On 30 September 2025 the Bulgarian power exchange ran its first day-ahead auction with quarter-hour prices. From delivery day 1 October 2025, the IBEX day-ahead market no longer sets 24 hourly prices a day but 96 prices, one for each quarter-hour. For a solar plant, whose output rises and falls within every hour, it is one of the more consequential market changes in years. Here is what changed and what to check.
Key terms
- IBEX (БНЕБ) – the Independent Bulgarian Energy Exchange, which runs the Bulgarian day-ahead and intraday markets.
- SDAC – Single Day-Ahead Coupling, the EU mechanism that clears the day-ahead markets of the participating European countries in one daily auction. IBEX is part of it.
- MTU – market time unit, the length of the interval that gets its own price. It is now 15 minutes.
- ESO (ЕСО) – the Electricity System Operator, Bulgaria’s transmission system operator (TSO), which receives schedules and settles imbalances.
- EWRC (КЕВР) – the Energy and Water Regulatory Commission, the regulator.
- Imbalance settlement period – the interval in which ESO measures and prices deviations from schedule: 15 minutes in Bulgaria.
What changed and when
| Item | Delivery until 30 Sep 2025 | Delivery from 1 Oct 2025 |
|---|---|---|
| Day-ahead prices per day | 24 | 96 |
| Length of a day-ahead price interval | 60 minutes | 15 minutes |
| Imbalance settlement period | 15 minutes | 15 minutes |
The switch was European, not only Bulgarian. According to IBEX, the 15-minute MTU went live in SDAC across all European bidding zones, with 30 September 2025 as the first trading day and 1 October 2025 as the first delivery day. The European Commission links the change to the Electricity Regulation (EU) 2019/943 and notes that the finer granularity is particularly important for variable sources such as wind and solar. ESO announced the successful launch of 15-minute products in both the day-ahead and the intraday segments.
In IBEX’s revised operational rules, each delivery day consists of 96 consecutive 15-minute intervals, as summarised by the law firm Dimitrov, Petrov & Co.. On the two days a year when the clocks change, ESO’s schedule instruction expects 92 or 100 intervals instead.
Orders are accepted until 12:00 CET, 13:00 Bulgarian time, under the IBEX operational rules. Because the calculation is now heavier, Nord Pool reports that preliminary results now come at 12:55 CET instead of 12:45 CET – 13:55 in Bulgaria.
Why it matters for a solar plant
Prices follow the solar curve more closely
A PV plant’s output climbs through the morning, peaks around noon and falls in the afternoon, changing noticeably within each hour. With hourly prices, that shape was averaged away. With 96 prices, the price of 10:00–10:15 can differ from the price of 10:45–11:00. On days with a solar surplus, low or negative prices can start and end on any quarter-hour, not only on the full hour.
Curtailment decisions are now per quarter-hour
If you curtail at low or negative prices, the decision unit is now 15 minutes. A plant can stop for exactly the negative quarter-hours and produce in the positive ones around them. The same applies to the minimum price you set – see negative electricity prices for how to choose it.
Trading and settlement use the same unit
Imbalances in Bulgaria have been settled per 15-minute period since 1 October 2022, under the definition in EWRC’s Trading Rules. Until 30 September 2025 the day-ahead product was still an hour. Now the traded unit, the schedule interval and the settlement period are all the same 15 minutes. In Economic.bg’s coverage of the switch, a trading executive of Electrohold noted that metering at 15-minute resolution now lines up with trading, and Dimitrov, Petrov & Co. list reduced exposure to imbalance costs among the expected benefits.
Four times as many values to forecast
The same Economic.bg article quotes Electrohold’s chief commercial officer: the frequency of the whole planning, forecasting and purchasing process increases fourfold. For a PV plant, that means a production forecast with 96 genuine values a day, not an hourly curve split into four equal parts. See Day-ahead production schedules for what a good schedule contains.
Contracts that refer to “the hourly price”
Dimitrov, Petrov & Co. point out that the change may affect contracts, including financial instruments such as swaps, that reference the IBEX day-ahead hourly price. The same question applies to PPAs, trader agreements and fee formulas. IBEX continues to publish base-load, peak and off-peak averages, but if a contract defines its price per hour, check how that is calculated now. Do not assume a particular averaging method – ask your trader or the contract counterparty.
Negative prices after the switch
Negative prices were rising before the switch and continued afterwards. An analysis of ENTSO-E data by Capalo AI counts 170 hours with a negative day-ahead price in Bulgaria in 2025 and 166 in just over seven months of 2026. In late April 2026 Sega reported a day with zero or negative prices from 8:00 to 17:00, down to about −€100/MWh in some intervals. Note that statistics now differ depending on whether they count hours or quarter-hours.
What to check now
- Read prices per quarter-hour, not as hourly averages, when you review revenue and curtailment.
- Make sure your curtailment rules work per 15-minute interval.
- Confirm with your trader that forecasts and schedules contain 96 real values a day.
- Review PPAs, trader contracts and fee formulas for references to an hourly IBEX price.
- Remember the time zone: IBEX and SDAC deadlines are usually quoted in CET, ESO deadlines in Bulgarian time, which is one hour ahead of CET.
TokPirate reads all 96 quarter-hour prices every day and compares each one with the plant’s minimum price, so curtailment follows the market quarter-hour by quarter-hour.