Recap: The Setup

In 2012, we constructed our home with a strong emphasis on energy efficiency. From day one, our residence has utilized a Drexel+Weiss Aerosmart X2 heating and ventilation system paired with a ground heat pump, which has kept our heating energy use remarkably low (around 1.2 kW on the coldest days).

Over the years, we gradually incorporated the following systems:

• 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 vehicles, each with a 75 kWh battery

A Loxone building control system manages the major power consumers (vehicles, washing machine, heating), optimizing based on solar energy availability, battery charge levels, and seasonal changes.

Recap: Self-Sufficiency Throughout the Year

Since 2017, what levels of self-sufficiency have we achieved with our installations? First, let's examine our yearly consumption data:

Our average self-sufficiency has consistently exceeded 70% annually—even with two electric cars.

Focusing on monthly data (using 2020 as a reference), we maintained nearly 100% self-sufficiency from mid-February to mid-October. However, the latter half of February and the first half of October showed significant weather-related variations.

During the mid-October to mid-February period, self-sufficiency typically drops to 20–30%, largely influenced by weather conditions.

Problem: Winter Charging Challenges

Charging vehicles during winter becomes challenging if relying on solar energy. The self-sufficiency yield chart supports my realization that solar power is limited in Central European winters.

Previously, my building control system operated under this high-level charging logic:

• “Summer Mode”: From February 15 to October 31, charge any connected vehicle during periods when solar energy production exceeds 3.5 kW.
• “Winter Mode”: From November 1 to February 14, charge all connected vehicles after 8 PM when electricity is cheaper, utilizing a maximum power of 12 kW.

Examples of summer mode charging patterns illustrate its efficacy:

These patterns demonstrate that we can operate independently from the grid during summer.

In contrast, here are some typical winter mode charging patterns:

The winter charging logic was inefficient, leading to significant depletion of our house battery from 8 PM to 9 PM, resulting in minimal self-sufficiency overnight. This pattern, compounded by the losses during battery charging and discharging, was not optimal for winter self-sufficiency.

It took five years to pinpoint the flaws in this winter charging approach, with a crucial nudge from Geopolitics.

Now, discussions about natural gas shortages and cold winters are prevalent across Central Europe. Reports highlight the unreliability of solar and wind energy, which, combined with significant discrepancies between daytime and nighttime energy production, can destabilize electricity grids and potentially lead to power outages.

Thus, why charge vehicles at night? Considering our consumption patterns, vehicles represent the largest energy draw in our household. Even in winter, renewable energy is more accessible during the day than at night, making daytime charging a more viable option—despite the potential for slightly higher electricity costs.

New High-Level Charging Logic

Here’s the revised charging logic for my building control system:

• “Summer Mode (unchanged)”: From February 15 to October 31, charge any connected vehicle during periods of at least 3.5 kW solar excess production.
• “Winter Mode (adapted)”: From November 1 to February 14, charge all connected vehicles from 10 AM to 3 PM, when solar output is highest, utilizing a maximum charging power of 12 kW.

This adaptation preserves the benefits of summer mode while significantly enhancing winter mode:

• On a global scale, vehicle charging must occur during sunlight hours, promoting grid stability and preventing power shortages.
• Locally, avoiding battery cycling mitigates the inefficiencies caused by charging/discharging, increasing overnight self-sufficiency.

Here are some adapted winter mode charging patterns:

While still not achieving complete self-sufficiency, the refined winter mode marks significant progress. It allows our house battery to sustain us during the night rather than being depleted by vehicle charging.

Future Improvements

• As an early adopter of electric mobility, my Keba Wallbox charging stations limit charging to three phases, resulting in a minimum of 3.5 kW. Newer options, such as the Fronius Wattpilot, can alternate between one and three phases, lowering the minimum charging power to 1.4 kW. This would enable the same summer charging strategy to be employed in winter—charging vehicles exclusively with excess solar energy.
• To facilitate daytime charging for all electric vehicles without relying on nighttime electricity, we need to develop large-scale solar charging facilities at workplaces. Such initiatives must be prioritized at the political level in response to the current geopolitical shifts in the energy sector.

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