Day-ahead market

Across most of the EU the day-ahead market runs as Single Day-Ahead Coupling (SDAC), a joint scheme of power exchanges (NEMOs) and transmission system operators. Every day one algorithm, PCR EUPHEMIA, matches buy and sell orders for the following day across all coupled bidding zones and allocates cross-border capacity at the same time. Its outputs are clearing prices, matched trades, scheduled cross-border exchanges and the net position of each bidding zone.

Orders close at 12:00 CET on the day before delivery (D-1). Since trading day September 30, 2025, with first delivery on October 1, 2025, the market time unit is 15 minutes, so a normal delivery day has 96 periods. Orders can still be placed in 15, 30 or 60-minute resolution. The current price limits are -600 to +4,000 EUR/MWh: Nord Pool's product specification effective September 30, 2025 set them at -500 and +4,000 EUR/MWh, and from trading date May 28, 2026 the NEMO Committee replaced the -500 floor with a harmonized minimum clearing price of -600 EUR/MWh. Negative prices occur in normal operation, so they have to be handled as regular data.

In Poland the day-ahead market is RDN (Rynek Dnia Następnego) on TGE, the Polish power exchange. RDN lists 15-minute, hourly and block contracts, and TGE's TGe24 index is the arithmetic mean of the 15-minute contract prices for a delivery day.

The intraday market lets participants correct positions after the day-ahead auction closes, when wind, solar and demand forecasts change. In the EU it runs as Single Intraday Coupling (SIDC), launched in June 2018, which works on a shared order book across coupled zones.

It has two parts. Continuous trading matches orders one by one; for the next delivery day it opens after day-ahead prices are published, normally at 14:00 CET. Each zone has its own gate closure: in Nord Pool's September 2025 specification it is 60 minutes before delivery in Poland, 5 minutes in Germany and 0 minutes for local trades in several western zones. Intraday auctions (IDAs) have run since June 13, 2024, three times a day: at 15:00 and 22:00 CET for the whole next day, and at 10:00 CET for 12:00 to 24:00 the same day. All IDA products are 15 minutes long.

The cross-zonal intraday gate closure is moving closer to real time. Regulation 2019/943, as amended in 2024, requires it to be no more than 30 minutes ahead of real time from January 1, 2026, and ACER's Decision 11/2025 brought the gate closure methodology in line. A national regulator may grant a TSO a derogation until January 1, 2029, which can be extended once by up to two and a half years. All Polish borders use such a derogation, with the move to 30 minutes planned for the end of 2028.

Any system that uses day-ahead prices, for cost forecasts, battery schedules or bidding, has to handle the following:

• A day has 96 prices, not 24. Store them per delivery period with UTC timestamps. On daylight saving changeover days a day has 92 or 100 quarter-hours. History before October 2025 is hourly, so charts, averages and indices must handle mixed resolution.
• Results come after the 12:00 CET gate closure and can be delayed. Fetch jobs need retries, completeness checks and a backfill process for missing periods.
• The ENTSO-E Transparency Platform publishes prices per bidding zone through an API. Exchanges such as TGE, EPEX SPOT, Nord Pool, OTE or OPCOM publish their own data, often under paid licenses. A scraper of a public web page breaks whenever the page layout changes.
• Price fields and optimization logic must accept values below zero and respect the current harmonized limits, which change over time.
• Polish prices are quoted in PLN and most others in EUR, so the system needs a stated exchange-rate source and date for every conversion.
• If the software submits orders, it needs order types, deadlines in CET/CEST, confirmations, position tracking and an audit trail.

Since 2020 we have been building a platform for Enerace, an energy procurement advisory. It pulls market data from more than a dozen sources, including ENTSO-E (spot prices, generation and load), TGE (first a crawler, later an API), ICE, OTE, OKTE and OPCOM, for markets such as Poland, Czechia, Slovakia, Romania, Austria and Croatia. We adapted it to the 15-minute market time unit of the day-ahead market from October 1, 2025, then added 15-minute price imports from ENTSO-E in 2026 and a fetch that runs after the day-ahead results are out.

For Zeronest we built an energy management engine that decides every minute whether a home battery charges, discharges or sells to the grid. It weighs day-ahead and intraday prices against tariffs, consumption and the weather forecast, and since October 2025 it reads RDN prices from TGE. Earlier, in 2022 and 2023, we built a tool for Nuvve to monitor and bid on the Danish energy market.