12 months after the Iberian blackout: the system absorbs the same, pays differently

Updated 2026-05-21 · Thanks to Sergio B. for the expert feedback. We initially called RT3 “tertiary reserve” — RT3 is actually the consumer-side cost of technical constraints in the day-ahead market (REE redispatching the OMIE clearing to keep synchronous units online for system security). All data and the ×2.5 step are correct; the mechanism is now precisely identified as the consumer-side print of REE’s Operación Reforzada introduced after April 28, 2025, as documented in PwC’s February 2026 report.

On April 28, 2025 the Iberian power system went down for five hours. Twelve months later, with full data, we can ask what actually matters: what changed in how the grid is operated day to day?

The most direct indicator is the daily cost of technical constraints in the day-ahead market (RT3) — the line item consumers pay when REE has to redispatch the OMIE clearing to keep synchronous units online for system security. Before the blackout it moved within a narrow band (~3-8 €/MWh). The month of the blackout itself shows the first visible step. Since then, the baseline has settled into another regime: the post-blackout average exceeds 14 €/MWh, double the pre-blackout average of the prior year. The mechanism behind this step is now well-documented — REE introduced Operación Reforzada with substantially stricter security criteria, forcing more synchronous capacity (mainly combined cycles) to be online at all times to prevent another collapse.

What’s interesting is that this change coexists with others pointing in opposite directions. Wholesale price (OMIE) has dropped by 23%. The renewable share is exactly the same — 61.5% pre, 61.7% post. Hours with negative prices have quadrupled. The system absorbs the same amount of renewable, but the mechanics by which it does so have completely changed.

The operational mix: combined cycle replaces nuclear, not renewables

The initial intuition seeing “more RT3” would be to think REE has held back renewables to make room for firm generation. The data doesn’t back that up.

Comparing total generation by technology over the 365 days of each window, the moves are:

• Combined cycle: +9.5 TWh (+32%) — the single largest change in absolute terms. Goes from 11.4% to 14.7% of the mix. This is the technology REE has leaned on hardest to satisfy Operación Reforzada’s security requirements.
• Nuclear: −2.9 TWh (−5.4%) — scheduled outages and an aging fleet (Almaraz set to close in 2027, etc.). Drops from 20.7% to 19.0%.
• Solar PV: +5.5 TWh (+12%) and wind essentially flat — renewables keep growing, they’re not being held back.
• Cogeneration: −2.8 TWh (−17%) and coal −1.7 TWh (−25%) — both in continued structural decline.

Adding it up: inverter-based generation (PV + wind) rises from 40.7% to 42.1%; total synchronous drops from 59.3% to 57.9%. There was no shift toward synchronous generation in the day-ahead market. What did happen: combined cycles being redispatched (via technical constraints) on top of the OMIE clearing, to cover the nuclear decline and provide the firm capacity that the new security policy requires available. PwC quantifies this: CCGT volume used to resolve technical constraints rose +39% (May–Dec 2025 vs 2024).

The immediate question — shouldn’t more CC running push OMIE up? — has an answer that comes from the intraday profile.

The intraday paradox: OMIE collapses at midday, RT3 rises all day

If you average the full day, OMIE drops 23% and RT3 rises 147%. But the hourly mechanics are more revealing.

OMIE — the wholesale curve has been flattened at its solar valley:

• The price in peak-solar hours (13-16h) falls from €42-46/MWh to €21-26/MWh — half.
• The evening peak (22-23h) barely drops (€106 → €91-95), because there’s no solar there.
• The result is a much deeper “duck curve”: the gap between solar valley and evening peak goes from ~€62/MWh to ~€74/MWh — the system has amplified its intraday volatility.

RT3 — constraints cost rises in every hour:

• The largest absolute increase happens at midday (+11.4 €/MWh at 14h) — paradoxically, when OMIE is lowest. REE is paying for expensive redispatch precisely in the hours when the wholesale market is giving energy away. The reason: with so much PV winning the OMIE clearing at midday, synchronous capacity gets displaced exactly when the grid needs more of it for stability — so redispatch costs concentrate there.
• The largest relative increase happens in the evening ramp (+344% at 20h, +300% at 21h) — when the sun sets and thermal plants need to come online quickly, and Operación Reforzada requires them to already be warm.

The physical reading: in the post-blackout regime, REE keeps redispatching synchronous units all day long — including the midday valley hours when renewable generation is maximal — precisely because the abundance of renewables in those hours leaves the grid short of inertia and reactive power. The constraint isn’t against the renewable being there; it’s against the synchronous machines not being there to provide stability services the market doesn’t price.

Negative-price hours have quadrupled

When a power system has more renewable than it can absorb, the wholesale price goes to zero or below: you pay to place your energy, instead of being paid for it. It’s the rawest signal of structural oversupply.

• Pre-blackout: 733 hours with negative OMIE over 365 days.
• Post-blackout: 3,195 hours with negative OMIE over 365 days. ×4.4 in twelve months.

The distribution is very concentrated. In the post period, hour 16h registers 437 negative-price events over 365 days — more than one per day on average. Hours 14-17h all have more than 400 events. Practically every day of the year after the blackout had several hours of negative OMIE in the solar window.

This is the symmetric reverse of the RT3 increase: the OMIE market clears massively-renewable, the price goes negative, and simultaneously REE is paying more and more (via technical constraints) to keep thermal plants warm and dispatchable for security. The renewable transition hasn’t slowed down; it’s become operationally more expensive, and the cost is split between whoever receives OMIE on the way down and whoever pays tolls on the way up.

What the blackout changed

Four quantitative facts, all defensible against a symmetric 365-day window around April 28, 2025:

1. The daily cost of technical constraints in the day-ahead market (RT3) has multiplied by 2.5 (5.93 → 14.68 €/MWh average). The first step is visible in the month of the blackout itself, and the mechanism is REE’s Operación Reforzada — stricter security criteria requiring more synchronous capacity online at all times. PwC corroborates this directly: monthly cost of technical constraints rose +55% over May-Dec 2025 vs the same months of 2024.

2. The share of renewable generation absorbed has not changed (61.49% pre vs 61.68% post). REE hasn’t slowed the transition. The apparent mix reorganization is internal to synchronous generation — combined cycle +32% substituting for declining nuclear (−5%), mostly via redispatch on top of OMIE.

3. Midday wholesale price has collapsed by 50% (from €42-46 to €21-26/MWh between 13h and 17h). The solar valley of the “duck curve” has deepened dramatically.

4. Hours with negative wholesale price have quadrupled (733 → 3,195). Hour 16h has more than one negative-price event per day on average.

The system hasn’t reversed the renewable transition. What it has done is reorganize its costs: less OMIE for the consumer who pays based on the wholesale market, more RT3 (tolls and charges) for whoever pays based on the total system cost. The blackout didn’t stop renewable integration; it triggered a regulatory tightening (Operación Reforzada) that made the cost of stability services explicit and shifted it onto the consumer bill.

This is what we see twelve months in. If the convexity observed in RT3 — the most recent renewable penetration points cost disproportionately more, as the structural cost analysis suggests — continues, the next twelve months will tell us whether the system has found a new equilibrium or is still adjusting.

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The same queries are available in ESIOS Data — explore for yourself what changed since April 28, 2025.