Return reaches financial close on 200 MW / 800 MWh Sirius BESS under Vattenfall tolling agreement

Return and Vattenfall have signed a long-term agreement for Sirius, a 200 MW / 800 MWh battery energy storage project in Winschoten, Groningen. The commercial agreement was announced alongside financial close, moving the four-hour system into its delivery phase.

Sirius represents an investment of approximately €180 million and is scheduled to begin commercial operations in the fourth quarter of 2027. Huawei, Hanab and Omexom are involved across battery technology, construction and installation.

A fixed-fee toll anchors the revenue model

Under the long-term tolling agreement, Vattenfall will obtain access to the full charging, storage and delivery capacity of Sirius in exchange for a fixed fee. Vattenfall will determine when the asset charges and discharges, within the boundaries of the commercial contract and the grid arrangement.

The structure separates ownership and physical delivery of the battery from commercial dispatch and portfolio optimisation. Return gains a more predictable contracted revenue stream, while Vattenfall gains a flexible asset that can help manage renewable generation, customer positions and short-term electricity-market exposure.

The parties have not disclosed the term, pricing formula or detailed risk allocation. It is therefore not possible to determine exactly how much revenue is contracted, how upside is shared, or which party carries the costs associated with dispatch restrictions and technical availability.

Five banks support a wider storage platform

Sirius has been financed under Return’s broader BESS portfolio framework. ING, NatWest, Deutsche Bank, ABN AMRO and Rabobank have committed up to €400 million to support investments across the platform.

The figures describe two different levels of the transaction: approximately €180 million is the stated investment in Sirius itself, while the commitment of up to €400 million applies to Return’s wider battery portfolio financing.

The participation of five major lenders is a useful marker of how the European storage market is maturing. Banks are not financing capacity in isolation. They are underwriting a package that includes a credible counterparty, contracted revenue, an executable grid position, defined delivery responsibilities and a risk structure that can withstand downside scenarios.

Flexible transmission rights become part of bankability

Sirius will connect to TenneT’s Meeden substation under Time-Dependent Transmission Rights, known as TDTR. The battery will adapt its charging and discharging to the capacity available on the transmission system instead of relying on unrestricted firm access at every hour.

Under the general Dutch framework described by regulator ACM, TDTR provides access to maximum transport capacity for at least 85% of the hours in a year. TenneT may restrict access during the remaining periods to relieve pressure on the network.

That trade-off can be workable for storage because dispatch is inherently flexible. A battery can move charging and export into different periods if the commercial strategy, state-of-charge management and revenue assumptions are designed around the network constraints.

Sirius therefore illustrates a broader point: flexible grid access does not have to be treated only as a temporary compromise. If the curtailment rules are clear and the revenue contract accommodates them, non-firm access can become part of the bankable project structure in a congested power system.

The lesson for Southeast European projects

The Dutch model cannot be copied directly into Bulgaria or another Southeast European market. Connection rules, ancillary-service markets, network tariffs and contractual practice differ. The underlying investment logic is nevertheless highly relevant: a permit and a headline connection capacity are not enough without a coherent commercial and operational model.

For a BESS acquisition, investors and lenders need clarity on grid charging and export rights, actual restrictions at the connection point, remaining network works, route-to-market arrangements, availability obligations, degradation assumptions, warranty coverage, EPC responsibility and the treatment of operator-imposed limitations.

A long-term contract with a creditworthy counterparty can improve revenue visibility, but it does not eliminate due diligence. Value still depends on the toll, indexation, minimum availability, cycling parameters, losses, balancing costs and the way market upside and downside are allocated.

From merchant project to infrastructure asset

Sirius expands an existing relationship between the two companies. In 2025, Vattenfall contracted 50 MW of Return’s 100 MW / 200 MWh Antares battery project. The new agreement gives it access to the full capacity of a 200 MW / 800 MWh four-hour system.

The transaction points toward the next phase of Europe’s BESS market. The strongest projects will increasingly be assessed as infrastructure assets in which physical capacity, contracted revenue and grid rights are designed together. That combination, rather than megawatts and megawatt-hours alone, is what allows storage to move from development into construction and long-term financing.