Using an electric car as a home backup battery

How does vehicle to home charging work?

An ordinary home charger only moves power one way, from the grid into the car. A bidirectional charger can also move power back out. In a V2H setup, the car's battery supplies the house through that charger and a switching device that isolates the home from the grid so that no power flows back to the lines during an outage. When the grid returns, the system reconnects and charges the car again.

Two related terms are worth separating. Vehicle-to-load, or V2L, is an outlet on the car for plugging in tools or appliances, which is simpler and smaller scale. Vehicle-to-grid, or V2G, exports energy to the grid for payment or grid support and needs utility programs that are not available everywhere. This page is about V2H.

What equipment and approvals does V2H need?

Four things have to line up.

• A compatible vehicle. Only some electric vehicles support exporting energy to a house, and support can depend on the model year and on the manufacturer's warranty terms.
• A bidirectional charger. Certified for the vehicle and for use in Canada, sometimes sold as part of a package with a home energy gateway.
• Home integration. A transfer switch or energy gateway and a backup panel, installed by a licensed electrician under permit.
• Utility and code approval. The connection must meet the electrical code and the utility's interconnection rules.

Rules and product availability are changing quickly, so confirm each item with the automaker, the installer and your utility. The permitting and safety points in battery safety and installation rules apply to V2H too.

How long could a car power a house?

Electric car batteries are large compared with home batteries, often several times larger. Take an example with assumptions stated. A vehicle with a 60 kWh pack, kept above a 30 percent reserve so that you can still drive, offers about 42 kWh to the house. If the protected circuits average 1.25 kW, which is 30 kWh a day, the energy lasts a bit over a day. With a gas furnace, a fridge and lights drawing 0.5 kW on average, it lasts closer to three days.

Real results will be lower after conversion losses and depend on the charger's power rating, which limits how many appliances you can run at once. These are illustrative figures. Compare them with a stationary battery in home battery backup and with the options in backup power during outages.

What are the limits and risks?

The car must be home and plugged in. If you drive to work, it is not available during a weekday outage, and you may not want to leave the house without a charged car. Cold weather reduces battery performance and driving range, so a winter outage means the reserve matters even more. Exporting energy adds cycles to the battery, which raises questions about degradation and warranty; the battery degradation and warranty guide lists what to ask.

Equipment cost, installation and a possible need for a new panel add up, and you still need a second vehicle or a plan if the car is the only source of power. For most households V2H is an emerging option to watch rather than a settled purchase.

Is V2H worth considering, and where does solar fit?

It is most interesting for homeowners who already plan to buy an electric vehicle and want a large backup source without a second battery, particularly in regions with frequent outages. A car that charges from rooftop solar by day and powers the house at night can also shift energy between day and evening; the charging side is covered in EV charger and solar. For larger whole-home loads, see whole house battery backup.

Ask your installer and your utility about availability in your area, written specifications and warranty positions before you commit. Prices and support change quickly, so quotes and manufacturer statements decide. The comparison with ordinary storage is in the solar battery guide.

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