidirectional charging?
The car's onboard charger works in reverse; the car then has to meet the grid's rules.
What did the 2026 amendment of ISO 15118-20 add for the AC case?
ISO lists an AC DER service, together with an MCS service and improved security.
- Inverter Power electronics that turn the battery's direct current into alternating current for the motor, and back when the car regenerates. eMobility and EV basics, stop 03 Glossary
- Onboard charger The converter in the car that turns AC from a wallbox or public AC charger into DC for the battery; its rating limits AC charging power. eMobility and EV basics, stop 04 Glossary
- ADAC: Bidirektionale Wallbox: Welche Modelle es gibt und was sie können (26 June 2026): DC wallbox or bidirectional onboard charger. https://www.adac.de/rund-ums-haus/energie/versorgung/bidirektionale-wallbox/. Checked 13 Oct 2026.
- ADAC: Bidirektionale Wallbox: Welche Modelle es gibt und was sie können (26 June 2026): DC wallbox or bidirectional onboard charger. https://www.adac.de/rund-ums-haus/energie/versorgung/bidirektionale-wallbox/. Checked 13 Oct 2026.
- VDE FNN: Hinweis Bidirektionales Laden, version 1.0, February 2024: V2L, V2H in island and parallel operation, V2G, and the rules for generation units. https://www.vde.com/resource/blob/2303260/4430c923f56d66ba8f5b41ca5db2a480/bidirektionales-laden-download-data.pdf. Checked 13 Oct 2026.
- ADAC: Bidirektionale Wallbox: Welche Modelle es gibt und was sie können (26 June 2026): DC wallbox or bidirectional onboard charger. https://www.adac.de/rund-ums-haus/energie/versorgung/bidirektionale-wallbox/. Checked 13 Oct 2026.
- ISO: ISO 15118-20:2022 and Amendment 1:2026 (AC DER service, MCS service, improved security). https://www.iso.org/standard/77845.html. Checked 09 Oct 2026.