Understanding BESS
Battery storage, in plain terms.
A BESS – a battery energy storage system – stores electricity in batteries and returns it later. On an industrial or commercial site, it serves three purposes: shaving power peaks, consuming the site’s own solar output, and securing critical loads. Its size is always described by two numbers: a power in kW and an energy capacity in kWh.
Facts verified on 2 October 2026.
What is a BESS made of?
A BESS brings together six subsystems. Comparing suppliers means knowing what varies from one to another: the cell sets useful life and thermal behaviour; the power electronics and the controls set what the asset will be able to do once installed.
| Subsystem | Role | What varies |
|---|---|---|
| Cells, modules, racks | The storage itself. Cells are assembled into modules, modules into racks. | The cell chemistry and the cell supplier. This is the component that determines useful life, thermal behaviour and the availability of parts over time. |
| BMS – battery management system | Monitors every cell: voltage, temperature, state of charge. Balances the cells and disconnects if they drift. | The resolution of the measurements and how open the data is. Accessible data leaves the operator free to choose its monitoring tools over time. |
| PCS – power conversion system | Converts the batteries’ DC into the grid’s AC, in both directions. | The power, the round-trip efficiency and, above all, the operating modes it supports: the PCS determines which market mechanisms the asset will be able to take part in. |
| EMS – energy management system | Decides when to charge and discharge, according to the site’s consumption, the tariffs and the market signals. | The strategy, and the signals it can read. This is the part that separates a battery that shaves peaks from a battery that earns revenue – and the one whose quality shows soonest on the bill. |
| Thermal management | Keeps the cells within their temperature range. Air or liquid cooling. | Liquid cooling allows higher densities and higher power; it adds a circuit to maintain. |
| Fire safety and detection | Detection of gas, temperature rise and thermal runaway; extinguishing; smoke extraction. | The level of integration and the certifications targeted. This is also the component that shapes the administrative review of the project. |
What the EU battery regulation requires
BMS data
Since 18 August 2024, the BMS of a stationary storage system must hold the data that determine the battery’s state of health and expected lifetime. Its buyer, or a third party acting on their behalf, has read-only access to them at any time – notably to make the battery available to independent aggregators.
Regulation (EU) 2023/1542 concerning batteries and waste batteries, article 14, 12 July 2023 Battery passport
From 18 February 2027, every industrial battery above 2 kWh – the category stationary storage systems belong to – is linked to a digital passport as soon as it is placed on the market or put into service. The passport holds information on the model and on the battery itself, including information resulting from its use.
Regulation (EU) 2023/1542 concerning batteries and waste batteries, article 77, 12 July 2023
BESS, inverter, backup battery, generator set: what are the differences?
Four pieces of equipment are regularly confused because all of them touch a site’s electricity. Only one stores energy for economic reasons, and that is what separates an investment from an insurance policy.
Do not answer the question
- Solar inverterDoes not store
- Backup battery, UPSFor availability
- Generator setCuts no bill
The answer
BESS
Stores for savings
The first confusion to clear up sets the BESS against the backup battery, because both contain cells and look alike on a drawing. A backup battery is sized for a number of hours of autonomy and only works on the days the grid fails; a BESS is sized from a load curve and works every day. A site that buys one thinking it is buying the other discovers the gap on its first bill.
Shaving a peak: which line of the bill?
Peak shaving acts on the network charges (tarif d’acheminement) – TURPE, the French tariff for the use of the public electricity networks – and not on the price of the electricity consumed. A site pays for a subscribed power (puissance souscrite, the power level contracted with the network operator) in each time category (plage temporelle) of the tariff, and pays a surcharge as soon as it exceeds it. Reducing the peak means reducing the power to subscribe: the saving is structural, and depends on no market price.
What TURPE sets
Entry into force
TURPE 7 HTA-BT, the distribution-network part of the tariff, entered into force on 1 August 2025, for around four years. Its price schedule is updated each year on 1 August.
CRE (the French energy regulator), deliberation no. 2025-78, 13 March 2025 Peak hours
At HTA (medium voltage, 1–50 kV), peak hours run from December to February inclusive: two hours in the morning between 8:00 and 12:00, and two hours in the evening between 17:00 and 21:00.
