Car battery backup for a house during power cuts

Speculative concept
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

This is an idea, not a product. Nothing here has been built or tested unless a linked technology profile says so.

The problem

Households may want a few essential circuits kept running during a power cut. An electric car battery could in principle help, because the vehicle-to-grid profile lists vehicle-to-home use during a power cut as a speculative application. The profile does not report any home-charging product that does this, so the need is not confirmed.

The idea

A first version would pair a bidirectional home charger with a V2G-capable car, so the car could supply part of a home's everyday load while the grid connection is up, keeping the battery above a reserve level the owner sets. Operating during a grid outage, with the house isolated from the grid and fed from the car, would be a separate later step, limited to a small set of essential circuits. It would only be considered once safe switch-over has been shown on a test rig.

Enabling technology

  • Vehicle-to-grid: Vehicle-to-grid (V2G) lets an electric car send power from its battery back to a home or the grid, not only take power in. A few UK and Dutch schemes are running, but the evidence on whether discharging speeds up battery wear is mixed, and most of it comes from lab cells or simulations.

What stands in the way

  • Battery wear under V2G is not settled. A lab study on commercial cells reported faster capacity loss with daily discharging, and a simulation reported that setting the state of charge before V2G operation reduced total ageing. Neither measured a real car in everyday use, so an outage reserve that is drawn on regularly could age the battery faster.
  • The V2G profile does not cover isolated operation during a grid outage. Disconnecting a house from the grid and reconnecting it safely is not covered there.
  • Eligibility is narrow. The UK tariff named in the profile lists one qualifying car and charger pair, requires a smart meter and a G99 certificate for export, and expects regular plug-in hours. An outage feature would therefore depend on a narrow set of hardware.
  • ISO 15118-20:2022 specifies messages for bidirectional power transfer. The profile's source is the public abstract only, because the full text is paywalled, and the abstract does not show whether the standard covers an isolated backup mode.
  • Backup alone may not pay for the hardware. A UK residential V2G project modelled a typical 7 kW charger earning about 100 pounds a year, and above 400 pounds only in ideal circumstances, so charger and metering costs would need a case beyond outage protection.

A cheap way to test it

Bench test only, with no connection to a live grid or a household, and no human trials. A qualified electrician should set up and supervise it under the distribution network operator's connection rules. Use a bidirectional charger, a battery test pack and a resistive load bank, with a switchable isolator standing in for the grid connection. Before testing starts, write a fixed list of grid-loss events and the pass limits for isolation time and for load voltage and frequency, citing the connection rules the team relies on. For each event, record the time from grid loss to isolation and the load voltage and frequency throughout. Pass only if every event isolates within the written time limit and voltage and frequency stay within the written limits. This test does not measure battery wear, which needs a separate and longer study.