Who generates what in Spain? A company-by-company view of the electricity market

Six companies generate most of Spain’s electricity. But their portfolios look nothing alike. Iberdrola leans on nuclear and hydro. Endesa mirrors that mix with less wind. Naturgy balances hydro, wind, and combined cycles. Repsol bets on renewables and flexibility.

The I90 settlement data reveals these differences at full resolution — not just annual totals, but how each company participates across every market program. This article maps the generation landscape of Spain’s electricity market, company by company, over a full year of settlement data from March 2024 to February 2025.

Monthly generation by company

The monthly view shows how each company’s generation volume evolves across seasons.

Iberdrola towers over the field. At 9,400 GWh per month on average, it generates more day-ahead volume than the next three companies combined. Endesa sits at a distant second with 4,600 GWh. The gap between them is nuclear — Iberdrola owns stakes in five of Spain’s seven operating reactors, while Endesa holds three.

Below the top two, the competition tightens. Axpo Iberia, Gnera, and Naturgy each hover around 1,700-2,200 GWh per month. These mid-tier companies rely on different mixes — Axpo on wind and solar, Gnera on renewables and cogeneration, Naturgy on a blend of hydro, wind, and combined cycles. The “Other” category captures dozens of smaller generators, collectively producing more than any single mid-tier company.

Portfolio fingerprints

Volume alone misses the strategic story. The technology mix reveals each company’s core bet.

Endesa is the most concentrated — 61% nuclear, 27% hydro. Two technologies account for nearly nine-tenths of its output. Iberdrola is similarly nuclear-heavy at 49%, but supplements with hydro (22%) and wind (12%), creating a broader base.

Naturgy stands apart. No nuclear, no single dominant technology. Hydro leads at 40%, wind follows at 37%, combined cycles contribute 12%. This is the most diversified portfolio among the top four — and it creates a different relationship with the market.

Repsol’s portfolio reflects a company in transition. Wind (24%) and solar PV (21%) lead, followed by pumped hydro (17%) and gas cogeneration (16%). No nuclear, no conventional hydro dominance. Among the major generators, Repsol has the highest renewable share — and, as the constraints data will show, that comes with a cost.

Constraints resolution: who earns, who gets curtailed

PDVP — the technical constraints resolution process — reveals which companies the system operator (REE) calls on for reliability, and which ones get curtailed. The diverging bars split upward redispatch (green) from curtailment (red).

Endesa dominates upward redispatch at +9,100 GWh — the go-to provider when the system needs more power in congested zones. Naturgy follows at +6,500 GWh. Both companies earn premium prices through PDVP, compensated above market rates for their dispatchable capacity.

The curtailment side tells the opposite story. Repsol loses -3,800 GWh to curtailment, netting -3,400 GWh overall. Gnera, Gesternova, and Galp — all renewable-heavy portfolios — face similar treatment. Their wind and solar output gets cut when transmission constraints bind.

Iberdrola’s position is the most striking. Despite being the largest generator by far, it nets just -387 GWh in PDVP. Its 3,300 GWh of upward redispatch nearly offsets 3,700 GWh of curtailment. Nuclear plants rarely get curtailed, but Iberdrola’s growing wind fleet pulls the aggregate down.

Market flexibility by company

Beyond PDVP, how much does each company trade across all market programs? The flexibility chart measures each program’s volume as a percentage of the company’s day-ahead schedule.

Naturgy is the most active trader. Its non-PDBF activity reaches 153% of its day-ahead volume — for every GWh scheduled in the auction, it trades another 1.5 GWh across subsequent programs. PDVP alone adds 88% on top of PDBF, and PHF1 intraday corrections add another 20%.

Iberdrola’s 61% total adjustment comes primarily from PHF1 at 24% — intraday corrections where it repositions after the day-ahead close. Its PDVP contribution is small in relative terms (-5%), despite large absolute volumes, because the PDBF base is so massive.

Acciona and Repsol show negative totals (-41% and -16% respectively). Their non-PDBF activity is dominated by curtailment, not trading. These companies schedule in the day-ahead market, then lose volume to constraints — the opposite of the flexibility premium that Naturgy and Endesa capture.

Six fingerprints in the data

Spain’s electricity market is not a monolith. Six companies dominate generation, but they do so with fundamentally different strategies. Iberdrola and Endesa anchor the system with nuclear and hydro baseload. Naturgy bridges traditional and renewable with a balanced portfolio. Repsol is making a renewables bet — and paying the price in curtailment.

The I90 settlement data captures these differences in granular detail. Not just how much each company generates, but where they earn, where they’re constrained, and how actively they trade across market programs. Every company leaves a fingerprint in the data — and no two look alike.

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To see how each market program works from a technology perspective, see Inside Spain’s Electricity Market Programs. For a plant-level deep dive, see How a Combined Cycle Plant Navigates Spain’s Electricity Markets.

All data comes from the datons library. Reproduce every chart with a few lines of Python.