In early October 2026, Neoen inaugurated the Breizh Big Battery in Pleyber-Christ, Brittany: 80 MW connected to the transmission grid by RTE in March 2026 according to RTE, for an installation announced by Neoen at 92 MW and 183 MWh. It is the largest battery in Brittany. What makes it relevant for the whole sector is the trial it hosts: for the first time in France, RTE is testing the conversion of an existing battery to grid-forming mode.
DOES YOUR SITUATION LOOK LIKE ONE OF THESE?
Behind the technical term lies a very practical question for storage developers: will this mode remain an optional service, or become a condition for connecting new projects?
GRID FOLLOWING VS GRID FORMING IN TWO SENTENCES
In grid-following mode, a battery inverter locks onto the grid's voltage and frequency, then injects or draws the power requested. It follows the grid. Every battery in operation in France works this way today.
In grid-forming mode, the inverter behaves more like a conventional generator: it helps set voltage and frequency itself, reacts within milliseconds to disturbances and can provide a form of synthetic inertia. It helps hold the grid together.
As renewables and batteries grow, fewer large rotating machines remain to stabilise the system. Hence grid operators' interest in the technology.
WHAT RTE IS TESTING IN BRITTANY
Neoen and RTE signed a trial contract in January 2026. According to published information:
Conversion by retrofit: an update of the inverter control systems, without rebuilding the installation
A pilot phase of at least one year, with tests from the second half of 2026
Goal: measure the stability benefit of grid forming, and check that an existing asset can be converted
The second point matters. If the retrofit works, part of the existing fleet could one day provide these services without being rebuilt. If it shows limits, grid-forming capability will have to be designed into new projects from day one.
Neoen has done this before: in 2022, its Hornsdale battery in Australia was one of the first in the world to provide inertia services to the grid at scale.
WHAT EUROPE IS PREPARING
The draft revision of the European network code on requirements for generators (RfG 2.0) introduces grid-forming requirements for non-synchronous power generating modules and electricity storage modules. In November 2025, ENTSO-E published a technical report detailing these requirements.
According to the specialist press, the framework would cover new storage and generation plants above 1 MW, as well as substantial modifications of existing plants, with precise requirements: current response under 10 milliseconds, oscillation damping, ride-through of voltage sags and phase jumps.
Each country will then set its own implementation rules and timetable, likely with transition periods
Existing plants would only be affected in case of substantial modification
RTE's trial therefore comes at the right time: it should inform how France applies these rules.
WHAT IT MEANS FOR A PROJECT DEVELOPER
Inverter choice. Not every power conversion system can run in grid-forming mode, and those that can sometimes need extra current headroom or energy reserve. A project commissioning in 2028 or 2029 should raise the question when tendering.
Supplier contract. A clause committing the supplier to offer a software upgrade to grid forming, with its conditions and cost, can avoid a heavy investment later.
Grid connection. Technical connection requirements, compliance tests and the simulation models requested by the grid operator may change. Build this into the schedule and the studies.
Business model. While grid forming is not mandatory, it can become a paid service. Once mandatory, it becomes a cost. Nobody knows the final balance yet, but a project that anticipated it will have more options.
QUESTIONS TO ASK YOUR SUPPLIER NOW
Is conversion a software update, or does it require hardware changes?
What current headroom and energy reserve should we plan for?
Will you commit contractually to a future upgrade, and at what price?
Do you have the detailed simulation models the grid operator may request?
OUR VIEW
Results from Pleyber-Christ will not be known before 2027. Until then, the prudent course is to keep the door open: choose equipment that can evolve and write it into the contracts. For a battery project as for a data center, grid connection issues are easier to solve upstream than at commissioning.
See also how we support battery storage projects in France.
Sources: RTE (post by the West regional delegate, October 2026); Neoen press release, 13 January 2026; ESS News and pv magazine, January 2026; Les Énergies Renouvelables, 16 January 2026; ENTSO-E Phase II technical report on grid forming, 4 November 2025; ESS News, 12 November 2025; Neoen and pv magazine Australia on Hornsdale inertia services, July 2022.
FREQUENTLY ASKED QUESTIONS
What is grid forming?
An inverter operating mode in which the battery itself helps set grid voltage and frequency, like a conventional power plant, instead of simply following them (grid following).
What is RTE testing on the Breizh Big Battery?
The conversion of one inverter of an existing battery from grid-following to grid-forming mode, through a control-system update, during a pilot phase of at least one year starting in the second half of 2026.
Will grid forming become mandatory?
The draft EU network code RfG 2.0 provides for grid-forming requirements for new storage installations. It has not yet been adopted, and each country will then set its own implementation timetable.