Construction – Evergreen

From Installed Capacity to Available Flexibility: What 636 Days of V2G Operation Taught Us

Introduction System Evergreen AG has published EVERGREEN® Technical Note No. 2, From Installed Capacity to Available Flexibility: A Practical Comparison of Smart Charging, V2H, V2G and Dedicated Battery Energy Storage in Zero-Energy Buildings. The paper compares the main flexibility options available to a highly electrified residential building and tests its assumptions against 636 days of measured operation of an electric vehicle connected to a bidirectional (V2G) charger in one of our condominiums, as part of the Swiss-European GLOCALFLEX project.

The Question Behind the Paper

Electric vehicles carry large batteries, so it is tempting to treat every parked car as extra storage for the building. But a car battery is first of all a mobility asset. Whether it can help the building depends on whether the car is parked, how much charge it has when it comes home, how much the driver needs for the next trip, and whether the owner is fairly compensated. The paper therefore asks a more practical question:

Not “Can an EV battery provide flexibility?”
But “When does using an EV battery add value to a building that already has its own stationary battery?”

The Valleverde Pilot Site

The measurements come from the Valleverde pilot site, a condominium of 12 flats whose electricity demand also covers heating and cooling. The site includes:

A 27 kWp photovoltaic array.
A 44 kWh building battery.
A 10 kW bidirectional V2G charger.
A Nissan Leaf with a 62 kWh battery, monitored from 1 January 2025 to 28 September 2026.

Over the period the car travelled 34,738 km. Its presence, state of charge, charging energy and the energy it returned to the building were recorded continuously.

A Clear Hierarchy for Building Flexibility

The paper proposes an operating order for zero-energy buildings with stationary storage:

1. Use solar energy directly in the building.
2. Store the surplus in the building battery.
3. Charge electric vehicles smartly, choosing when they charge.
4. Use V2H/V2G discharge only as a supplementary, contracted resource.

In this view, the EV is first a flexible heavy user of the building’s energy system and only secondarily a battery for it.

Key Results

Charging flexibility is abundant

On 99.8% of the days the car was at home, it stayed plugged in longer than it needed to recover its driving energy.
The median margin was 19.1 hours, and on 450 days it exceeded 12 hours.
Presence was highest overnight: the car was at the building on roughly three nights in four.

In practice, charging almost never has to start the moment the car arrives. It can wait for solar surplus, cheaper hours or energy stored in the building battery, without discharging the car at all.

Discharge flexibility is limited

The car usually came home with 40–50% state of charge, leaving little energy above a reasonable mobility reserve.
Only 60 days in 2025 met the paper’s reference V2G eligibility rule (evening presence and at least 50% charge on arrival).
On those days, about 16.5 kWh was available on average, less than half of what fleet models typically assume.
The car returned 2.04 MWh to the building in 2025 and 4.02 MWh over the whole period, mostly between April and September.

The solar surplus is real, but its timing is the problem

From April to September 2025, the building battery was full while solar energy was still being exported on most days.
At the same time, 44% of the car’s charging energy still came from the grid, largely because the car is often away at midday and charges after sunset.

What It Means Economically

Using the paper’s reference prices and the measured utilisation:

V2G yields a gross operating return on its incremental investment of about 2% (about 4% under the more optimistic fleet assumption).
A dedicated building battery reaches 15–24% under the same price assumptions.
With the illustrative financing structure, V2G’s return on equity stays negative, while the dedicated battery remains positive in every scenario.

This is not a general verdict on V2G. It shows that, in this configuration and for this vehicle, routine discharge does not justify the extra infrastructure compared with smart charging and dedicated storage.

Next Step: The Figino Pilot

The results point to a different next experiment. Instead of discharging cars more often, the building battery can act as a buffer between daytime solar production and night-time EV charging:

Store the solar surplus in the building battery during the day.
Reserve what the flats need for the night, using a day-ahead solar and load forecast.
Charge the parked car overnight with the remaining energy, at a local tariff between the export price and the retail price.
Free the building battery for the next solar day.

