Executive summary

On 24 April 2026, Sustainable Energy, Grids and Networks published a paper on grid inertia estimation and FAESR — Fast-Acting Energy Storage Resource — requirements for the Indian power system. The work is relevant not for its geography but for its methodology: it offers a reproducible framework to quantify how much fast-response storage is needed for a given drop in rotational inertia level.

The quantitative question, formulated for India, is the qualitative question that Europe is already answering through regulation. ENTSO-E has closed the Phase II report of the new Network Code RfG 2.0 with binding grid-forming requirements for all new BESS or renewables above 1 MW. Germany published the first price for the inertia services market on 28 January 2026. The UK Pathfinder already has operational commercial projects. And the Iberian Peninsula experienced, one year earlier, a major blackout that the European technical literature interprets as the case study the Indian paper models from theory.

The industrial consequence is direct. The design metric for a BESS PCS is no longer stored kWh or round-trip efficiency at steady state. It is the capacity to sustain a sub-100-millisecond response to a grid event over a transient of a few seconds, maintaining a minimum damping ratio of 5 %, full ride-through of phase-angle jumps, and conformity with the ENTSO-E mechanical starting time method.

The FAESR concept — and why the design metric changes

The term FAESR — Fast-Acting Energy Storage Resource — designates a storage resource that responds to a grid event on a sub-second timescale, with the explicit purpose of compensating for the mechanical inertia lost when a conventional synchronous machine is replaced by a renewable source connected via inverter. The rotational inertia of a thermal alternator is not programmed: it exists physically as kinetic energy stored in rotating parts, and is released automatically upon a frequency drop.

The synthetic equivalent — the virtual synchronous machine — emulates that behaviour from a bidirectional power converter with downstream storage, but only if the inverter-plus-battery assembly can inject the necessary power within the correct timeframe. That timeframe is demanding: ENTSO-E places the current response time below 10 milliseconds in the Phase II report of NC RfG 2.0.

This shifts the relative weighting of five design metrics:

The paper’s contribution — and its limits

The paper proposes a quantitative estimation methodology for the residual inertia level of a power system under different renewable penetration scenarios, and from that level derives the minimum FAESR needed to keep frequency within the acceptable operating range during a generation-loss transient. The valuable contribution is the methodology, not the Indian case: the work explicitly articulates the relationship between the admissible RoCoF — Rate of Change of Frequency — the available equivalent inertia, and the required FAESR power with its temporal profile.

Our critique is threefold. First, the Indian case is not linearly transferable to Europe: the energy mix, grid age, geographic dispersion and quantity of residual synchronous reserves are different. Second, the methodology does not cover the actual worst case observed — the Iberian event of 28 April 2025 showed that cascades can be qualitatively different from academic scenarios. Third, the paper models the FAESR response as if it came from a single ideal source; in practice, it comes from hundreds of distributed PCS units, each with their own computation times, communication delays and potential fleet-level degradations.

Transposing to Europe — what changes operationally in 2026

European regulation has closed, within six months, the technical requirements boundary that the paper models theoretically:

• ENTSO-E NC RfG 2.0 Phase II: binding grid-forming requirements for all new BESS or renewables >1 MW. Current response time <10 ms. Damping ratio ≥5 % on electromechanical oscillations. Full ride-through. Effective application expected 2028–2029 with national transitional periods.
• Germany: inertia services market launched on 28 January 2026 with four product categories (positive/negative response × base 30 % / premium 90 % availability). Mandatory VDE FNN certification. Multi-year contracts with remuneration indexed to verified availability.
• United Kingdom: Pathfinder programme with its first commercial operational plant (Blackhillock), demonstrating system-strengthening performance under real conditions.
• Iberian Peninsula: the event of 28 April 2025 — the only documented cascade in the European continental grid — remains the most-read operational reference for FAESR sizing assumptions across the European technical community.

Implications for PCS design

Three families of power conversion assets in the Premium PSU portfolio map directly onto the FAESR requirement space:

The catalogue alone is not enough. Technical rules have tightened within six months and TSOs now demand compliance traceability. A Design for Compliance (D4Co) approach articulates, from the design phase, the dossier that the operator will present to the regulator to make the inertia service contract remunerable. That discipline turns a technically good PCS into a contractually bankable PCS.