Ancillary-service commitments and EBX's outage cover

What EBX guarantees when a unit fails unexpectedly during an ancillary-service commitment, where that guarantee ends, and what lies beyond it.

In short

When your battery is committed to ancillary services, its capacity forms part of a delivery commitment to the transmission system operator (TSO). EBX fulfils that commitment through its pool: a group of assets operated together to provide the service.

Reliable operation is the starting point. Because technical failures cannot be ruled out entirely, EBX provides collateralisation, which is replacement capacity that protects the pool’s commitment when a unit fails unexpectedly.

Who this is for

Asset owners and O&M teams whose assets are committed to ancillary services through the EBX pool. For the practical steps during a fault, see Managing asset outages with EBX.

The commitment is full availability

Your asset is expected to remain available and fulfil its agreed ancillary-service commitments throughout the delivery period.

There is no planned allowance for an asset to fail.

For frequency containment reserve (FCR) and automatic frequency restoration reserve (aFRR), the asset must keep the committed capacity available and deliver the response required by the service.

Collateralisation applies when a failure affects an active ancillary-service commitment. EBX manages replacement delivery while the asset owner and O&M team address the fault.

EBX guarantees cover for the largest technical unit

EBX prepares for the unexpected failure of any one participating technical unit. Its guaranteed collateralisation is sized to the largest technical unit (LTU) of the asset. This is the N−1 principle:

The pool must still be able to fulfil its commitments with one unit unavailable.

EBX secures replacement capability from other pool assets or contracted external resources. The cover needs sufficient power, enough energy to sustain the response, and the technical capability required by the service.

This cover carries the collateralisation fee agreed in your customer contract.

Does N−1 mean that one inverter is allowed to fail?

N−1 describes the failure the pool is prepared to cover. It does not make an inverter outage an acceptable operating condition. The asset should remain fully available, and a failed component should be restored promptly.

Whether an inverter corresponds to a technical unit depends on the site’s technical design and the units defined in its prequalification and collateralisation arrangements. A fault affecting one inverter may be contained; a shared transformer or controller fault can affect several parts of the battery at once.

For your O&M team, the important distinction is between an isolated failure within the agreed cover and an outage that exceeds it. EBX can help clarify how this applies to your asset.

Beyond the LTU: best-effort support and greater risk

An outage larger than the LTU exceeds the guaranteed cover.

EBX will seek replacement capacity on a best-effort basis, but full replacement is not guaranteed.

The collateralisation fee still applies, together with any additional penalty specified in your contract.

Unexpected technical outage	EBX cover	Commercial treatment
One technical unit, up to the LTU	Guaranteed collateralisation	Agreed collateralisation fee
Outage exceeding the LTU	Best-effort cover	Collateralisation fee plus any contractual penalty

Preventing larger outages is particularly important because the pool may be unable to compensate for the full shortfall. Delivery or availability failures can prompt TSO requests for clarification and corrective action. Serious or recurring failures can put the asset’s prequalification at risk and, in the worst case, lead to its withdrawal for the affected service.

The TSO assesses compliance with the applicable requirements; exceeding the LTU is not an automatic trigger for loss of prequalification. Repeated smaller failures also need investigation and corrective action.

Keeping participation reliable

Early communication helps EBX and your O&M team respond together. Share known restrictions before commitments are made, report unexpected faults promptly, and keep recovery estimates current. Planned maintenance and other foreseeable restrictions need to be managed separately from unexpected-failure cover.

For practical steps, see the companion article Managing asset outages with EBX.

Official guidance

The German TSO prequalification documents set out availability and collateralisation requirements, particularly sections 2.6 and 2.18 of the PQ conditions. The TSO battery redundancy guidance explains component redundancy in battery systems.

These sources describe the technical requirements. EBX’s guarantee and the commercial treatment described above are part of its customer arrangements; fee rates and penalty terms are defined in your contract.