Vehicle-to-home backup charging

Supplying household loads from a compatible EV, including the extra design needed for power-cut operation.

V2H (vehicle-to-home) uses a compatible EV battery to supply loads in the home through fixed, purpose-designed equipment. It can be used while the public supply is present to reduce grid import, and some systems can also support selected loads during a power cut.

Those are two different operating states. Grid-connected V2H needs controlled reverse power flow. Power-cut backup additionally needs the home to separate safely from the public network and operate as a local electrical island.

V2H is not the same as V2L

V2L (vehicle-to-load) provides an outlet for appliances through the vehicle or an approved adaptor. It can be useful for portable equipment, but it does not make the vehicle a fixed supply for the house.

Never connect a V2L outlet to a household socket in an attempt to energise the wiring. That can bypass protection, create an unsafe neutral and earthing arrangement, energise circuits beyond the outlet’s design and backfeed the public network.

A V2H system instead uses equipment designed for connection to the installation. The vehicle, charger, changeover or gateway, protective devices and controls must be assessed as one system.

A compatible connector is only the starting point

Some V2H designs use a DC bidirectional charger containing the inverter. Others use bidirectional AC conversion in the vehicle. In either case, the vehicle must permit discharge through that charging interface and exchange the required control information.

Compatibility depends on the exact:

vehicle model, market version, model year and software
battery and vehicle warranty terms
charger model, power stage and firmware
connector and supported charging mode
communication implementation
grid-interface approval and installation design

A Type 2 or CCS socket does not prove V2H support. Nor do “ISO 15118 capable”, “bidirectional ready” or “future ready” prove that reverse power is enabled in the UK. Support has to cover the complete vehicle, software and charging-equipment combination.

Grid-connected home supply

While the grid is present, V2H can discharge enough power to offset some or all of the home’s demand. The control system normally measures power at the grid connection and adjusts vehicle output around an import or export target.

The bidirectional converter is still capable of operating as embedded generation. The installer must establish whether G98 or G99 applies and account for existing solar, stationary batteries or other generation. If export is restricted, a G100 customer limitation scheme may be required. A setting labelled “zero export” does not by itself prove compliance or remove the connection process.

Sensors and controls also need to coordinate. If the house has PV, a home battery and a bidirectional car, several systems may respond to the same measurement. Poor coordination can make them fight one another, oscillate around the target or charge one battery from another without achieving the intended saving.

What power-cut backup adds

The ordinary grid-connected inverter must stop energising the grid when the public supply fails. To keep the home running, the backup design needs:

a compliant changeover or gateway that disconnects the installation from the grid
a source able to establish and regulate local voltage and frequency
a defined neutral and earthing arrangement in island mode
protection that still disconnects faults with the available backup fault current
circuits and loads that stay within the V2H system’s output limits
a safe sequence for entering and leaving island mode

These requirements cannot be inferred from a charger’s normal charging power. Backup output may have different continuous, surge, phase and temperature limits. Some systems support only a dedicated group of circuits. Others may supply more of the house but still need automatic load shedding or careful user control.

Transfer may be automatic or manual and may include an interruption. Any transfer-time claim belongs to the exact tested product and installation. Equipment that must remain uninterrupted, such as medical or critical IT equipment, may still need its own uninterruptible power supply.

The car has to be there

Unlike a fixed home battery, the vehicle may be away when an outage starts. Its remaining charge must also cover the next journey.

A useful design starts with the loads that genuinely need backup and the energy reserve the driver will not allow the house to use. It should consider:

likely vehicle location and plug-in routine
minimum driving reserve and departure schedule
overnight and seasonal household demand
heating, cooking, pumps and other high-power loads
whether solar can operate and recharge the vehicle while islanded
conversion losses and system standby consumption
what happens when the vehicle reaches its reserve or is unplugged

Battery capacity alone does not determine backup duration. Two homes with the same vehicle can have very different results because their loads and usable reserve differ.

Protection and installation

The charging circuit must meet the applicable EV installation requirements, including the RCD and open-PEN protection selected for the equipment and earthing arrangement. Bidirectional and islanded operation add operating modes that the protection design must cover explicitly.

Commissioning covers normal charging, grid-connected discharge, loss of communications, export control and every backup transition the product supports. Handover information identifies:

which circuits are backed up
continuous and short-duration output limits
loads that must not be used in backup mode
manual isolation and emergency shutdown
the minimum vehicle reserve and how to change it
how to return to normal operation after a fault or outage
DNO acceptance, settings and the single-line diagram

Cost, tariffs and battery wear

There is no durable typical payback for V2H. The result depends on equipment and installation cost, the difference between charging and avoided-import prices, energy losses, how often the car is available, and whether backup rather than daily arbitrage is the main purpose.

Daily discharge increases battery throughput. The resulting degradation depends on chemistry, temperature, state-of-charge range, power and the vehicle’s own controls. Battery warranties, leases and finance agreements can place separate limits on that use. Occasional emergency use and daily tariff cycling are not equivalent duties.

Vehicle and chargepoint availability changes quickly, so a permanent list of “compatible” models would be misleading. Compatibility belongs to an exact combination of vehicle, software, chargepoint, conversion equipment and service documents.

Vehicle, charger and backup design

A functioning V2H system depends on:

exact vehicle and software support for V2H rather than V2L or unspecified future capability
whether reverse conversion occurs in the vehicle or the chargepoint
separate approval for grid-connected discharge and outage backup
the means of preventing the system energising the public network during a cut
the source of the local neutral, earth reference, voltage and frequency
supplied circuits, continuous output and surge output
coordination with solar and any stationary battery
operation if the car is unplugged, reaches reserve or loses communication
a DNO decision covering every generating source at the property
vehicle warranty or lease terms covering the intended use