Spain’s 140 MW Balearic Battery Enters Final Commissioning Phase

A 140 MW battery storage system spanning two Spanish island substations has entered its final commissioning phase, making the Balearic Islands home to southern Europe’s largest Storage as Transmission Asset installation. The system, split across sites on Menorca and Ibiza, is the first project of its kind in Spain.

The project sits where two grid pressures meet on island systems. There is the need to squeeze more value from existing submarine cable links without laying expensive new ones, and the need to absorb rising renewable output without destabilising grids that are only weakly connected to the mainland. The Storage as Transmission Asset (SATA) classification is the distinction that matters here: this is not a standalone battery farm but a piece of the transmission network itself.

Here is how the Balearic project came together, what makes its regulatory classification significant, and what the commissioning timeline tells investors and grid planners watching the European battery energy storage market.

A transmission asset disguised as a battery: what the SATA classification actually means

Most readers have encountered battery farms and grid-scale storage. Far fewer have met a battery that is legally and functionally a piece of the transmission network.

That is what a Storage as Transmission Asset is. A SATA battery is classified not as generation and not as a standalone flexibility service, but as a component of the transmission grid, the same category as a power line or a substation. That single classification changes how the asset is planned, how it is regulated, and how its costs are recovered.

Island grids are the natural proving ground for this idea. Physical isolation, constrained interconnectors, and the sheer cost of laying new submarine cables create the exact conditions where storage-as-transmission makes more economic sense than building new physical infrastructure.

The renewable integration imperative driving Red Eléctrica’s decision sits within a wider structural shift: system operators across Europe are increasingly treating storage not as an add-on to renewable build-out but as a prerequisite for grid stability at high penetration levels.

Why Red Eléctrica chose batteries over cable

Red Eléctrica applied a specific rationale to the Balearic system. Rather than replace the existing submarine links between the islands or lay new ones, the batteries at Mercadal and Sant Antoni are designed to squeeze more throughput out of the cables already in the ground.

The architecture breaks down as follows:

• Total system: 140 MW power and 105 MWh of energy storage across two sites
• Mercadal (Menorca): 50 MW / 37.5 MWh, connected at a 132 kV substation
• Sant Antoni (Ibiza): 90 MW / 67.5 MWh, connected at a 66 kV substation

Red Eléctrica’s stated purpose is to optimise the existing submarine cable interconnectors, increase inter-island exchange capacity, and reduce both system costs and CO2 emissions.

Red Eléctrica’s stated purpose The batteries are designed to maximise the contribution and benefits associated with the existing submarine links between the islands.

The SATA label is not marketing shorthand. It signals that this battery will be planned, regulated, and cost-recovered like a power line, which has direct implications for the revenue model and long-term contractual certainty. If you are assessing this project’s economics, that classification is the prerequisite: it separates the Balearic system entirely from merchant batteries chasing volatile spot prices or capacity-auction revenue.

From tender to two substations: how a €99.87 million contract was built and delivered

The build was a sequence of decisions, each one locking in the project’s final shape.

The procurement clock started when the tender closed in December 2023. NHOA Energy won the competitive process and was awarded the contract in November 2024, with a value of €99,874,250.09. Execution activities began on 18 November 2024.

From there, the two-site build was staged rather than launched simultaneously. Construction at Mercadal on Menorca started in February 2025, carrying a capital investment of €50 million for two 25 MW units. Sant Antoni on Ibiza followed in mid-May 2025, with a budget of €76 million for its larger 90 MW installation.

Staggering the two islands was a deliberate risk-management choice. Each battery can be installed, tested, and commissioned independently, which reduces the operational exposure of the Balearic grid at any single point during construction.

The engineering choice underneath the numbers is just as telling. Both sites use a modular cabinet architecture built around CATL battery cabinets, rather than the conventional large-container BESS format. Each cabinet runs its own independent liquid cooling system, a design intended to extend equipment lifespan and simplify maintenance.

