France has taken a major step in expanding its energy storage capacity with the activation of a 100 MW/200 MWh battery energy storage system (BESS) at the port of Nantes-Saint-Nazaire.
Developed by UK-based Harmony Energy, the project represents the largest BESS currently operational in the country, underscoring the accelerating role of large-scale storage in stabilizing grids with growing renewable penetration.
The installation connects directly to France’s high-voltage transmission network, Réseau de Transport d’Électricité (RTE), at 63 kV. This voltage level positions the system to deliver a range of balancing services, from frequency regulation to peak shaving, at a scale that supports both regional demand and national system stability. With France targeting a significant increase in renewables under its Programmation pluriannuelle de l’énergie (PPE), grid flexibility has emerged as a central constraint, making storage assets such as this critical.
The facility utilizes Tesla’s Megapack technology, paired with the company’s Autobidder platform. The combination allows automated optimization of charging and discharging cycles based on market signals, a capability increasingly vital in balancing intermittent solar and wind resources. By doubling its storage capacity relative to output—200 MWh for 100 MW—the plant can sustain full discharge for two hours, aligning with peak demand windows on the French grid.
Harmony Energy’s decision to site the project at Nantes-Saint-Nazaire reflects the port’s importance as a hub for offshore wind development. France’s first commercial-scale offshore wind farm was commissioned near the same port in 2022, and additional projects are in planning. Co-locating large-scale storage near generation assets reduces transmission bottlenecks while ensuring a more resilient supply for industrial and residential consumers.
The project’s scale places it well ahead of France’s current battery storage fleet, which has been relatively modest compared to European peers. According to RTE data, France had less than 1 GWh of operational grid-scale storage capacity as of early 2025, a fraction of what is being deployed in the UK and Germany. The Nantes-Saint-Nazaire BESS alone boosts that capacity by more than 20%, indicating how individual large projects can shift national totals in markets still scaling up.
Full commercial operation is expected by the end of 2025. When complete, the facility will provide not only balancing services but also contribute to congestion management and ancillary reserves, functions that are increasingly monetized within Europe’s evolving electricity markets.
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