The car under the target: why the electric car raises the winter wall

Image above: AI impression, not a construction drawing.

Everything I wrote about self-sufficiency until September was about houses. Electricity, heat, water. The car stood outside the balance, and in the Netherlands that is a large absence: a petrol car uses more energy per household than the house itself.

So I built it in. Not an average charging profile, but charging sessions per car: when does it come home, how far has it driven, when does it plug in, with a fixed random seed so that every run repeats. Seven cars for eight homes, Breda's car ownership, 12,300 kilometres a year each. Two of my three hypotheses fell within an hour.

Hypothesis one: the car breaks the grid. Seven 11 kW chargers make 77 kW on a 55 kW connection, so that looked like arithmetic. The model says no. Zero hours above the connection limit, across three weather years. The highest simultaneity is five to six cars, the charging peak 36 to 38 kW, and the import peak of the whole neighbourhood goes from 10 to 40 kW.

The car does not break the cable. It eats the margin. From 45 kW of headroom to 15. Grid-aware charging, 4 kW per charge point between five and eleven in the evening, brings that peak to 30 kW. Whoever sizes on an average profile does not see the margin vanish; whoever sizes on the worst case buys a connection they do not need.

Hypothesis two: the car deepens the winter deficit by ten to fifteen percent. Measured: 52 percent. From 4.2 to 6.4 MWh. The cars charge 13.8 MWh a year at home and nearly a third of that falls in December to February, the months without sun. Exactly the gap that seasonal storage has to bridge.

In euros, robust across three weather years and at 100 euros per kWh of storage: the 90% neighbourhood without cars costs 8,090 euros per home per year. With cars, 14,116. The seasonal store grows from 8.1 to 14.1 MWh. Under the 90% requirement the car's marginal kilowatt-hour costs over three euros, against thirty cents from the grid.

Then I asked the question differently. Why should the car fall under the promise made for the house? House and heat winter-proof, and the car simply charges from the grid in January, only in the hours it is charging. That is the same logic by which I already leave the last ten percent of the house to the grid.

Result: 8,473 euros per home per year. The installation is identical to that of the neighbourhood without cars. The car only costs its grid electricity, 383 euros. Under the requirement it cost 6,026 euros. Sixteen times as much, for the same kilometres.

My first version of that sum was a leak, by the way. I wrote the requirement as an annual total and got 751 euros per home, with no seasonal storage. Too good. Electrons carry no labels: the model fed the house in January from the car's summer charging allowance. The requirement had to be hourly. Found within an hour, thrown out, and it is in the register.

Then the ladder. The target per demand item saves forty percent. Demand design, shared cars and less commuting, saves another quarter on top and brings the charging peak from 36 to 11 kW and the parking land from 22.5 to 1 square metre per household. Smart charging, letting the car choose when it charges, saves four percent. Sizing and target definition carry the outcome. Control is the last few percent. Seasonal storage had taught me that already in July.

The weakest points, for anyone who wants to attack this:

• I calibrated the charging session model on ElaadNL's weekday profile: the distance went from 7.2 to 1.8 percentage points per hour. What would not fit away: ElaadNL measures a quarter of charging after midnight, I get 16%. Their 2019 fleet drove 18,000 kilometres and charged 18 kWh per session; my Breda car 11. Short sessions end earlier. For the same reason grid-aware charging cuts my evening peak by 14% and ElaadNL's by 44%. That number belongs to a mileage, not to a technique.
• The engine of demand design is the KiM rule: fewer kilometres means fewer cars, not less driving per car. If people keep their car and drive less, the peak gain evaporates. That is behaviour, not physics, and I do not know.
• The shared car charges in the model like a private car, at home at night. A shared car doing 144 kilometres a day does not only do that.
• No vehicle-to-grid, and the storage price of 100 euros per kWh once again carries the euros, not the direction.

This article builds on seasonal storage breaks the winter wall. The cost curve without cars is on the explorer; the curve with cars shifts far to the right and is not on it yet.