The choice of CATL battery cabinets for both Mercadal and Sant Antoni connects the Balearic project to a broader shift in the CATL supply chain, where the manufacturer’s 2026 agreements have locked in 270 GWh of capacity commitments across European grid-scale projects.

That modularity is not an abstract feature. At Mercadal, delivering the 50 MW / 37.5 MWh rating takes 176 individual modules, which shows what modular scale looks like in practice. The format lets installation be tailored per site and keeps maintenance logistics manageable.

A unified control platform ties the two islands together, so Mercadal and Sant Antoni operate as a single coordinated resource rather than two isolated batteries.

The procurement and build sequence ran as follows:

1. Tender closed (December 2023)
2. Contract awarded to NHOA Energy (November 2024)
3. Execution activities began (18 November 2024)
4. Mercadal construction start (February 2025)
5. Sant Antoni construction start (mid-May 2025)
6. Commissioning phase (2026)
7. Anticipated full completion (early 2027)

For anyone tracking European transmission procurement, this contract is a working template for how a system operator moves from competitive tender to modular build, and it is a template more operators are starting to follow.

Commissioning in progress: where the project stands in September 2026

The project is now in the phase where hardware becomes a functioning grid asset.

As of 17 September 2026, Tera Batteries is carrying out commissioning and equipment verification at both sites, according to reporting by Pilar Sanchez Molina for pv magazine España and El Periódico. The project is described as progressing as planned, with no reported amendments, cancellations, or revised delivery terms on the NHOA Energy contract.

Commissioning is not a single switch-on moment. For a SATA installation of this scale, it means technical compliance verification, grid integration testing, and confirmation that the unified control platform linking both islands functions exactly as designed before the system is handed to Red Eléctrica for operational use.

The remaining timeline is short. Full completion is anticipated in early 2027, which puts the gap between now and handover at a matter of months rather than years.

The characterisation Red Eléctrica and NHOA describe the Balearic system as one of the most relevant energy storage projects developed in Spain.

That NHOA is delivering this on schedule matters more when you see what else the firm is building at the same time:

• Kallo, Belgium: 400 MWh BESS, groundbreaking May 2025
• Drogenbos, Belgium: 80 MW / 320 MWh, construction started May 2026
• Italy (MACSE-backed): 90 MW / 600 MWh across two projects, secured March 2026

A clean commissioning narrative, no delays and no public contract modifications, alongside a credible early-2027 date, tells you this project is executing on schedule. That is not a given at the 100+ MW scale. For anyone tracking contractor capacity in the European BESS supply chain, the Balearic project’s progress alongside NHOA’s concurrent Belgian and Italian deliveries is a live data point on execution reliability.

Where the Balearic project sits in the wider European battery storage landscape

The superlative attached to this project needs careful handling, because it can carry more weight than it should.

The Balearic system is the largest Storage as Transmission Asset in southern Europe and the first SATA deployed in Spain. It is not, however, among Europe’s largest batteries by total energy capacity. Those are two different claims, and conflating them misreads the project’s significance.

The scale comparison makes the point clearly.

By raw energy, Coalburn 1 (described as currently Europe’s largest BESS), Vilvoorde, and the planned GigaBattery Jänschwalde 1000 all dwarf the Balearic system. That is exactly why the SATA distinction matters: the Balearic project’s importance is about function and classification, not size.

The broader trend it exemplifies is the real story. Transmission and system operators across Europe are increasingly procuring batteries as quasi-grid assets, tied into capacity mechanisms or integrated directly into transmission planning, rather than treating them as merchant or standalone plays.

The broader trend it exemplifies is the real story: the European battery storage market recorded 78% growth in recent additions, yet grid connection constraints are increasingly acting as the ceiling that modular, TSO-integrated assets like the Balearic SATA are specifically designed to push against.

The policy scaffolding is already visible. Belgium’s capacity remuneration mechanism and Italy’s MACSE scheme both remunerate storage as a long-term grid asset with revenue certainty.

Institutional recognition ENTSO-E’s Ten-Year Network Development Plan and cost-benefit analysis guidelines now recognise storage projects alongside transmission projects, formally folding batteries into the transmission-planning toolkit.

