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Even after a BESS is fully built and connected to the grid, it cannot immediately start providing ancillary services. To ensure grid stability, Transmission System Operators (TSOs) have extremely high standards. This is where Ancillary Services Prequalification comes in — a rigorous testing and certification process that unlocks access to these critical markets.

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Why Prequalification Is Essential

Ancillary services directly impact grid frequency and voltage on a moment-to-moment basis. Any provider must be completely reliable. Prequalification builds the trust required by demonstrating that a BESS can react precisely and consistently to TSO signals, within very tight timeframes and technical specifications. The process is carried out exclusively via the TSO’s prequalification (PQ) portal.

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Step 1: Defining Your Units

During construction, the BESS asset is structured into specific units for the purpose of prequalification:

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Step 2: The Operating Test (Doppelhucker)

Every RPU or RPG must complete an operating test to verify it complies with prequalification requirements. This test is often called the “Doppelhucker” (double-peak curve) due to the characteristic shape of the power measurement graph during activation.
  • Purpose: Determines the prequalified power (PQ power) of the RPU/RPG and evaluates whether it can reliably reproduce a specific operating behaviour.
  • Process: The test involves the activation of balancing reserve on a test basis. It is normally carried out independently by the asset owner using its own control system. For assets prequalifying 150 MW or more, coordination with the reserve connecting TSO is required.
  • Timing: This test must be carried out after COD, as it requires the physical asset to be connected to the grid and fully operational.

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Step 3: The Control System Test

The Control System Test verifies that the asset’s pool is correctly connected to the TSO’s control system, including:
  • Correct transmission of real-time measurements from the asset to the TSO.
  • Receipt and implementation of TSO activation signals.
  • Robustness of the pool’s provision, including handling simulated technical challenges such as the failure of a TU.
This test is required during the initial setup of a pool, upon fundamental changes to pool composition or control, and approximately every five years thereafter.

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Step 4: Documentation

A comprehensive set of documents must be compiled and submitted via the TSO’s PQ portal, including:
  • Operating log: Detailed records from the operating test.
  • Equipment data sheet: Master data for all TUs and RPUs/RPGs.
  • Technical concept: Detailed description of the BESS site’s technical design.
  • IT concept and checklist: Documents confirming IT meets TSO requirements for data exchange.
  • CSO confirmation: Confirmation from the local DSO regarding the grid connection.
  • BRP confirmation: Confirmation that the TU is allocated to a balancing group and the BRP is informed of balancing reserve marketing.
  • Operator/owner confirmation: Agreement that the TU owner consents to the reservation and activation of balancing reserve from their unit.

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Expected Timeline

Prequalification can be renewed at any time up to the expiration of the existing certificate.
Successfully completing prequalification certifies the BESS as a reliable and capable contributor to Germany’s grid stability, unlocking access to FCR and aFRR ancillary service markets.
Last modified on April 20, 2026