CRE, deliberation no. 2025-78, § 5.2.1.5 – HTA voltage level, 13 March 2025 Subscribed power
At HTA, subscribed power is chosen in multiples of 1 kW for each of the five time categories of the tariff, each category having to be greater than or equal to the previous one.
CRE, deliberation no. 2025-78, § 5.2.1.5 – HTA voltage level, 13 March 2025 Overrun
At HTA, subscribed power overruns are measured at ten-minute intervals. The coefficient is calibrated so that beyond a hundred hours of overrun sustained at constant power, subscribing additional power becomes the better deal.
CRE, deliberation no. 2025-78, § 4.3.3.3, 13 March 2025
Beyond a hundred hours of overrun sustained at constant power, subscribing more costs less than overrunning. A site that overruns rarely but heavily and a site that overruns often but slightly therefore do not have the same problem – nor the same battery sizing.
Why is selling to the grid no longer enough?
For a long time, selling surplus solar output was enough to balance a project. Two regulatory moves have restricted it for industrial- and commercial-sized installations, and a third, in June 2026, brought the surplus tariff down to 1.1 c€/kWh.
What has changed for selling to the grid
Applications against target
Between the end of 2021 and the end of 2024, the guichet ouvert – the feed-in scheme open as of right without a tender – received 16 GWp of contract applications against a target of 4.8 GWp, more than three times the goal. The 100–500 kWp band alone accounts for 12.6 GWp of them, against 3.4 GWp targeted.
CRE, deliberation no. 2025-69, p. 2, 6 March 2025 Leaving the guichet ouvert
Since 22 September 2025, solar installations of 100 to 500 kWp have left the guichet ouvert: their support now goes through a dedicated call for tenders, known as “Petit PV”.
CRE, deliberation no. 2026-106, 21 May 2026 36–100 kWp band
The full-export feed-in tariff for the 36–100 kWp band fell from €112.6/MWh in the first quarter of 2025 to €70.0/MWh in the second quarter of 2026, a 38% drop.
CRE, deliberation no. 2026-92, § 1.2, 29 April 2026 9–100 kWp band
Complete grid connection requests in the 9–100 kWp band rose from 116 MWp in the second quarter of 2025 to 291 MWp in the third, against a quarterly target of 92.25 MWp. The CRE reads this as projects shifting down ahead of the closure of the guichet to installations above 100 kWp – though the second quarter covers only two months.
CRE, deliberation no. 2026-92, § 1.2, 29 April 2026 Feed-in tariff
Up to 100 kWp, injected solar output – whether surplus or full export – is bought at a tariff of 1.1 c€/kWh, indexed by 2% a year, within a cap of 1,600 full-power hours a year; the self-consumption premium has been withdrawn. For these installations, selling to the grid no longer factors into the equation: self-consumption becomes the default use of the output.
Order of 1 June 2026, articles 8 and 10, Journal officiel (France’s official gazette) no. 0129 of 4 June 2026, 5 June 2026
- 100–500 kWp12.6 GWp requested · target 3.4 · ×3.7
- 0–9 kWp2 GWp requested · target 0.6 · ×3.3
- 9–100 kWp1.4 GWp requested · target 0.9 · ×1.6
- All bands16 GWp requested · target 4.8 · ×3.3
“slow the development of PV installations through the guichet ouvert and redirect it towards self-consumption”
This shift is not a reading of the market: the regulator itself describes it. For a site that has solar, or could have it, value moves from selling to self-consumption – and consuming your own output means storing whatever the site does not use at the moment it is produced.
Where does the French fleet stand?
Stationary storage has become infrastructure in its own right in France. The figures that follow come from RTE, the transmission system operator, and from the national register, and change from one year to the next.
2,159 MW
in service on 31 July 2026
The national register counted 2,159 MW of non-hydro storage in service on 31 July 2026. In the first seven months of 2026 alone, 555 MW were commissioned – more than in the whole of 2025.
14 GW
of projects in the connection queue at end 2025
Around 14 GW of high-power battery projects were in the connection queue at the end of 2025 – projects whose grid access is already contracted – mostly on the transmission network.
40 to 50%
of frequency containment reserve needs
Batteries cover 40 to 50% of the French power system’s frequency containment reserve (FCR) needs.
