In Bulgaria a solar plant does not simply produce whatever the sun allows. Every day its trader commits, on its behalf, to a quantity for each quarter-hour of the next day, and ESO uses that commitment to run the grid. The document behind the commitment is the production schedule. It decides how much of your output was sold as planned and how much is settled as imbalance – sometimes at a far worse price. This guide explains who sends what, by when, and what makes a PV schedule accurate.
Key terms
- ESO (ЕСО) – the Electricity System Operator, Bulgaria’s transmission system operator (TSO). It collects schedules, dispatches the grid and settles imbalances.
- EWRC (КЕВР) – the Energy and Water Regulatory Commission, the regulator that issues the Electricity Trading Rules quoted below.
- Production schedule (график за производство) – the planned amount of electricity a plant will feed into the grid in each delivery interval of a given day (art. 77 of the Trading Rules).
- Delivery interval – 15 minutes, equal to the imbalance settlement period. A normal day has 96 intervals; the days when clocks change have 92 or 100.
- Balancing group coordinator (координатор на балансираща група) – the licensed company, usually your trader, that is responsible to ESO for the imbalances of a group of producers and consumers. See Imbalance and balancing groups.
- D-1 and D – the day before delivery and the delivery day.
- MMS – ESO’s market management system, where schedules are notified in the ENTSO-E scheduling format. Gross production schedules are called PPS files and trading (exchange) schedules TPS files.
Who sends what, to whom
The Trading Rules (art. 77) and ESO’s day-ahead instruction, in force since 1 November 2025, split the work as follows:
| Who | Sends | To |
|---|---|---|
| PV plant on the distribution grid | Production schedule, in the coordinator’s format, on D-1 and on D | Its balancing group coordinator |
| Coordinator | Gross schedules for its distribution-connected producers, aggregated by technology, on D-1 | The distribution system operator |
| Coordinator | Trading schedules of the group, by 15:30 on D-1 | ESO, via MMS |
| Plant on the transmission grid | Gross production schedule, by 15:30 on D-1 | ESO, via MMS, directly or through its coordinator |
Three details in the ESO instruction matter for PV owners. First, all deadlines are in Eastern European time – Bulgarian local time. Second, every schedule must contain a value for every delivery interval of the day, expressed in MW (average power), not MWh. Third, ESO never enters trading schedules on a participant’s behalf. Your coordinator’s own deadline for receiving your forecast will be earlier than 15:30 – it is set in your contract, so ask.
For plants of 500 kW and above, coordinators also e-mail the daily trading schedule of each renewable producer to the Electricity System Security Fund (ФСЕС), which pays renewable premiums, according to the same instruction.
How the schedule fits the day-ahead timeline
- Morning of D-1. You or your trader forecast output for each quarter-hour of day D.
- By 13:00 Bulgarian time (12:00 CET). The trader submits orders to the IBEX day-ahead auction, which closes at 12:00 CET under the IBEX operational rules. Since 1 October 2025 the auction prices each 15-minute interval separately.
- About 13:55 Bulgarian time. Preliminary results are published (12:55 CET, according to Nord Pool). Now you know which quarter-hours will have low or negative prices.
- By 15:30 Bulgarian time. Trading schedules reach ESO’s MMS, reflecting what was sold and any planned curtailment.
- Day D. Updated schedules can be sent on the delivery day (art. 77). IBEX also runs an intraday market for adjusting positions closer to real time (ATEB).
- After D. Meter data is compared with the registered schedules, and imbalances are settled weekly (preliminary) and monthly (final).
Why accuracy matters
It decides your imbalance. Producers, including renewable ones, pay imbalance costs only when their registered schedule and actual production differ. If a producer sends no schedule at all, the coordinator prepares one on its behalf (art. 155).
ESO plans the system with it. ESO uses the production and consumption schedules, together with its own forecasts, to size the reserves it needs for the next day (art. 77).
It can become a ceiling. In April 2024, with far more solar output expected than the system needed, ESO ordered PV plants not to exceed their trading schedules and, at times, to stay at or below 80% of them (Smart Energy Trade). On such days the schedule effectively caps what the plant may produce, so an unrealistically low schedule costs output.
What a good PV schedule includes
- All intervals – 96 quarter-hour values (92 or 100 on clock-change days), never an hourly profile copied four times; solar output ramps within the hour.
- Weather – irradiance and temperature forecasts for the plant’s location, refreshed on the morning of D-1.
- Real capacity – inverter capacity, export limits and any grid-connection limits, not just the panels’ peak power.
- Availability – planned maintenance, known inverter or string faults, and partial outages.
- Planned curtailment – zero or reduced values in quarter-hours where the price is below your minimum; see negative electricity prices.
- ESO limitations – any instruction already received for day D.
- Intraday updates – a revised schedule when the weather or plant availability changes materially.
Common mistakes
| Mistake | Consequence |
|---|---|
| Curtailing without updating the schedule | You are short against the schedule in those intervals |
| Ignoring a known inverter fault | A predictable shortfall is settled as imbalance |
| Scheduling at panel peak power | Systematic shortage whenever inverter or export limits cap output |
| Flat values inside each hour | Deviations at the morning and evening ramps |
| Sending nothing | The coordinator prepares a schedule for you, and deviations from it are still yours |
Checklist for owners and asset managers
- Confirm with your trader who prepares the schedule, in what format and by what time.
- Agree how intraday updates, outages and curtailment are reported.
- Check that schedules are in 15-minute resolution and in MW.
- Compare schedule and metered output each week to see where the deviations come from.
- Review export limits and capacity data after every change at the plant.
TokPirate sends each plant’s day-ahead schedule to your trader automatically every day, with your curtailment settings already applied.