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In short:
Amsterdam, 31 August 2026 - Return and Vattenfall have signed a long-term agreement for Sirius, a 200 MW / 800 MWh battery storage facility in Winschoten, Groningen. Sirius connects to TenneT's substation in Meeden. Under the long-term agreement, Vattenfall secures access to the facility's charging, storage and power delivery capacity in exchange for a fixed fee. Commercial operations are expected to start in the fourth quarter of 2027. The project represents a total investment of approximately €180 million, making Sirius one of the largest battery storage projects under development in the Netherlands. The agreement is expected to be the largest fixed-price battery storage tolling agreement in the Netherlands to date.
A long-term agreement, built for flexibility
Wind and solar power supply fluctuates, while most demand does not. Batteries help close that gap. They store surplus power and release it when it's needed, and they can respond quickly when conditions on the grid change. Under the agreement, Vattenfall will decide when Sirius charges and discharges. This lets Vattenfall use the battery to balance its wind and solar generation, manage its customer portfolio, and support TenneT in balancing the grid in real time.
The agreement builds on the existing partnership between the two companies. In 2025, Return and Vattenfall signed a long-term agreement covering 50 MW of Return's 100 MW / 200 MWh Antares battery facility in Waddinxveen. Sirius significantly expands the collaboration, providing Vattenfall access to a 200 MW / 800 MWh four-hour battery system.
Funding secured
Financing for Sirius is now in place under Return's wider BESS portfolio financing framework, which provides commitments of up to €400 million to support investments across Return's battery storage portfolio. Sirius itself represents a total project investment of approximately €180 million. Alongside the existing Return investors, the lending group includes ING, NatWest, Deutsche Bank, ABN AMRO and Rabobank.
ING Corporate Finance advised Return on the transaction, while ING also participated in the financing as a lender. Clifford Chance acted as legal adviser to Return and Dentons acted as legal adviser to the lenders. Sirius is being delivered with Huawei, Hanab and Omexom, covering the battery technology, construction and installation.
Supporting grid flexibility through TDTR
Sirius operates under a Time-Dependent Transmission Rights (TDTR) agreement with TenneT. Under this arrangement, the battery adjusts when it charges and discharges based on how much capacity is available on the grid at that moment. This helps ease congestion and makes better use of existing grid infrastructure.
Battery storage as infrastructure
The Dutch electricity grid is under growing pressure as renewable generation, electrification and demand all increase. Grid capacity is becoming one of the central challenges of the energy transition. Large-scale batteries like Sirius give the system flexibility: they help balance supply and demand, absorb surplus renewable power, and support reliability.
Long-term agreements like this one help accelerate that build-out, by giving developers the revenue certainty they need to finance and build new storage capacity. With this agreement, Vattenfall also moves further toward its goal of contracting at least 1.5 GW of battery capacity across the markets where it operates.
"Battery storage is rapidly becoming essential infrastructure for the Dutch electricity system," said Sjoerd Bazen, Managing Director of Return. "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. Sirius shows how large-scale storage can move from development into critical infrastructure through strong commercial collaboration."
"As wind and solar account for a growing share of our electricity supply, flexibility is becoming a crucial building block of the energy system," said Erik Suichies, Head of Wholesale Customers at Vattenfall. "Batteries such as Sirius can respond quickly to changes in supply and demand, helping us make the most of renewable electricity. This agreement is an investment in storage capacity, and in a reliable energy system for the future."
Together, the long-term tolling agreement, the portfolio financing and the TDTR arrangement show how commercial certainty, institutional financing and grid integration are turning large-scale battery storage into established energy infrastructure in the Netherlands.
