Portugal makes grid-forming mandatory for large battery storage

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Portugal has set the technical conditions battery storage must meet to connect to the public grid. The rules were published on 22 October 2026 and took effect the following day. They apply to connection requests submitted from that date and to existing storage plants only where they are significantly expanded, modernised or re-equipped. Pumped hydro storage is outside the grid-forming and inertia requirements.

Grid-forming scales with plant size

The rules sort storage plants into four classes by maximum capacity and connection voltage. Plants of 45 MW or more, or connected at 110 kV or above, must operate in grid-forming mode and provide synthetic inertia. They must also hold an energy reserve for that inertia response on top of the storage they use in normal operation. Any plant connected to the transmission network carries the same obligation whatever its size, on terms the network operator sets in coordination with the system operator.

Below 45 MW the rules leave more to the individual connection. For these plants, grid-forming applies where the grid capacity reservation title, the permit or the documents of the tender that awarded the capacity provide for it. For plants from 1 MW to 45 MW the system operator can also specify their contribution to synthetic inertia. Plants on the distribution grid must be able to run in grid-following mode as well, so the operator can choose the mode that suits the local network.

How synthetic inertia must perform

The rules set a clear technical bar. Plants must respond to frequency deviations between 47.0 Hz and 52.5 Hz and to rates of change of frequency of up to 2 Hz per second. The emulated mechanical starting time must be adjustable between 1 and 20 seconds, with a default of 12 seconds unless the grid operator sets another value. Plants must also damp active power oscillations between 0.1 Hz and 10 Hz by at least 10%.

Frequency response and ancillary capability

The frequency requirements apply to storage of 500 kW or more. While charging from the grid, these plants need an under-frequency protection set at 49.2 Hz by default, within a range of 49.1 Hz to 49.8 Hz that the operator can adjust. Changes in output that are not part of a reserve activation must ramp over 10 minutes unless the system operator instructs otherwise.

With the exception of plants below 1 MW, storage must be technically able to provide FCR, aFRR and mFRR, as well as voltage and reactive power control. Plants above 1 MW also need real-time monitoring and remote control links. Please note that when the operator instructs a plant to respond, the plant answers with the capacity it actually has available. The rules impose no obligation to hold energy reserves or state-of-charge margins for this.

What it means for battery projects

For large plants the rules move grid-forming from a design option to a connection condition. Inverter specification, control settings and the energy set aside for inertia now need to be fixed at design stage. That inertia reserve sits on top of the storage the plant uses in normal operation, which affects how the battery is sized. For plants below 45 MW, the grid capacity title or tender documents decide how far the same requirements apply, so they are worth reading closely before the plant is sized.

The ancillary capability requirement points the other way. Every new plant from 1 MW upward will be technically ready to bid into FCR, aFRR and mFRR, which keeps the full revenue stack open from the first day of operation. The government also states that the rules do not prevent Portugal from implementing the revised EU grid connection code once it is published, so developers have a defined national standard to build to in the meantime.

Synertics tracks balancing and ancillary service prices alongside PPA and capture prices across European markets in SEMT. If you are sizing a Portuguese battery project and want to test how the inertia reserve and ancillary capability affect its revenue stack, you can start with SEMT at https://synertics.io/signup/ or reach us at info@synertics.io to discuss it in more detail.