Using an electric vehicle as a home battery storage system

Interest in home batteries has increased dramatically recently. Many of our existing customers are choosing to swap their old solar inverter for a hybrid model, to which we can connect a set of batteries. This way, they're ready for the world after the net metering scheme is abolished.

Large storage capacity

But there are also people who wonder if it wouldn't be better to use their electric car as energy storage instead of home batteries. After all, electric cars often have very large batteries, and there's increasing talk of "bidirectional" charging stations, which can draw power from the car and feed it back into the home.

Regarding those large batteries: most cars certainly have batteries with a larger capacity than most home batteries. The home batteries Zonnefabriek installs are on average around 10 kWh, and in any case, we never install systems larger than 20 kWh in homes due to safety requirements (see this article for more information). Full electric vehicles, on the other hand, often have batteries with a capacity of over 60 kWh. Incidentally, plug-in hybrid cars have smaller batteries because they also contain a gasoline engine and fuel tank.

An electric car often has a battery with more capacity than a home battery (source: KIA)

V2H, V2L, or V2G?

Certainly not all cars are capable of transferring the power stored in their batteries back to the home. Only cars with the so-called "V2H" option can do this. V2H stands for "Vehicle-to-Home. You can also find options like "V2L" and "V2G." V2L stands for "Vehicle-to-Load". This means that the car has a built-in electrical outlet that can power one or more devices (a bit like the cigarette lighter of the past, but much more powerful). This is useful for camping trips, for example, so people can use the power from the car battery to power lights and tools. However, a car with only the V2L option cannot transfer power back to the grid.

V2G stands for "Vehicle-to-Grid". It's essentially the same as V2H, since the power is fed back into the public grid, either via a home (V2H) or directly to the grid (e.g. via a public charging station). You can check with the manufacturer or on a website like EV-database.org to see if your car supports V2H.

This Renault 5 E-Tech is suitable for Vehicle-to-Home discharging via AC according to the EV database

AC or DC?

Also note that not all cars discharge in the same way. Some only discharge using DC (Direct Current), while others use AC (Alternating Current). While almost all cars can charge via both AC and DC, discharging, if it's possible at all, usually only works in one these two ways. You can also find this information for various car models on EV-database.org.

Once you know whether your (future) car has the 'V2H' option and how it works (AC or DC), you'll of course also need to get a bidirectional charging station installed at your home. The charging stations currently on offer at Zonnefabriek are equipped for AC bidirectional charging; this means that with a new electric Renault or Kia, for example, you should be able to use the car battery for charging at home.

Wear and Losses

However, using a car as a home battery also has some disadvantages. First of all, there's the added wear and tear on the battery: a battery used solely for driving will last longer than one that also stores solar power. That's why some car manufacturers restrict the discharge function after a certain throughput energy has been reached.

Furthermore, there are losses: when converting power from the panels to the grid (DC to AC), then to the car, and then within the car to the car battery (AC to DC), some energy is lost at each step. We also lose a small amount when feeding power back from the car battery to the grid. All in all, these losses can add up considerably.

And finally, there's a practical point: the car isn't always parked at the charging station when there's surplus solar power. After all, it's there to drive around in. And in the evening, when grid power is at its most expensive, someone might just be using it again.

The SMA eCharger is in principle suitable for bidirectional AC charging and discharging

Better get a home battery instead of an electric car with a bidirectional charging station?

The biggest obstacle to using a car as a home battery, however, is for now the absence of clear regulations. Currently, it's completely unclear what the technical and regulatory requirements will be. That's why the SMA eCharger, for example, is "bi-di ready", as they say, but not yet suitable for discharging in practice.

At SMA, they expect that bi-directional charging won't be truly feasible before 2027.

A home battery system on the other hand can be installed today, is efficient, and—not unimportantly—can also provide backup power. Therefore, we believe it's worthwhile for most people, including those with an electric car, to purchase a home battery now. A bi-directional charging station can then be considered a possible valuable addition in the future. This way, you keep your options open and continue to build on optimal use of your own solar power, and on better protection against rising energy costs.