The fleet, by size and pace of growth
Fleet at end 2025
On 31 December 2025, 1.6 GW of batteries were in service in France, around a third of it connected to the transmission network. Those on the transmission network average 25 MW, against nearly 75 MW for the projects waiting to connect to it.
RTE, Bilan électrique 2025, chapter “La production”, pp. 50–51, March 2026 Annual average
The fleet grew by around 300 MW a year on average over 2023 and 2024.
RTE, Bilan prévisionnel 2025–2035 (RTE’s generation adequacy report), § 3.5.2, December 2025
2,159 MWconnected as at 31 July 2026, with the first seven months of 2026 above the whole of 2025
- before 202016.4 MW
- 202078.7 MW
- 2021218.9 MW
- 2022211.5 MW
- 2023362.3 MW
- 2024231.7 MW
- 2025484.1 MW
- 2026 – seven months, to 31 July555.4 MW
Where the grid accepts a battery
Solar density
In February 2025, RTE observed that few batteries were connecting in regions with a high density of solar, for lack of any incentive to locate where they would relieve grid congestion.
RTE, SDDR 2025, sheet 8, p. 91, 27 February 2025 Connection with time restrictions
Approved by the CRE on 4 February 2026, a connection with seasonal time restrictions lets RTE connect a battery at the substations it designates without reinforcing the grid, and therefore faster. In return, the battery gives up injecting every day from 10:00 to 18:00 from March to October, or withdrawing from 7:00 to 13:00 and from 17:00 to 21:00 from November to March, depending on the substation.
CRE, deliberation no. 2026-32 of 4 February 2026, § 3.2.2, 4 February 2026 Low-voltage grid connection
No generating installation can be connected at low voltage (BT) above 250 kVA. The ceiling for an HTA grid connection is 17 MW in mainland France, and 12 MW in the non-interconnected zones.
Order of 9 June 2020, article 24, 9 June 2020
Does a BESS fall under ICPE?
Yes, under heading 2925-2 of the ICPE nomenclature (battery charging workshops) – ICPE being the French regime for installations classified for environmental protection – with three distinctive features: the threshold is expressed in charging power, not in energy capacity; it is calculated by adding up everything the site charges; and no regime exists above declaration.
The three counter-intuitive points
Threshold in kilowatts
The threshold is expressed in kilowatts of charging power, not in energy capacity: the heading has no threshold in MWh or in tonnes.
Annex to article R511-9 of the French Environmental Code, note (1), 28 October 2019 Cumulative threshold
A battery energy storage system falls under ICPE heading 2925-2, in the declaration regime, as soon as the site’s cumulative charging power exceeds 600 kW: the storage and the charging points not open to the public run by the same operator are added together.
DGPR (the French ministry’s risk prevention directorate), question-and-answer sheet on heading 2925, revised on 14 April 2025, 14 April 2025 Heading 2925
Heading 2925 has only one regime, declaration: there is neither registration nor authorisation, whatever the power. A 35 MW site stays under the same regime as a forklift charging workshop.
DREAL Nouvelle-Aquitaine (the regional environment authority), report of the Inspection des installations classées (the classified-installations inspectorate), pp. 1–2, 14 January 2025
The wider framework
No national text
No national text yet sets siting rules specific to stationary lithium storage. Setback distances and firefighting resources are set by prefectoral order, département by département.
DREAL Nouvelle-Aquitaine, report cited above, p. 3, 14 January 2025 Substations
Since 4 March 2026, substations of 63 kV and above are no longer subject to case-by-case environmental screening. Until then, the substation was what could make a storage project connected at HTB (high voltage, above 50 kV) subject to that screening.
Decree no. 2026-146 of 2 March 2026, articles 2 and 9, 2 March 2026
Why do siting rules vary from one département to the next?
The order setting the requirements for heading 2925, of 29 May 2000, targeted the hydrogen given off by lead-acid batteries, not the thermal runaway of lithium, which the heading has only distinguished since 2019.
With no national text specific to lithium, setback distances, installation design and firefighting resources are set by prefectoral order. An identical project therefore does not face the same constraints in two neighbouring départements, and the values published here and there do not apply generally.
How do you size a BESS?
Sizing starts from the site’s load curve; the catalogue comes next, to choose the equipment that matches it. Four questions determine it, and each is answered site by site.