The Problem

Electric vehicles (EVs) powered by renewable energies could significantly contribute to a lower carbon footprint of the transportation sector. They would allow sustainable mobility with cars in a Mobility-as-a-Service setup. One of the main reasons why electric vehicles did not get traction yet is the lack of sufficient charging infrastructure.

The best time to charge an electric vehicle is at night or when the driver is at work. In both cases the car is often parked in a shared underground car park or parking garage where there are usually not sufficient charging points.

“So what’s the problem here? Just add a thicker cable and some charging points!”

© photoschmidt, 123RF

Unfortunately it can be a bit tricky with the “thicker cable”. Even if the power company would be able to provide more capacity, adding additional cables and electricity meters to an existing garage would usually require official permits, owner approvals, and cause major construction efforts and costs. Most often it will be a no-go from a profitability point of view.

In most European garages only one 11 kW three-phase AC cable is available and already partially used for existing power consumers. So a maximum of 11 kW is available for a car park with often up to 20 cars. If you consider that a Tesla S would need about 10 hours to fully charge in this setup, you can charge how many cars in this garage overnight? Right: only one single car. The next morning you will have one happy Tesla driver and 19 grumpy neighbors 🙁

Even if you also consider smaller cars, charging only up to 80% of the battery capacity and using load management, you cannot support much more than three cars for overnight charging with just one 11 kW cable – unless you “hack” the existing grid.

The Garage Hack

Your garage has one charging point directly connected to the grid? Then you have a standard setup which limits your garage to a small number of electric vehicles. A load management could help to distribute it better but you would still be limited to the power available in the one cable.

© Creative Commons icons: batteries: Adrien Coquet, cars: Viktor Ostrovsky, power line: Jakub Ukrop,

Add Batteries to the Micro Grid

What if your garage could store electricity when it is not needed and have it available when all cars want to get charged at once? With batteries such as the Tesla Powerwall, sonnenBatterie or the E3/DC power storage systems of this size are already available off the shelf, even their power management is not built for this use case. Such batteries could also supplement the grid capacity. If the batteries would be able to charge the cars with direct current (DC), loading times and conversion losses might even be further reduced.

Like every battery also car batteries lose performance over time. But even if they are not good enough for cars anymore, they might still be good enough for a second life as a garage battery. This would improve the life cycle of a battery significantly.

Scale the Grid With the Number of Cars

If you would add one of these batteries in front of each electric vehicle jointly with a charging point, you would have enough energy to charge each vehicle sufficiently for at least a reasonable range.

Let Charging Points Support Each Other

If you would also add a smart load management and settlement to the entire system, these batteries could be orchestrated to support other charging points independently from the external grid. It could also bridge power outages.

In such a micro grid each battery could work as a power supplier. The owners of the batteries could charge the recipient of the electricity provided via automated settlement e.g. using Blockchain technology. At the end of a month all these micro transactions would be settled.

Such a “Power Sharing” approach is also interesting from a regulation and legal point of view in the highly regulated power industry. The requirements to “sell electricity” to the grid are unbearable for small “one battery suppliers”. But if power is “shared” within a closed community, other regulations might apply. And if hundreds of these batteries would be combined into one provider, the effort to sell it to the grid could be shared, more advanced control technology applied and power fluctuations compensated. The sonnenCommunity practices such an approach in a supra-regional grid with hundreds of batteries scattered across many households.

Stabilize the Grid and Save Money

With the ability to store electricity the batteries could get charged when there is an oversupply in the grid and when power is cheapest. This could reduce charging costs and help to stabilize the power grid at once. It follows and extends the Vehicle-to-grid (V2G) concept, where car or bus batteries become part of the grid and can be loaded using smart meter technology. Using this Sector Coupling or Integrated Energy as a specific Power-to-X concept is key for electric vehicles to become “green” mobility. Only if EVs are fueled by renewable energies, their carbon footprint will be lower than these of combustion engine-powered vehicle.

When batteries can take oversupply from the grid, it can help to avoid wind power plants to be shut down and so increase their utilization. The energy doesn’t get wasted. Other than vehicle batteries, garage batteries will always be connected to the grid and so become a reliable asset in the grid.

Stabilize the Grid Further and Earn Money

More complicated from a technical and legal point of view would be an additional, bidirectional option to provide peak capacity to the grid and so help to further stabilize it – and earn money for selling peak power at once. With the Hornsdale Power Reserve in Australia Tesla has proven that battery systems can bridge power outages of major power plants. A large number of well-orchestrated “garage power storages” would have the same capacity at no extra costs.

In times where water, wind and solar power plants cannot produce enough electricity, the batteries can help to fill the gap, which otherwise fossil-powered power plants would need to fill.

Power grid operators might be able to make use of such combined “swarm battery systems” in their dispatch and re-dispatch planning of power supply for the upcoming days. With the help of machine learning, the grid operator could learn when the system could demand electricity from the grid or provide electricity to the grid.


Adding battery buffer storage to the loading infrastructure of car parks in existing garages could help to maximize the utilization of the existing power cables and so allow to charge more electric vehicles without the need of major construction works. This is vital for the acceptance of electric vehicles in cities.

Combined batteries could also deal as micro grids within the garage or help to stabilize the power grid by demanding or providing electricity when the grid requires it. This would allow more renewable energies in the power mix.

Of course this concept is oversimplified and experts will see many good and valid reasons why it won’t work – yet. But this is why we need experts. To make it happen and tap this potential. And I will ask them for their expertise and creativity as I’m about to implement a pilot of such a system in a parking garage I co-own. I will keep you posted.

Garage-to-Grid: EV Charging in Parking Garages

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