Electricity is a strange product: you can’t store it easily at the scale of a country, and it has to be produced the instant it is used. So somewhere, someone has to match supply and demand minute by minute. In Spain and Portugal, the place where most of that matching happens is called OMIE. It runs two kinds of auctions - the day-ahead market and the intraday market - and together they set the price of almost every megawatt-hour consumed on the peninsula. For a battery, understanding how OMIE works is the first step to understanding how it earns money.
The day-ahead auction
The day-ahead market is a pay-as-cleared, uniform-price auction for the following delivery day. Generators, retailers and cross-border traders submit buy and sell curves, and OMIE matches them against each other for every delivery slot of the next day. The clearing price is the intersection of aggregated supply and demand in each slot. Until 1 October 2025, slots were hourly - a single price for each of the 24 hours. From 1 October 2025 onwards, the day-ahead market clears in quarter-hourly resolution, one of the biggest structural changes the Iberian market has seen in a decade.
A battery dispatching against the day-ahead curve no longer has 24 prices to optimise against; it has 96. Morning ramps, midday solar troughs and evening peaks all now appear at 15-minute resolution. The shorter clearing window typically widens intraday spreads (sharper peaks and troughs are no longer averaged into hourly blocks) and, in principle, makes a fast-responding asset more valuable.
24 hourly
96 quarter-hour
Intraday auctions and the continuous market
OMIE's second layer is the intraday market, which lets participants adjust positions after the day-ahead has cleared. Since 13 June 2024, Iberia participates in the three pan-European intraday auctions (IDAs) that run across the SIDC (Single Intraday Coupling) area, alongside the continuous cross-border XBID order book. The IDAs clear at fixed gate times each day; the continuous market runs between gates and closes a short time before delivery.
Intraday is where most short-notice corrections happen: a solar plant revising its output forecast, a wind farm adjusting to a weather update, or a battery repositioning itself around a price spread it did not capture in the day-ahead. Since March 2025, the intraday continuous market and its auctions trade in quarter-hourly products, and day-ahead followed in October 2025. Intraday spreads relative to day-ahead have been reported at ±€50/MWh in Spain during late 2025, confirming that arbitrage opportunities exist; liquidity remains lower than in the UK or German continuous markets, which is a practical constraint for high-frequency algorithmic strategies.
Balancing and ancillary services
Energy markets (day-ahead and intraday) are only part of the picture. After gate closure, Red Eléctrica de España (REE), as Iberian TSO, operates balancing and ancillary mechanisms - secondary and tertiary reserves, congestion management, voltage and frequency control. Batteries are increasingly eligible for these services, and the reform path Spain has taken since 2024 is pointed at opening more of these mechanisms to storage. For most standalone BESS projects today, the economic anchor is still energy arbitrage in day-ahead and intraday, with ancillary revenues layered on top as the regulatory framework opens up.
What it means for a battery
A two-hour battery in Spain now dispatches against 96 price blocks per day across three temporally overlapping venues - day-ahead, intraday auctions, and intraday continuous - each with different liquidity, different closing times and different strategic purposes. The day-ahead is where most capacity is still booked. The intraday auctions are where forecast corrections land. The continuous market is where algorithmic, short-notice trading captures residual spreads. Optimising across all three simultaneously, while respecting the battery's own state of charge and degradation cost, is the core operational problem for any Iberian BESS.
| Venue | Cadence | Closes | Resolution | What a BESS uses it for |
|---|---|---|---|---|
| Day-ahead | 1 auction / day | 12:00 D-1 | 15-min, since 1 Oct 2025 | Anchor schedule. Most capacity booked here. |
| Intraday auctions | 3 auctions / day (IDA1-3) | 15:00, 22:00, 10:00 | 15-min, since Mar 2025 | Re-clear after wind/solar forecast updates. |
| XBID continuous | order book, ~24/7 | ~1h before delivery | 15-min | Capture residual spreads, hedge imbalance. |
| Balancing & ancillaries | real time | delivery hour | seconds to minutes | FCR, aFRR, mFRR, RR - layered on top. |
The quarter-hourly transition in 2025 is structural. It raises the value of fast, accurate forecasting and of low-latency execution, and it pushes the industry closer to the operational tempo of the ancillary markets. Expect spread structure, capture prices, and the value of flexibility to continue shifting as the market settles into the new tempo through 2026.