Don’t charge your cars at night just because you can save a few cents on electricity.

As described in a previous article, solar self-sufficiency in a modern single-family home is feasible, even with electric cars in moderate climates such as Switzerland.

This article illuminates my recent changes to the charging patterns of my cars during the last winter season. With the energy crisis in full swing, it is intended to give as many home and electric car owners thought and time to prepare for the next winter.

Recap: The Setup

We built our home on the green field back in 2012, paying close attention to energy efficiency. The house has a Drexel+Weiss Aerosmart X2 combined heating/ventilation system with a ground heat pump since the first day, which has helped us keep energy consumption for heating very low (when running on full power on a cold winter day, the consumption of the heating system is around 1.2 kW).

After living in the house for some years and learning about our energy consumption, we added the following systems step-by-step:

• 8 kWp east-west solar panels on the carport
• 14 kWp east-west solar panels on the house
• 12 kWh Fronius Solar Battery
• 2 Tesla Model 3 with a 75 kWh battery each

A Loxone building control system is controlling the large power consumers (cars, washing machine, heating), taking into account solar power availability, charging levels of the batteries, and the seasons.

Recap: Self-Sufficiency over the Year

So, what self-sufficiency levels have we reached since 2017 with the installation mentioned above? First of all, let’s look at the annual consumption values:

Average self-sufficiency levels over the years have been above 70% for the whole year — this is with two electric cars!

Drilling down to the months (I am using 2020 as an example), self-sufficiency is pretty much 100% between mid-February and mid-October. We have seen quite some variations in the second half of February and the first half of October — the weather in these periods can be anything from foggy all day to bright sunshine all day.

The period of mid-October to mid-February typically yields 20–30% self-sufficiency only — again, self-sufficiency levels vary greatly based on weather and temperature.

Problem: Charging the Cars in Winter

Charging cars in winter poses a problem if you want to use your own solar energy to do so. The above self-sufficiency yield chart confirms my acceptance that cars can’t be charged on solar power during the Central European winter months.

Here is the pre-war high-level charging logic of my building control system:

• “Summer Mode”: Between 15th February and 31st October, charge whichever car is connected during times when there is at least 3.5kW solar power excess production.
• “Winter Mode”: Between 1st November and 14th February, charge all connected cars after 8 pm, when power is cheap, using the maximum charging power of 12kW.

Here are some typical summer mode charging patterns:

Beautiful, both on crisp and variable days. We don’t need the grid in summer mode, neither to operate the house nor to charge the cars.

However, here are some typical winter mode charging patterns:

Ouch. My logic sucks empty the house battery between 8 pm and 9 pm, which leaves me basically with no self-sufficiency throughout most of the nights. Add the loss from charging and discharging the house battery, and you’ll understand that this pattern is not the best available to achieve the highest self-sufficiency during the winter months.

It took me roughly 5 years to identify the shortcomings of this winter charging pattern, and it took an external factor: Geopolitics.

Now suddenly everyone in Central Europe is talking about natural gas shortages, cold winters, and power shortages. Depending on the political color of the reports, solar and wind power play a significant role in creating power shortages: They are not reliably available and there are large yield differences during daytime and nighttime. Such yield differences render electricity grids unstable, and under certain conditions, large-scale power outages might occur.

So, therefore, why would you want to charge cars at night? If you look at the above consumption patterns, cars are by far the largest power consumers in our household. Even in winter, renewable power is more readily available during the daytime than at night, so I suggest you charge your cars during the daytime. Even if this means spending a few cents more on electricity.

So here is the new high-level charging logic of my building control system:

• “Summer Mode (unchanged)”: Between 15th February and 31st October, charge whichever car is connected during times when there is at least 3.5kW solar power excess production.
• “Winter Mode (adapted)”: Between 1st November and 14th February, charge all connected cars between 10 am and 3 pm, when solar power is strongest, using the maximum charging power of 12kW.

Whilst keeping the advantages of the summer mode alive, I improved the winter mode significantly, both on a global and a local level:

• On a global level, charging electric cars needs to occur during hours of sunshine, irrespective of the season or the weather. This helps avoid power shortages and increases the stability of electric grids.
• On a local level, avoiding the cycling of the house battery eliminates deficiencies from charging/discharging, and increases the overnight self-sufficiency.

Here are some adapted winter mode charging patterns:

Whilst still far away from full self-sufficiency, the adapted winter mode is a large step in the right direction. In terms of resilience, it allows the house battery to power us through the dark hours as opposed to being sucked empty by charging the cars.

Future Improvements

• Since I am an electric mobility pioneer since 2017, I don’t have the latest charging stations installed. My Keba Wallbox charging stations only allow cars to be charged on 3 phases, therefore the minimum charging power is 3.5 kW. More modern products such as the Fronius Wattpilot can switch between charging on 1 and 3 phases, thus reducing the minimum charging power to 1.4 kW. This would help using the same winter charging pattern as in summer — only charging the cars with excess solar power.
• If we want to charge all those electric cars without using lots of electricity during the nighttime hours, we need to be able to charge them during the daytime. Since not everybody has a mobility pattern to charge their cars at home during the daytime, large-scale solar car charging facilities at workplaces are needed. Such an initiative needs to be pushed on the political level — and it should be tackled soon in response to the geopolitical shifts that are currently happening in the energy sector.