Design and sizing / Battery and backup sizing / Vehicle-to-home storage overlap

D-011·Design and sizing / Battery and backup sizing

Vehicle-to-home storage overlap

How EV batteries compare with fixed home storage.

An EV and a fixed home battery can both store electricity, but they are not interchangeable products. Vehicle-to-home, usually shortened to V2H, needs a compatible vehicle, bidirectional charging equipment, a suitable electrical installation and controls that decide how much energy the car may release.

The useful comparison is not simply which battery has the larger nameplate capacity. Availability, output power, conversion losses, backup capability, warranty, grid approval and the driver’s required range all affect how much of the vehicle battery can perform the job of stationary storage.

V2L, V2H and V2G are different functions

The terms describe different electrical arrangements:

  • V2L (vehicle-to-load) supplies portable appliances from a vehicle outlet or approved adaptor.
  • V2H (vehicle-to-home) supplies loads within a property through purpose-designed equipment.
  • V2B (vehicle-to-building) is the equivalent arrangement for a larger building.
  • V2G (vehicle-to-grid) permits controlled export through the property’s grid connection.

A car with V2L does not necessarily support V2H or V2G. A charger described as smart, ISO 15118 capable, bidirectional ready or future ready is not necessarily able to reverse power today. Reverse operation depends on the exact vehicle, market version, software, charger power stage, firmware, connector, communications and UK grid-interface approval working as a supported combination.

Never connect a vehicle outlet to a household socket to energise the fixed wiring. That can bypass the required changeover, protection and earthing arrangements and may backfeed the public network.

Compare available energy, not badge capacity

The vehicle’s traction battery may be much larger than a home battery, but not all of it is available to the house. The driver needs a minimum state of charge for the next journey, the vehicle may impose its own reserve, and discharge may stop at limits set by the vehicle or charger.

A simple planning calculation is:

energy available to the home = usable vehicle energy above the driving reserve × discharge-chain efficiency

The input values must come from the supported system, not from the vehicle’s gross advertised battery capacity. The calculation should also allow for energy needed before the next departure.

A fixed battery is normally present and connected. A vehicle may be away, unplugged or below its intended reserve when the home needs energy. If the requirement is dependable daily load shifting or power-cut resilience, availability can matter more than capacity.

Power can be the binding limit

Battery capacity is measured in kWh. The power that can be supplied at one moment is measured in kW. A vehicle may hold enough energy for a long period but still be unable to start or run every household load together.

The design must use the bidirectional system’s supported continuous and short-duration output, not the car’s driving power or the charger’s one-way charging rating. It should check high-demand and high-inrush loads such as electric heating, cooking equipment, pumps, compressors and motors.

A home battery faces the same capacity-versus-power distinction, but its inverter and backed-up circuits can be selected around the property. With V2H, the vehicle and bidirectional charger pairing narrows the available choices.

Conversion losses belong in the comparison

The traction battery stores DC while the home uses AC. Some systems put the bidirectional inverter in the charger; others use compatible conversion equipment in the vehicle. In either arrangement, charging and discharging lose energy and the controls consume some power.

Compare whole-chain energy at a consistent boundary. For example, grid energy measured on import should be compared with energy later displaced at the supply meter. Cell efficiency, charger efficiency or a single conversion-stage figure cannot represent the whole round trip by itself.

For tariff shifting, the same arithmetic applies to both stores:

delivered stored-energy cost = charging energy price / measured round-trip efficiency

Any benefit must then exceed that delivered cost and an allowance for battery wear. For solar charging, include the value of export that would otherwise have been earned.

Cycling and warranty need product-specific answers

Vehicle and stationary-battery warranties are written for different products. Relevant conditions may include age, mileage, retained capacity, energy throughput, use of approved charging equipment, commercial use and permission for bidirectional discharge.

Do not infer approval from the cell chemistry or from participation in an earlier trial. Obtain the current warranty and a manufacturer-supported compatibility statement for the exact vehicle and charging system. Establish whether V2H or V2G use affects the traction-battery warranty, vehicle lease, charger warranty or insurance.

A fixed battery has its own restrictions. Its warranty may distinguish self-consumption from deliberate export, third-party control or unusually high cycling. Economics should be assessed after those restrictions are known.

Grid-connected V2H is an embedded-generation system

When the bidirectional converter can supply the property while it remains connected to the public network, it is capable of operating as generation. The installer must follow the applicable ENA connection route and include the existing solar PV, stationary batteries or other generation at the site.

Depending on the equipment and aggregated registered capacity, the route may be G98 or G99. An export-limitation scheme may form part of an accepted design, but a software setting labelled zero export does not remove the connection assessment. Vehicle charging load control, which limits import, is a different function from a compliant site export limit.

The accepted application or notification, type-test evidence, single-line diagram, export settings and commissioning results should remain with the installation records.

Supplying a home is not automatically power-cut backup

A grid-connected V2H system may reduce household import while the public supply is healthy yet shut down during an outage. Backup requires the property to disconnect safely from the public network and operate as an electrical island.

That adds a suitable changeover or gateway, a source that can establish local voltage and frequency, a defined neutral and earthing arrangement, protection that works with the available fault current and a controlled reconnection sequence. The supported circuits must stay within the vehicle and charger output limits.

The transfer may include an interruption. If an appliance needs uninterrupted power, it may still need a separate uninterruptible power supply. The system also cannot provide backup if the car is absent, so a stationary battery or another source may still be justified where continuity is important.

Where the two forms of storage complement each other

The choice is not always one or the other. A small fixed battery can absorb routine solar surplus, cover short peaks and remain available for backup. A compatible vehicle can add energy while it is parked without forcing the home battery to be sized for rare long events.

If both are installed, one controller or a clearly defined control hierarchy should decide which store charges or discharges. Otherwise each system may react to the same current-transformer reading, charge from the other store or oscillate around the import and export target.

The complete arrangement is defined by:

  • the exact supported vehicles and chargers, and the party responsible for compatibility
  • the battery reserve for travel and the periods when the vehicle is normally connected
  • continuous and surge output through the V2H path
  • grid-connected and power-cut operating modes
  • the circuits supplied in island mode
  • coordination between solar, a home battery and V2H
  • vehicle, charger and battery warranty conditions
  • grid-connection and export conditions
  • safe operation after loss of internet access or the control service

Product availability and compatibility can change through software, model-year and market approvals. Check the complete supported combination at the point of design rather than relying on a list of vehicles or chargers copied into an evergreen article.

Applies to

EV charging, Battery

Last reviewed

22 Jul 2026