Micro-hydro plants and batteries in France: shifting output within the contract rules

Pairing a micro-hydro plant with a battery gives continuous generation something it lacks: a choice of when to inject. The question is a practical one for owners of small plants in France, whose contract has just changed again. Here is what the text says, what the battery really changes and how to size it.

Why put a battery on a run-of-river micro-hydro plant?

A run-of-river plant turbines the available flow at all hours. It has no reservoir in which to wait for a better moment: water passing through is either generated or lost to the plant. This regularity is its strength, and it gives the French fleet a solid position: 25,747 MW installed at the end of 2025 and 56.7 TWh of renewable generation in 2025, or 12.7% of mainland consumption, according to the Panorama of renewable electricity.

Market prices, however, are no longer regular. RTE counted 513 hours of negative prices in 2025. The solar side of this topic is covered in our article on negative prices and solar PV. For hydro, one point reads differently: the French energy regulator (CRE) estimates that small plants under feed-in contracts ran at an average of 44% of their capacity during these episodes in 2024, and 34% in 2025. The plant therefore generates a significant share of its capacity at those hours.

The battery comes in after the turbine. It stores part of the electricity generated and returns it later, without changing the turbined flow or the water regulations.

Which contract applies to a small plant?

The reference framework is the order of 13 December 2016, known as "H16", most recently amended by the order of 28 May 2026, published in the Official Journal of 1 August 2026. In its opinion of 5 March 2026, the CRE summarises the open-window schemes for installations under the authorisation regime:

Above 1 MW, a new installation goes through a call for tenders. The 200 kW threshold comes from decree no. 2025-498 of 5 June 2025, cited by the CRE: an older contract may have been signed under a different threshold. The first step is therefore to reread your own contract, with the version of the order that applies to it.

Can you store under an H16 contract?

Yes. Article 1 of the order allows storage devices to benefit from a feed-in contract or a contract for difference "only insofar as a technical device makes it possible to guarantee that the stored energy comes exclusively" from the hydroelectric installation.

This has three practical consequences:

• Charging comes from the plant. The electrical layout and the control system must prevent any charging from the grid, and be able to show it.
• Pumped storage is a separate matter. An installation whose pumped storage needs energy to fill benefits from neither a feed-in contract nor a contract for difference. A battery does not fall into this case.
• The counterparty is informed upstream. The change to the layout and its treatment in the contract are checked with the counterparty before ordering equipment.

What is the effect on installed capacity?

This is the least discussed point, and it deserves checking from the pre-project stage. Under Article 3 of the order, installed capacity is the connection capacity, increased where applicable by the maximum active power generated above that connection capacity without being injected, in particular because it is stored.

Two configurations can be distinguished:

1. The plant generates within its connection capacity. The battery shifts part of this generation in time. The connection capacity remains the reference.
2. The battery is used to generate above the connection capacity. The extra power generated and stored is added to installed capacity.

The second case can cross a threshold in the table above. It is therefore calculated before setting the battery's power, from the maximum power the turbine can actually deliver.

What happens during negative-price hours?

It all depends on the contract. Under a contract for difference, Annex III of the order provides a premium for hours of strictly negative prices with no grid injection. The order of 28 May 2026 changed its two parameters:

The formula now reads: premium = 0.4 × connection capacity × reference tariff × number of hours concerned. These changes follow the recommendation of the CRE, which considered a 40% factor consistent with observed load factors. A contract already signed is read according to its own conditions: this should be checked with the counterparty.

The text focuses on the absence of grid injection. A battery absorbing generation during these hours therefore raises the question of how storage and the premium combine: the answer should be confirmed by the counterparty for the contract concerned, before building the control strategy on it.

How should the battery be sized?

The method has four steps and assumes no result in advance.

1. Obtain the generation curve, at the finest time step available and over several years if possible. Hydrology varies from year to year: a single year is not enough to describe a plant. In 2020 the CRE noted that 75% of the installations in its panel reported output of 2,000 to 4,300 full-load equivalent hours per year, which gives an order of magnitude to compare with the actual curve.
2. Choose the service. Shifting injection, avoiding injection at negative prices, supplying a neighbouring site or taking part in the markets: each calls for its own sizing.
3. Measure the energy to shift. This is generation during the target hours, not the turbine's rated power.
4. Separate the seasons. High water fills the battery quickly, while low water leaves more flexibility. Sizing based only on peak generation is often little used for the rest of the year.

A worked example

Take, as an illustration, a plant with 150 kW of connection capacity. It generates 40% of its capacity during a five-hour episode, or 60 kW for five hours: 300 kWh to absorb, before conversion losses. This calculation gives an amount of energy to store, not a gain: the value then depends on the contract, prices and the number of cycles.

At this scale, a VenturePro 250 cabinet (250 kWh, 125 kW) covers part of the episode, and a Fusion 313 cabinet from the Fusion 2 range (313 kWh, 156.7 kW) the whole episode, excluding losses. The final choice comes from the site study and the actual curve.

Which range for which plant?

The Fusion 2 range is listed in the catalogue for generating plants, hydro included. The choice comes down to the connection point, the space available near the plant and the temperature of the room or platform.

Can the battery provide other services?

Yes, subject to the contract and the project study. A battery can take part in the electricity markets through an aggregator: our article on how a battery is remunerated details the mechanisms. The charging rule in Article 1 must still be respected as long as the battery falls within the plant's contract.

Another use under study is reducing mechanical stress. Tests on a reduced-scale platform, published in 2024, compared a Kaplan turbine paired with a battery and a turbine converted to variable speed to provide frequency reserve, with the aim of limiting mechanism fatigue. They concern machines on a different scale from a micro-hydro plant and do not transfer directly.

Backup or islanded operation is validated project by project: it depends on the electrical layout, the turbine and the protections.

Where to start?

Three documents are enough to open the study: the current contract, the generation curve and the connection diagram. With them, the project assessment sets the target service, checks the contract thresholds and proposes an initial battery size. For the general method, see also our guide to sizing a storage battery.