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Can I use my electric vehicle battery to power my home?

Often, I am asked “Why can’t I use my electric car to run my refrigerator or get hot water when the utility grid is down,” or “Why do I need an energy storage system (ESS) when I have a big battery in my electric car?” This is often called “Vehicle to Grid” or VTG. In this blog I will discuss the policies, practical applications, and costs of VTG as a replacement to a dedicated ESS and their possible roll in augmenting ESS. While these technologies are coming, I do not know of any jurisdictions in Sonoma County that are currently approving this technology.

To begin this discussion, it may be beneficial to first discuss how an ESS system works.  When the utility grid is functioningnormally, my storage system charges from solar in the mornings when PG&E costs are low, and discharges in the early evening when PG&E rates are high. (We call this peak shaving.) This is one of the primary ways an ESS lowers home energy costs. An ESS can also provide homes with backup power when the utility goes down or is shut down for fire prevention. When my ESS senses no grid power, the automated transfer switch disconnects from PG&E and the ESS provides power to my home. When the utility power is restored, the transfer switch automatically reconnects to the grid and we return to normal operations. This ability to minimize the impact of the ever-increasing grid outages that we experience in northern California is the primary reason most of our customers add storage to their solar.

So now that we understand how a traditional ESS system works, let’s look at how it would differ with a VTG setup. The biggest problem is the battery’s dual use of both driving an EV (electric vehicle) and using it as the home battery back-up system. The battery cannot be in two places at the same time. Let’s look at a few examples:

  • Recently during the Glass Fire, the residents of Oakmont were ordered to evacuate their homes. For the battery in their EV to operate as the ESS, these homeowners would need to leave their EV behind. This requires that they did not need this vehicle to evacuate and are willing to leave it in harm’s way so the vehicle’s battery could provide backup to the home.
  • In the Kincade Fire, homeowners were not allowed back into the evacuation area for an extended period. If you were in your EV and not able to return to your home, there would be no back-up power to your home (leaving spoiled food in refrigerators and freezers).
  • Let’s assume you drove your car to San Francisco and back, leaving just a minimum remaining charge in the vehicle. If the power goes out that evening, you will have very little battery for your home use, and then be stuck with a drained car battery.

So, one very big difference is reliability – with a VTG there is no guarantee the vehicle will be present, have available charge or be rechargeable from solar, all significant draw backs.

And the biggest benefit often brought up in VTG applications versus installing a separate backup battery is the cost savings – let’s take a look at that.

Enphase Solar Battery Storage System are several components of an ESS installation that would also be required in a VTG application. You would still need to install the following components which can make up well over half the ESS system cost:

  • A transfer switch which disconnects the system from the utility in an outage and protects the grid, home electronics and utility lineman.
  • An inverter to convert the DC power of the car to the AC power of the home.
  • The electrical integration with either a connection to the entire home, or a protected load panel (PLP).
  • Possible replacement of the existing car charger to a bi-direction charger since most chargers only allow power to flow one way – from the grid to the EV.
  • The labor, permits, inspections and design costs.
  • Online monitoring or automation of this system so it could work even if you were not at home, but the EV was.

Other considerations include the following:

  • For a homeowner to receive the Residential Federal Solar Tax Credit on the VTG or ESS system, it must be charged from a solar system. Since most EVs are charge from the grid, they would not fall under the Residential Federal Solar Tax Credit.
  • One of the benefits of ESS is the ability to charge from solar when utility rates are low and then to allow a homeowner to discharge from the ESS system when utility rates are high. VTG systems would not easily be able to do this since they would be driven during the day, and most likely charged from the grid at night.
  • Would the EVs warranty be voided when it is used in a VTG application?
  • Also, many jurisdictions have more stringent rules for home storage systems, and in some cases will not allow them to be installed in garages, which could preclude the use of VTG in these communities.

It is my belief that using VTG to replace a ESS system is not viable today, but as the technology changes, I do see a place for VTG integrating to ESS for residential use. The integration could provide multiple benefits.

  • When the utility is down, the ESS system could control or limit charging to the EV so power from the ESS could first be used to run the home, and extra solar power (once battery are fully charged) could be used to charge the EV’s battery.
  • Adding the EV battery to the ESS would allow for a greater resiliency (more KWH of battery available) for low light days from rain, fog or heavy smoke.
  • Having portable energy storage – the ability to drive the car to another community, charge it and return home with more storage during long outages.

Summary:

some stand-alone VTG system do exist today which allow customers to plug in a refrigerator or charge a phone, they are very customized and include running extension cords to appliances.  Most jurisdictions have not approved these applications which does raise safety concerns. While VTG is an emerging technology, currently it is years, maybe decades out before it is a viable alternative to or component of home ESS. It is my belief that VTG will have limited application but will become a critical add-on option to home energy storage systems.