System Evergreen AG proposes to test this forecast-constrained battery-to-EV charging strategy at the Figino pilot site, measuring how much export it avoids, what it saves the EV owner, and whether the building’s own supply is always protected. Read the Full Paper The complete Technical Note, with methods, figures and the full economic comparison, is available here: EVERGREEN® Technical Note No. 2 (PDF) Learn more about the project at www.glocalflex.eu

Giuseppe Macario

CEO & Senior Executive Project Director at System Evergreen AG

Miro Rava

Data Science, AI and Energy Engineer at System Evergreen AG

Key Considerations in Designing PV Systems: Fire Safety, Maintenance, Wind Resistance

Introduction

Understanding the importance of integrating photovoltaic (PV) systems into building roofs to harness solar energy as a renewable source in the near term and before the pivotal year of 2050, Evergreen is committed to ensuring the safety and maintainability of every installation. It is crucial not to overlook the technical aspects of PV systems, including fire safety, structural integrity, and mechanical support and anchoring of the photovoltaic surfaces.

Ensuring Adequate Power and Energy Sharing

When designing a PV system, it is essential to install a capacity that matches the building’s real energy consumption patterns throughout the year. This approach ensures optimal utilization of the generated energy. Energy storage solutions, while useful, can be costly and often have limited durability. Therefore, designing a system that aligns closely with actual energy needs can mitigate these drawbacks. Additionally, wherever possible, sharing energy production with adjacent users through energy self-consumption communities can enhance efficiency and reduce costs.

Evaluating Roofing Materials and Fire Risks

A predictive evaluation of the roofing materials is necessary to understand the potential risks and impacts associated with the PV system, particularly in the event of a short circuit or fire. This evaluation should include an assessment of how the PV system interacts with the existing roof materials and the potential consequences of any electrical malfunctions. Ensuring that the PV installation does not compromise the fire safety of the building is paramount.

Considering the Permanent Load

Photovoltaic surfaces add a permanent load to the building’s roof. While this load typically results in minimal additional stress on the underlying structure, it is crucial to account for it during the design phase. The structural integrity of the roof must be evaluated to ensure it can support the added weight without compromising its stability or performance. This consideration is vital to prevent structural failures that could result from overloading.

Assessing Wind Effects

The effects of wind on the PV system must be thoroughly evaluated to prevent potential failures that could cause material damage to third parties, especially given the increasingly extreme weather events. Wind can exert significant forces on the PV panels and their mounting structures, so it is essential to ensure that the system can withstand these forces. This assessment includes understanding the local wind patterns and designing the anchoring and support structures accordingly.

Hail Resistance and Panel Durability

The hail resistance classification of the photovoltaic panels is another critical factor to consider. The mechanical properties of the glass that protects the photovoltaic cells play a crucial role in the panel’s durability. Ensuring that the panels can withstand hail and other environmental impacts without significant damage is essential for maintaining the system’s longevity and performance.

Ensuring Stability with Appropriate Mounting Techniques

Ensuring the stability of PV systems typically involves two primary approaches: the use of ballasts and direct attachment to the roof support structure. Ballast systems add weight to the PV installation to secure it in place, while direct attachment involves mechanically fastening the PV system to the existing roof structure. Both methods require a thorough evaluation of the load-bearing capacity of the existing structure and the potential local consequences, such as punching or deformation of insulating layers, which could compromise the roof’s waterproofing.

Facilitating Maintenance and Ensuring Accessibility

Facilitating maintenance is vital to reduce the costs associated with periodic cleaning and inspection of the PV system. During the design phase, it is crucial to ensure that the PV system is designed with easy access points and safe spaces for cleaning and maintenance activities. This includes compliance with current regulations, which often mandate the provision of guardrails and safety lines. Ensuring that maintenance can be performed safely and efficiently helps to maintain the system’s performance and prolong its lifespan.

Conclusion

At Evergreen, we are committed to providing reliable and safe technical solutions for integrating photovoltaic systems into building roofs, thereby contributing to the transition towards a sustainable energy future. By addressing these key considerations, including energy matching, material evaluation, load assessment, wind and hail resistance, and maintenance accessibility, we ensure that our PV installations are not only efficient but also safe and durable. These efforts align with our mission to advance sustainable energy practices, ensuring that our PV systems support a low-carbon future while maintaining the highest standards of safety and reliability.

Miro Rava

Data Scientist and Certified EU Drone Operator at System Evergreen AG