Europe is advancing toward unified grid-forming inverter standards, aiming to streamline cross-border deployment of long-duration energy storage (LDES). The European Network of Transmission System Operators for Electricity (ENTSO-E) released its Phase II technical report in November 2025, mandating that new storage systems exceeding 1 MW incorporate grid-forming capabilities-including voltage control, inertia response, and frequency regulation that emulate synchronous machines. Pending adoption of NC RfG 2.0, these requirements will be incorporated into national grid codes, subject to transitional periods. The ENTSO-E report establishes these standards for new installations and significant upgrades. Member states will implement the measures according to domestic regulatory timelines. Meanwhile, Belgium's transmission system operator, Elia, began a fast-track public consultation in early 2026 to introduce grid-forming requirements for battery energy storage systems (BESS), focusing on type D BESS and large inverter-based loads (≥75 MW), ahead of NC RfG 2.0's enactment.
Background
Grid-forming inverters actively establish and regulate grid voltage and frequency, distinguishing them from conventional grid-following inverters that only synchronize with existing grid conditions. This function is increasingly vital as inverter-based resources proliferate across Europe. The absence of standard definitions and testing procedures has hindered sector progress. In response, Germany's Fraunhofer ISE launched the GFM Benchmark project in 2025, developing a practical test procedure that contributed to Germany's VDE FNN guideline, "Grid-Forming Capabilities," effective since May 2025. This guideline establishes certification pathways and enables participation in upcoming instantaneous reserve markets. ENTSO-E is also preparing a European implementation guide to assist with adoption of grid-forming requirements.
Details
According to ENTSO-E's Phase II report, new storage systems and substantial retrofits over 1 MW must implement grid-forming controls to provide inertia, voltage regulation, and frequency stability-including during grid disturbances. These obligations will be embedded in the NC RfG 2.0, with phased rollout across EU member states. Elia's public consultation seeks to align Belgian regulations promptly with European standards, anticipating increased inverter-based generation and the need for grid stability. Fraunhofer ISE's benchmark testing demonstrated variation in device performance, guiding the development of the VDE FNN guideline, which is now the normative foundation for certification.
Outlook
As NC RfG 2.0 moves toward implementation and national rules follow, developers and utilities will need to incorporate grid-forming requirements into project design and procurement. Certification, based on the VDE FNN guideline and future ENTSO-E guidance, will be essential for interconnection approval. The transition is expected to occur over several years, allowing time for technical adaptation. Standardized grid-forming requirements are anticipated to advance cross-border LDES integration and support a harmonized market for resilient, inverter-based grids.
