Power flow: V1G to V2G

A car’s battery can trade energy with the world in five ways, and what separates them is where the power stops. In V1G it stops in the battery: this is one-way smart charging, with the power modulated to match generation or ease congestion. In V2L it stops at a tool or appliance plugged into an outlet. In V2H and V2B it stops inside a home or a building, as outage backup or to cut bills. In V2G it crosses the meter and reaches the grid. The diagram above shows each in turn, and where the inverter that makes the AC sits.

The words are not used the same way everywhere. V1G is the IEA’s term; the US Department of Energy’s facilities programme says managed charging for the same idea. OCPP uses V2X as the umbrella, treats V2H and V2B as special cases of V2G, leaves V2L outside the protocol because it involves no OCPP traffic, and avoids V2G for power flow because ISO 15118 already uses it for the communication between car and charger. The CPUC’s 2019 subgroup went further and called any discharge V2G, exporting or not. The sources agree on the machines and differ on the labels, so the page says which source is speaking.

What matters to a utility is export. V2H and V2B are non-exporting: the CPUC subgroup compares them to a stationary battery that never feeds power out through the point of common coupling, and the 2012 Idaho National Laboratory review treats them as subsets of V2G for the same reason. V2G is the form that exports, and CHAdeMO describes it as grid services typically sold through an aggregator. The Department of Energy’s facilities programme adds two cautions: programmes that pay for such services are not yet widespread, and a site should check that its interconnection agreement allows reverse flow at all.

The other split is where the inverter sits. With AC bidirectional charging the car inverts its own battery, so the car’s power electronics carry grid compliance; with DC the charger does, and the car supplies DC. CHAdeMO, whose protocol is DC only, set out the trade-off in a 2019 slide, reproduced below as the Association’s own view, not a neutral one. Its argument is that an AC system is simpler for the charger and harder on the grid, and a DC system the other way round.

What certifies what follows the inverter. For DC, California’s Rule 21 already covers V2G, and UL 1741 can be applied to the stationary inverter in the charger. For AC in the United States, OCPP says the car must be certified to SAE J3072 and system operators also want the charger certified to UL 1741, so that it can watch the car and disconnect it. UL 9741 certifies a charger and a car together as a matched pair. UL 1741 SC, published in May 2026 according to trade press, instead certifies the charger as a supervisor for a J3072 vehicle, without certifying a specific pair. As of the CPUC’s April 2025 slides, California allowed AC V2G only through a pilot, and no source read here says whether its permanent process arrived on schedule. The SAE entries rest on secondary sources, because SAE’s catalogue could not be opened.