Madrid, Spain 20 July 2026, Return, an independent energy storage provider, and ENGIE, a global player in the energy transition, have signed a long-term flexibility agreement for a battery energy storage portfolio. Structured as a full tolling model, the partnership marks an important milestone for Spain's growing flexibility market and supports the deployment of battery storage needed to integrate increasing volumes of renewable electricity into the power system.
It covers three battery storage projects located in the Basque Country, Spain, with a combined capacity of 55 MW / 220 MWh. Return will develop, own and operate the assets, while ENGIE will optimize the portfolio across Spain's wholesale and ancillary services markets. It runs for ten years, with commercial operations expected to begin by the end of 2027.
Beyond the transaction itself, the agreement demonstrates how long-term commercial frameworks can help unlock investment in battery storage by providing revenue certainty for asset owners while creating access to flexibility for energy market participants.
Building on an existing partnership in Germany, the transaction expands the collaboration between Return and ENGIE into a second European market and demonstrates how innovative long-term contracting models can help accelerate battery storage deployment across Europe.
Supporting Spain's transition to a more flexible energy system
Spain is experiencing rapid growth in renewable generation, creating a growing need for flexibility solutions capable of storing electricity when renewable production is abundant and delivering it back to the grid when it is needed most. At the same time, electricity demand is evolving, driven by electrification, industrial decarbonisation and the rise of energy-intensive activities such as data centres. In this context, large-scale battery storage will be essential to maintain a reliable, affordable and sustainable power system.
The battery assets covered by this agreement will help absorb surplus renewable generation, support grid balancing needs and contribute to a more resilient and efficient electricity system. Beyond the assets themselves, the agreement establishes a long-term commercial framework that supports investment in battery storage and provides a scalable model for future storage deployment. Through the full tolling structure, Return benefits from long-term revenue visibility that supports project financing and investment, while ENGIE gains access to flexible storage capacity that can be integrated into its energy management activities and support the Group's ambition to deliver 24/7 carbon-free energy solutions to its customers.
Steve Sceery, Head of Return Spain, said: "This agreement reflects our confidence in the Spanish market and in the growing role that battery storage will play in the energy transition. Long-term contractual frameworks like this provide the investment certainty needed to deploy storage at scale and support the continued growth of renewable energy across the country."
Jean-Nicolas Lejeune, Managing Director of ENGIE’s Supply and Energy Management acitivities in Iberia, added: "Battery storage is becoming a cornerstone of modern power systems. As renewable generation continues to expand, flexibility solutions are essential to maintain system reliability, optimize renewable integration and support customers in managing increasingly complex energy needs. This agreement combines Return's storage expertise with ENGIE's leading energy management capabilities and reinforces our ambition to develop innovative flexibility solutions that create value for energy systems, customers and accelerate the energy transition."
Looking ahead
Construction of the three battery storage facilities is already underway. Once operational, they will support renewable integration, provide flexibility services to the electricity system and contribute to balancing supply and demand across the Spanish market.
As battery storage becomes an increasingly critical component of power systems, Return and ENGIE see significant potential to replicate similar partnership models across Europe, combining long-term investment certainty, flexibility and energy management capabilities to support a reliable, affordable and decarbonized energy future.
