Return Energy, a European independent provider of large-scale energy storage infrastructure, and Vattenfall (European energy company) have signed a long-term fixed-price tolling agreement for Sirius, a 200 MW / 800 MWh battery storage facility under development in Winschoten, Groningen, marking what is expected to be the largest fixed-price battery storage tolling agreement in the Netherlands to date.

The project has simultaneously reached financial close, with up to USD 464 million in portfolio financing secured from a group of five banks.

Project Overview and Commercial Structure

The Sirius facility will connect to TenneT's high-voltage substation in Meeden and is expected to begin commercial operations in the fourth quarter of 2027. The total project investment stands at approximately USD 208.8 million, making Sirius one of the largest battery storage projects currently under development in the Netherlands.

Under the terms of the tolling agreement, Vattenfall secures access to the facility's charging, storage, and power delivery capacity in exchange for a fixed fee. Vattenfall will retain operational control over when Sirius charges and discharges, allowing the company to use the battery to balance its wind and solar generation portfolio, manage its broader customer commitments and support TenneT in real-time grid balancing.

The fixed-price structure of the agreement provides Return with the revenue certainty required to finance and build new storage capacity, while giving Vattenfall a long-term, contracted route to large-scale battery flexibility.

Financing Arrangements

Financing for Sirius has been secured under Return's broader battery energy storage system portfolio financing framework, which provides commitments of up to USD 464 million to support investments across the company's battery storage portfolio. The Sirius project itself accounts for approximately USD 208.8 million of that total investment.

The lending group includes ING, NatWest, Deutsche Bank, ABN AMRO, and Rabobank, alongside Return's existing investors. ING Corporate Finance advised Return on the transaction and also participated as a lender. Clifford Chance served as legal adviser to Return, while Dentons acted as legal adviser to the lenders.

Grid Integration via TDTR

Sirius will operate under a Time-Dependent Transmission Rights agreement with TenneT. Under this arrangement, the battery adjusts its charging and discharging schedule based on available grid capacity at any given moment.

The structure is designed to help ease congestion on the network and to make more efficient use of existing grid infrastructure, rather than requiring new transmission build-out.

The combination of the tolling agreement, the portfolio financing and the TDTR arrangement is intended to demonstrate how commercial certainty, institutional financing and grid integration can work together to bring large-scale battery storage into operation within existing network constraints.

An Expanding Partnership

The Sirius agreement builds on an existing commercial relationship between Return and Vattenfall. In 2025, the two companies signed a long-term agreement covering 50 MW of Return's 100 MW / 200 MWh Antares battery facility in Waddinxveen.

Sirius represents a significant expansion of that collaboration, providing Vattenfall with access to a 200 MW / 800 MWh four-hour battery system, quadrupling the energy storage capacity compared to the Antares arrangement.

The agreement also advances Vattenfall's stated goal of contracting at least 1.5 GW of battery capacity across the markets where it operates.

Delivery Partners and Technology

Sirius is being delivered in partnership with Huawei, Hanab, and Omexom, covering battery technology, construction, and installation, respectively. The project is located in Winschoten in the province of Groningen.

Industry Context

The Dutch electricity grid is facing increasing pressure as renewable generation, electrification, and overall demand continue to rise simultaneously. Grid capacity constraints have become one of the central challenges of the country's energy transition.

Large-scale battery systems such as Sirius are designed to address this by storing surplus renewable power during periods of high generation and releasing it when demand requires, as well as responding rapidly to short-term fluctuations in grid conditions.

Sjoerd Bazen, Managing Director of Return, described battery storage as rapidly becoming essential infrastructure for the Dutch electricity system. "Long-term partnerships like this allow us to build the storage capacity needed to integrate more renewable energy, support system reliability and create greater flexibility across the power system," he said. "Sirius shows how large-scale storage can move from development into critical infrastructure through strong commercial collaboration."

Erik Suichies, Head of Wholesale Customers at Vattenfall, pointed to the growing share of wind and solar in electricity supply as the driver behind the agreement.

"Batteries such as Sirius can respond quickly to changes in supply and demand, helping us make the most of renewable electricity," he said. "This agreement is an investment in storage capacity and in a reliable energy system for the future."

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