ENTSO-E’s TYNDP 2026 project portfolio now includes 199 transmission and 69 storage projects, a composition that formally embeds batteries into the same European network planning framework as power lines and substations.

What this means for anyone assessing the European storage market: the trajectory from standalone merchant batteries toward TSO-integrated, SATA-type assets with long-term regulatory backing is a structural change in the risk and revenue profile of grid-scale storage. The Balearic project is one of the clearest current examples of that shift.

What the Balearic SATA completion means for island grid strategy and the investors watching it

Once operational, the system delivers a specific set of capabilities to the Menorca and Ibiza grids.

Red Eléctrica is targeting three concrete benefits, alongside one constraint worth naming plainly:

• Higher throughput on the existing submarine cables, increasing inter-island exchange capacity
• Improved frequency stability for the Menorca and Ibiza grids, drawing on the millisecond response batteries provide
• Greater renewable absorption without building new physical infrastructure
• The constraint: short duration at full discharge, discussed below

Here is the number that calibrates everything. At 105 MWh and a full 140 MW output, the system would sustain maximum discharge for roughly 45 minutes. This is not a system built to power the islands through a multi-day outage.

That 45-minute figure is not a design flaw. It tells you exactly what the system is built to do: smooth short-term congestion and frequency events on the submarine links, not provide backup energy supply. Expert literature reinforces the boundary. A NYSERDA-supported study found that adding up to 12-hour battery storage cuts supply-interruption risk by around 10 percentage points for island-mode operations, but that effectiveness falls away for longer-duration outages. Short-duration storage is a congestion and stability tool, not a seasonal energy solution.

For investors wanting to situate the Balearic project within the longer demand trajectory, our full explainer on global battery capacity growth to 2030 covers the IEA projections and the technology cost curves that make TSO-integrated storage increasingly cost-competitive against new cable construction.

The replicability question for Mediterranean and Atlantic island systems

Whether the Balearic model transplants elsewhere depends on the target grid’s characteristics.

The model applies most directly to island systems that already have submarine interconnectors, hold high renewable ambitions, and face tourism-driven demand peaks. These are the conditions where squeezing more from existing cable capacity beats laying new line.

It applies far less cleanly where the challenge is different. Islands facing multi-day storage gaps, heavy seasonal load variation, or the absence of any existing interconnector need additional or entirely different solutions, because short-duration storage cannot fill those gaps.

If the Balearic system completes on schedule and performs as designed, its early-2027 finish positions it as the reference case for any European TSO weighing SATA procurement in 2027 or 2028. Before projecting the model onto other grids, though, investors and planners need to hold both its demonstrated capability and its duration limits in view at once.

The grid operator’s bet on batteries over cables, and what early 2027 will confirm

Red Eléctrica has made its wager, and it is largely built. The bet is that integrating 140 MW of battery storage into the transmission network is a cheaper and faster path to island grid reliability than new submarine cable construction. That wager gets tested in live operation starting early 2027.

The stakes extend well past the Balearic Islands. The first large-scale SATA in Spain, completing on schedule with CATL hardware and NHOA integration, establishes a procurement and engineering blueprint that other transmission operators will study closely.

The framing claim Red Eléctrica and NHOA describe the system as the largest SATA in southern Europe and the first of its kind in Spain.

Three variables will decide whether the model is judged a success: actual throughput improvement on the submarine links, frequency stability metrics after commissioning, and whether renewable curtailment on the islands falls as projected. The €99.87 million contract remains in force with no public amendments, and commissioning partner Tera Batteries was active on site as of 17 September 2026.

Early 2027 is the near-term event to watch. It will give the European transmission community live operational data on whether battery storage can functionally substitute for physical interconnection at island-grid scale, either strengthening the case for SATA procurement across southern Europe or surfacing the limitations the engineering debate has already anticipated.

This article is for informational purposes only and should not be considered financial advice. Investors should conduct their own research and consult with financial professionals before making investment decisions. Past performance does not guarantee future results, and forward-looking statements are speculative and subject to change based on market developments and project performance.