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

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.

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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?”

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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.

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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.

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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.

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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.

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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