Return, an independent energy storage provider, and ENGIE, the global energy transition company, have signed a ten-year flexibility agreement covering a portfolio of three battery energy storage projects in the Basque Country, Spain.

The deal, structured as a full tolling agreement, gives ENGIE control over the optimization of the assets across Spain's wholesale and ancillary services markets, while Return retains ownership and operational responsibility. Combined, the three facilities will carry a capacity of 55 MW and 220 MWh, with commercial operations expected to begin by the end of 2027.

Structure and Commercial Terms

The agreement is built around a full tolling model, a structure in which the asset owner receives a long-term revenue stream while the counterparty gains access to the storage capacity to deploy as it sees fit across energy markets.

In this case, Return benefits from the revenue visibility required to support project financing and investment decisions, while ENGIE gains access to flexible storage capacity it can incorporate into its broader energy management operations and use to support what the company describes as its ambition to deliver 24/7 carbon-free energy to customers.

Construction of the three battery storage facilities is already underway. The projects are located in the Basque Country, a region in northern Spain, and are expected to be operational before the close of 2027. Once running, they will provide services including renewable integration support, grid balancing and flexibility across the Spanish electricity system.

The ten-year duration of the agreement is central to its function as an investment enabler. By locking in commercial terms over a long horizon, the deal provides the kind of revenue certainty that is typically required to attract financing for capital-intensive infrastructure projects like grid-scale battery storage.

Expanding a Partnership Already Established in Germany

The Spanish deal is not the first collaboration between Return and ENGIE. The two companies previously signed a flexibility agreement in Germany, announced in February 2026, covering 100 megawatts of battery storage capacity. The transaction in Spain marks the extension of that partnership into a second European market.

Both companies indicated that the Basque Country portfolio is intended to serve as a model that could be replicated across other European markets. The full tolling structure, they said, offers a scalable commercial framework that could be applied to future storage deployments as the two companies look to expand their joint footprint across the continent.

Spain's Renewable Integration Challenge

The agreement arrives at a moment of significant change for Spain's electricity system. The country has seen rapid growth in renewable generation capacity, creating an increasing need for storage and other flexibility resources that can absorb surplus power when wind and solar output is high and release it back to the grid when demand requires it.

At the same time, electricity demand in Spain is being shaped by several converging trends, including the electrification of heat and transport, industrial decarbonisation programmes and the expansion of energy-intensive facilities such as data centres.

Together, these forces are placing new and complex demands on the grid, making the availability of large-scale, dispatchable storage assets more important to system reliability.

The three projects covered by the ENGIE agreement are expected to contribute directly to addressing these pressures by helping to smooth out imbalances between renewable supply and consumer demand across the Spanish market.

What the Parties Said

Steve Sceery, Head of Return Spain, said the agreement reflects the company's confidence in the Spanish market and in the growing role of battery storage in the energy transition.

He described long-term contractual frameworks of this kind as providing the investment certainty needed to deploy storage at scale and to support the continued growth of renewable energy across the country.

Jean-Nicolas Lejeune, Managing Director of ENGIE's Supply and Energy Management activities in Iberia, said battery storage is becoming a cornerstone of modern power systems.

He pointed to the expansion of renewable generation as the key driver behind rising demand for flexibility solutions, arguing that such resources are essential to maintaining system reliability, optimising renewable integration and supporting customers as they navigate increasingly complex energy requirements.

Lejeune described the agreement as combining Return's storage expertise with ENGIE's energy management capabilities and said it reinforces ENGIE's ambition to develop innovative flexibility solutions that create value for energy systems and customers.

Return's Activity in Spain and Beyond

The Basque Country deal follows a broader period of activity for Return in Spain. In March 2026, the company announced the acquisition of a 318 MWh battery storage portfolio aimed at strengthening grid stability in the country. The new agreement with ENGIE adds a further 220 MWh of capacity under development, deepening Return's presence in the Spanish market.

Return's activity in Germany has also been progressing during the same period. In May 2026, the company held a groundbreaking ceremony at a battery storage site in Brietlingen, marking the formal start of construction at that facility.

The Spain agreement also follows a December 2025 framework deal between Return and engineering and consultancy firm Movares, described at the time as aimed at accelerating grid-enabling battery storage sites.

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