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What disappeared was voltage support. Not momentum.

At 12:33 local time on 28 April 2025, mainland Spain and Portugal went dark in their entirety. ENTSO-E, the association of European transmission operators, calls it the most serious blackout in Europe in more than twenty years. Solar's share of midday generation that day was extremely high.

In the immediate aftermath, the explanation that spread was "too much renewables, not enough momentum." Yet the Spanish government's report (17 June 2025), the transmission operator REE's report (18 June 2025) and ENTSO-E's final report (20 March 2026) all placed the primary cause elsewhere.

Voltage rose, and generators dropped off one after another

ENTSO-E's final report attributes the cause to a combination of grid oscillations, inadequate voltage and reactive-power control, differing approaches to voltage regulation, generators in Spain abruptly reducing output and disconnecting in succession, and uneven stabilising capability. The sequence went like this. Voltage begins to rise on the midday grid. There are too few generators playing the role of holding it down. To protect themselves from the excess voltage, plants disconnect via their protection devices. The more that disconnect, the further voltage rises. This chain ran its course in tens of seconds, the interconnector with France separated, and the whole system collapsed. The Spanish government's report notes that of the ten synchronous generators scheduled the previous day to perform voltage regulation, the number actually connected on the day was the lowest since the start of the year, and that some of the scheduled plants did not regulate voltage as required.

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How the blackout unfolded (as set out in ENTSO-E's final report)Midday: voltagebegins to riseHigh solar shareFew spinning machinesToo few playingthe holding-down roleOf the ten scheduled units,some did not perform (government report)Generators disconnecton protectionThe more disconnect,the further voltage risesThe chain runs on tosystem-wide blackout12:33 CESTAll of Spain and Portugal
Fig. 1 — The sequence of 28 April 2025. The primary cause was a shortage of voltage support. The Spanish government notes that of the ten synchronous generators scheduled the previous day for voltage regulation, the number connected on the day was the lowest since the start of the year, and that some did not regulate voltage as required. Sources: ENTSO-E final report, 20 March 2026; Spanish government (MITECO) report, 17 June 2025.

No report — not the Spanish government's, not REE's, not ENTSO-E's — names a shortage of momentum (inertia) as the primary cause. At the launch of the final report, ENTSO-E board chair Damian Cortinas said the problem was not renewables but voltage control, irrespective of generation type (Science Media Centre España).

Why a generator can hold voltage

Voltage is the pressure that pushes electricity out onto the line. Like water pressure in a pipe, equipment breaks if it runs too high or too low. A spinning generator can raise or lower that pressure by strengthening or weakening the magnetism inside it (excitation). Strengthen it when the push is insufficient; weaken it when it runs too high. Doing this automatically, without human instruction, is what voltage support means. Solar and batteries can do the same thing if their inverters (the devices that turn direct current into alternating current) are configured for it. In Spain at the time, however, it was not required of them.

Of the three free extras seen in 48-2, the one in short supply that day was voltage support. On a midday grid with fewer spinning machines, unless somebody takes on that role, the same thing happens.

Spain's answer came fast. Two months after the blackout, on 12 June 2025, it changed its operating rules so that solar and wind could take part in voltage regulation, with full implementation on 17 March 2026 (SolarPower Europe). In February 2026 the transmission operator REE disclosed that it has secured 16 GW of connection capacity for batteries. After the event, holding voltage changed from a free extra of spinning machines into a job somebody is asked to do.

In Japan

In Japan, voltage support is secured through the technical requirements of the network companies (the wheeling supply tariff and the grid interconnection technical requirements); it is not a priced market. Who holds voltage once spinning machines decline is being examined by OCCTO's Grid Code Study Group. A battery, if configured for it, is equipment usable for both voltage regulation and frequency regulation.

How an investor should read this
Incidents move regulation. Britain built a one-second product the year after its August 2019 blackout (48-4); Spain brought renewables and batteries into voltage regulation two months after its 2025 blackout. In Japan, who should be asked to perform the momentum and voltage roles is under examination. Either way, one of the resources able to take on that role is battery storage — and because it can serve both voltage and frequency, its range of use is wider than other non-spinning resources.
The questions in this series
I. Why the grid ran without batteries in the past
  1. 48-2The grid used to run fine without batteries, didn't it?
  2. 48-3What disappears from a grid with fewer spinning machines? (this article)
  3. 48-4Why "ten seconds"?
  4. 48-5Does more renewables mean more balancing power is needed?
II. Why thermal, nuclear and pumped storage are not enough
  1. 48-6Can't thermal just do the balancing?
  2. 48-7If nuclear grows, do we stop needing batteries?
  3. 48-8Wouldn't more pumped storage be enough?
  4. 48-9Where does the balancing power that the market failed to buy come from?
III. Where demand and the generation mix are heading
  1. 48-10Why do data centres point to batteries?
  2. 48-11What happens to solar from here?
  3. 48-12Thermal volume, nuclear operation, demand — what next?
  4. 48-13How much battery capacity is actually coming?
IV. The money, and what the state really wants
  1. 48-14Does the government actually want more batteries?
  2. 48-15What does battery storage resemble as an infrastructure investment?
  3. 48-16They say the earnings will thin out. Is that true?
  4. 48-17Can you copy the overseas playbook and make money in Japan?
  5. 48-18How much battery storage will Japan ultimately need?
  6. 48-19Is a battery the same wherever you put it?
The one question beyond the 1848-20 What do you pay, and what comes back?The answer differs by project, so we have built a calculator whose dials are made only from published figures. You can move unit price, clearing rate, seat fee, arbitrage spread and construction cost yourself.

Sources

Supervised by
Shinya Nakashima(Representative Director, Science X Inc.; Ph.D. in Engineering)

Works on the development, sale and technical due diligence of grid-scale battery storage plants. This column is written and supervised on the basis of hands-on transaction and evaluation practice.