Our house has 8 kilowatts of solar generation and 27 kilowatt-hours of battery storage, and is on a time-of-day electricity plan where power is cheaper in the early part of the day and more expensive from 3pm until midnight. We use 70% of the battery capacity to power the house during the peak hours each day, reserving 30% in case of outage.
This does work, but one battery is filling two roles with requirements at least somewhat in conflict.
- For load shifting we would prefer to use more of the battery to reduce our energy bill.
- If there is a power outage, we'd prefer to have more of the battery held in reserve than 30%.
Yet the batteries wired in to the house are not the only substantial energy storage on site. We also have an electric vehicle, with as much battery capacity as the house. A more modern EV than ours would have even more battery capacity.
Vehicle to Grid
That the substantial battery power in EVs could be useful in grid emergencies has long been recognized. Vehicle-to-Grid (V2G) is an idea to make the charging port of the EV bidirectional: charging the EV from the grid as normal, but also able to supply to the grid by draining the EV. In very large numbers, EVs could provide enough power to stabilize grid operation while still having enough power to be used for transportation.
V2G has been slow to catch on, owing mainly to the number of parties involved. Because it entails connecting a new generation source to the grid, it must follow similar processes and permits as a rooftop solar installation. The timeline of a solar install is long: it can be months with roofers / installers / electricians in several waves, and having time enough to apply for the needed permits and interconnection agreement.
Yet when people buy an EV they need a way to charge it almost immediately. Waiting to install an EV charger isn't realistic, so it will be installed to only draw from the grid not supply back to the grid. Designing V2G capabilities into an EV charger in hopes the homeowner will followup with additional permitting? Most consumers wouldn't ever do so.
Vehicle to Grid Vehicle to LOAD
Vehicle-to-Load (V2L) is an idea with fewer stakeholders involved: allow the vehicle to supply power to locally connected loads, most commonly by having electrical outlets and an inverter built into the car. This is useful in many situations, like an EV at a job site powering tools or while camping or picnicking.
V2L also allows the EV's stored power to be used at home during emergencies, albeit somewhat awkwardly. It doesn't power the whole house, it can power appliances which are unplugged from the house and plugged into an extension cord from the vehicle. Keeping cell phones charged, running medical equipment, or powering freezers full of stored food is quite feasible. Running central air conditioning is not.
This concept works well for emergencies in that it can power essential needs from a very large battery, especially because the battery is mobile and could go somewhere to charge itself if needed. Natural disasters in the last several years have demonstrated this, notably Hurricane Helene in the US Southeast just last week. EVs are helping supply power in damaged areas.
California, my home state, has a large enough market for automobiles that it has often been able to influence the auto industry throughout the US such as via fuel efficiency standards. A law being considered would allow the state to mandate Vehicle-to-Load capabilties for vehicles sold as of some future date.
Transfer switch
We'd very much want our next EV to have this capability. Our use of the batteries built into the home would change to entirely load shifting to further reduce our demand on the grid during peak hours. During a power outage of appreciable length, we'd rely on the much larger batteries in the vehicle to keep food frozen and cell phones charged.
For this to work we'd need to plug appliances into an extension cord from the vehicle, and some of them are hard-wired into the house. The traditional way to handle this is a transfer switch, intended for a generator. This is expensive, and difficult to retrofit. There needs to be an easier way.