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 keeps the car unchanged: the wallbox does the conversion and meets the grid rules. The wallbox is larger and more expensive.
- AC bidirectional uses the car's in reverse. The wallbox stays simple, but the car's converter must behave like a small generator towards the grid.
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.
- 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
- 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.