At approximately 13:00 CET in MIBEL, an operator receives the next day's 24-hour price profile. This day-ahead release begins the daily planning cycle. How those prices are used before delivery starts determines the battery's initial position.
About 11 to 13 hours separate day-ahead publication from the next delivery day. In that interval, automated dispatch can set overnight charge targets, select discharge price windows, and define the parameters governing the battery through the following morning, before intraday sessions update the picture. These settings determine the starting position for every later decision.
What day-ahead prices tell you and what they do not
Each day-ahead clearing price is the market's shared forecast of that delivery hour's supply and demand balance. It includes generator bids, demand estimates from large consumers, and implicit cross-border interconnector capacity allocation. At clearing, it is the best available price signal.
Day-ahead prices exclude changes between 13:00 CET on day D and delivery on day D+1. These include wind forecast revisions, cloud-cover changes affecting solar generation across the Iberian Peninsula, an industrial facility returning after an unplanned shutdown, and cross-border interconnector capacity changes. MIBEL's six intraday sessions provide those updates, so day-ahead prices guide overnight positioning but do not settle the final plan.
Treating day-ahead prices as fixed and leaving positions unchanged through intraday sessions systematically forfeits value. This is most costly on days with the widest spreads, when intraday revisions are also more likely to be significant.
Reading the day-ahead profile for overnight charging decisions
Overnight charging depends on the profile's lowest-price hours and the intended discharge windows. In many European markets, especially those with high renewable penetration, overnight hours have the lowest average prices. In MIBEL on a typical winter night, 01:00 to 05:00 CET may clear 20 to 35 EUR/MWh below evening peak prices.
An automated system identifies overnight hours below a configurable threshold, builds the charge schedule, and sets a target SoC that reaches the first expected discharge window with enough capacity. With a midday dip at 12:00 to 14:00 priced at 20 EUR/MWh and an evening peak at 19:00 to 21:00 priced at 110 EUR/MWh, it infers when to charge and discharge from the profile and the battery's current SoC.
Solar forecasts change the calculation. If day-ahead data indicates strong solar output from 09:00 to 15:00, the overnight target is lowered to leave headroom for midday solar absorption. The battery can then capture low-cost solar energy instead of charging from the grid overnight and lacking capacity when the surplus arrives.
The MIBEL intraday session 1 window
MIBEL's intraday session 1 opens late on day D for delivery hours on the coming D+1 day. It is the first following-day intraday signal and often differs from the corresponding day-ahead prices.
For early morning delivery, the gap between day-ahead and session 1 prices is generally modest. Between 13:00 and 17:00 the previous afternoon, the generation and demand outlook for 03:00 the next morning usually changes little. Later sessions matter more as delivery approaches, forecasts update, and the morning demand profile becomes clearer.
Session 1 offers an early test of whether the day-ahead overnight pattern still holds. If the 02:00 to 05:00 window clears significantly above day-ahead prices, overnight charging may be less attractive than expected. The dispatch system should flag the change and may move charging to the midday solar window.
Overnight target calculation for multi-site fleets
At fleet scale, overnight targets must be calculated independently because each site's inputs differ. Northern Portugal may have different overnight price dynamics from southern Spain when wind exposure in the regional mix is greater. A 20 kWh battery and an 8 kWh battery targeting the same evening peak face different capacity constraints and therefore different optimal overnight charge levels.
Manually performing this calculation for every fleet site is impractical after day-ahead prices publish at 13:00. Checking the profile and setting targets for ten sites can take an operator into late afternoon, while some sites may already be pre-positioning for the evening under the prior day's settings.
Automated configuration runs across the fleet within minutes of day-ahead publication. For each site, it uses battery capacity, current SoC, local tariff structure, and the location-specific solar forecast to set the overnight target. Each site begins delivery in an individually configured, coherent position rather than under one uniform setting.
What intraday sessions add on top of the overnight position
The overnight charge target establishes a starting position, not a permanent plan. As sessions clear the next day, the dispatch system revises each site's parameters using the new prices. If session 2 places afternoon prices 25 EUR/MWh above the day-ahead signal, it brings discharge forward at sites with sufficient SoC. If session 4 indicates softer evening prices, it may retain SoC for a later opportunity instead of discharging into a lower-priced window.
Overnight planning and intraday response form a sequential, cumulative process. A strong initial position leaves more flexibility for session updates. Too much or too little charge limits the available responses. Although overnight planning comes first, it has a disproportionate effect on the day's final result.
