Spain is building batteries for a market you can't see

For most of the last century, noon was an expensive hour in Spain — peak demand, peak price. Solar inverted it. On a clear day in 2026 the wholesale price at 14:00 is routinely near zero, sometimes negative: the country makes more power than it can use while the sun is at full strength.

Near-zero intervals have gone from a few hundred a month in 2024 to well over a thousand in 2025 and again in early 2026. The midday glut is structural now. The obvious answer is storage — charge the worthless midday energy, sell it into the evening peak — and batteries are arriving fast. But when you go looking for what those batteries actually earn, the data does something strange: it goes dark.

Four questions about the glut and the batteries

1. How big is the midday solar glut, and is it still growing?
2. How much solar is turned down at noon — and why?
3. Do batteries actually charge the cheap hours and sell the peak — and does it pay?
4. If batteries are scaling, why can’t we see what they earn?

What the grid turns down at noon

Before storage, the system had one tool for unwanted solar: turn it off. The shape of that curtailment says everything about why batteries showed up.

Across 2024–2026 the grid curtailed 3.9 TWh of solar for technical restrictions — the network physically unable to take it — and 82% of it falls between 10:00 and 16:00, peaking right at 13:00. (A separate, larger economic-redispatch flow exists, but that’s the market re-balancing on price, not curtailment.) The grid-driven part is the harder number, and it lands squarely on the midday hours where the price has already collapsed.

What you can see: charge the hole, sell the peak

A battery’s answer is to move that midday energy in time — buy it when nobody wants it, sell it when everybody does. REE meters the whole fleet doing exactly that, in real time.

Across Oct 2025–Mar 2026, Spain’s batteries charged 5.4 GWh and discharged 4.6 GWh, and the daily shape is textbook: a deep charging trough through the midday solar glut and pre-dawn dip, then 69% of all discharge fired into the 18:00–21:00 evening peak, tracking the price curve almost exactly. This is the entire promise of grid storage in one day’s profile — soak up the glut, release it into scarcity. It’s metered, not modelled. What it can’t tell you is which units, or what they were paid.

And it pays — but it’s the minority stream

The trade pays, and not just for one unit. Every standalone battery in the data buys low and sells high.

Genera, Ignis, Iberdrola and EDP all capture a positive spread — from €36 to €97/MWh — charging at €18–40 and discharging at €54–116. UPXEN alone cleared about €66,000 of gross arbitrage margin in four months on 5 MW. Real money.

But it is the smaller half of the story, and the data is quietly misleading here. Bolt a battery onto a solar plant and the arbitrage uplift nearly vanishes — the plant’s grid connection is already saturated by its own panels, so there is no room to sell the stored energy on top. Energy arbitrage is simply what public data lets you price. It is not where batteries make their money.

Where the money really is — and the day it went dark

Where batteries actually earn is reserve — getting paid to hold capacity ready for the grid’s second-by-second balancing. In Spain that’s the secondary regulation band (aFRR). And that is exactly where the public record stops.

Until November 2024, REE published secondary-band assignments per unit — up to 229 units in a single month, the gray bars. Then, on 2024-11-20, Spain’s aFRR reform moved the reporting from per unit to per balancing-service-provider; full coupling to the European PICASSO platform followed on 2025-06-17. The per-unit series ends cold. Solar wasn’t barred from the market — plants like Valdesolar had been providing band all along — the granularity simply disappeared.

The timing is the story. The blue line is the grid-storage fleet, and 17 of its units came online after the cutoff. The technology built precisely for the reserve market scaled into a market that, in public data, had just gone dark.

How big is the blind spot? Spain holds about 1,200 MW of upward secondary band around the clock, paid roughly €27/MW each hour — on the order of €200 million a year. Against that, the entire arbitrage margin we can measure — every standalone battery, all of it — adds up to a few hundred thousand euros. Per megawatt, holding band is worth several times what shifting energy earns. The market that decides whether a battery pays for itself is both the larger one and the one no longer reported unit by unit — we can size it, but we can’t assign it to a machine.

A boom you can’t audit

Spain’s midday glut has an obvious destination: charge a battery when power is worthless, sell when the evening peak arrives. The standalone fleet already does it, profitably — a €40-to-€87 round trip you can see and price. And the fleet is scaling fast.

But energy arbitrage is the smaller half of the story. Batteries earn most from holding reserve — secondary-regulation band — and on 2024-11-20 that market moved to per-participant reporting, right as the boom began. The result is a storage build-out whose primary revenue is, by regulatory design, unobservable per unit. We can watch the batteries arrive; we can price the part that barely matters; and the part that does, the public data no longer lets us see.

And it isn’t only a Spanish quirk. We checked ENTSO-E, Europe’s transparency platform, too: its per-generation-unit feed excludes batteries below roughly 100 MW, its aggregate “energy storage” type is incomplete for Spain, and its balancing data is published per zone. No public source — national or European — resolves an individual battery’s performance. To know whether a Spanish battery pays for itself today, you need the operator’s own meter.