Every solar plant in Bulgaria that sells on the market commits the day before, directly or through its trader, to how much it will deliver in each quarter-hour. When reality differs, the difference is an imbalance, and somebody pays for it. For owners based abroad the mechanics can be hard to see, because the cost usually arrives as a single line on the trader’s invoice. This guide explains the rules as they stand in the Electricity Trading Rules issued by the regulator, in the version in force since 1 July 2025. Article numbers below refer to those rules.
Key terms
- ESO (ЕСО) – the Electricity System Operator, Bulgaria’s transmission system operator (TSO). It runs the balancing market and settles imbalances.
- EWRC (КЕВР) – the Energy and Water Regulatory Commission, the regulator that issues the trading rules and the licences.
- Imbalance (небаланс) – the deviation between the registered schedule and the metered values of a producer or consumer (art. 155).
- Settlement period – the time unit for calculating imbalances: 15 minutes. This definition has been in force since 1 October 2022.
- Balancing group (балансираща група) – market participants who have transferred their balancing responsibility to one coordinator, so that their imbalances are netted (ETE).
- Balancing group coordinator (координатор на балансираща група) – a licensed producer or trader whose licence EWRC has extended to cover this role, and which ESO has registered as coordinator (art. 58). It is financially responsible to ESO for the whole group’s imbalance (art. 6). Coordinators are mainly traders (ATEB), so for a PV plant this is usually its trader.
- Direct and indirect member – a direct member’s site has its own schedule notified to ESO. For an indirect member’s site, no separate schedule is notified to ESO – the coordinator covers it within a sub-group.
How the chain works
- Schedule. A plant connected to the distribution grid sends its production schedule to its coordinator on the day before delivery and on the delivery day itself (art. 77). Plants on the transmission grid notify ESO themselves or through their coordinator.
- Notification. The coordinator notifies ESO of the group’s schedules and forecasts for every settlement period. Under ESO’s day-ahead instruction, trading schedules are due in ESO’s market system by 15:30 Bulgarian time on the day before delivery.
- Metering. After delivery, the owners of the commercial meters provide validated measured values.
- Calculation. For every 15-minute period, ESO compares each group’s net contracted position with its net measured position (arts. 156–157). It sets a price for energy shortage and a price for energy surplus in each settlement period.
- Settlement. ESO runs a preliminary settlement every week and the final settlement every month (art. 169).
- Allocation. The coordinator pays or receives the group’s imbalance from ESO and passes each member’s share on under their contract (art. 6).
Where a PV plant sits
Renewable producers of 500 kW or more must sell their output on the exchange (ATEB). Within a standard balancing group, the rules separate renewable producers by size:
| Installed capacity | Position in the balancing group |
|---|---|
| 4 MW and above | Direct member of the renewables sub-group |
| 500 kW to 4 MW | Indirect member of the renewables sub-group |
| Below 500 kW, output bought by the end supplier | Special balancing group run by the end supplier |
The first two rows come from article 56c (чл. 56в in the Bulgarian text), the third from article 56a (чл. 56а). ESO’s instruction applies the same split when schedules are notified.
Short or long: what an imbalance means in practice
| Metered output vs schedule | Position | Typical PV causes |
|---|---|---|
| Lower | Short (shortage) | Unforecast clouds, curtailment missing from the schedule, inverter or string faults |
| Higher | Long (surplus) | Over-cautious forecast, clear sky after a cloudy forecast |
Shortage and surplus are priced by ESO for each 15-minute period under a methodology set by EWRC. These prices can move far away from the day-ahead price. When the current model was introduced in 2022, the traders’ association ATEB warned that extreme and even negative balancing prices could not be ruled out.
19 May 2024: when balancing prices spiked
On Sunday 19 May 2024, forecast shortage prices stayed above 1,500 BGN/MWh from 10:00 to 18:00 and reached 2,500 BGN/MWh (about €1,278/MWh at the fixed rate of 1.95583) in some hours (Mediapool). Over the whole of May the shortage price peaked at 6,342 BGN/MWh (24 Chasa). On 28 June 2024 EWRC adopted a one-off compensation mechanism and ordered a secondary settlement of that day (BPVA). The lesson for owners: one bad day can outweigh months of normal deviations, and a balancing group softens that risk but does not remove it.
How the cost is shared inside the group
- Published principles. To register, every coordinator must write “general principles for allocating imbalances” within its group and publish them on its website (art. 58). Read them before you sign.
- Only real deviations. Producers, including renewable ones, pay imbalance costs only when their registered schedule and actual production differ (art. 155).
- No schedule is not an option. If a producer does not send a schedule, the coordinator prepares one on its behalf (art. 155) – and the producer still answers for the deviation from it.
- Netting. One stated purpose of balancing groups is to aggregate members’ imbalances and soften the economic effect of balancing prices (art. 56). In a given quarter-hour, a plant that is short can be offset by a member that is long. How much of that benefit reaches you depends on the published principles and your contract – check whether you pay a fixed balancing fee or a share of actual costs.
Reducing your imbalance
- Forecast per quarter-hour and send a schedule every day instead of relying on a default profile.
- Update during the delivery day when the forecast changes – schedules can be sent on the delivery day too (art. 77).
- Put planned curtailment for negative-price intervals into the schedule.
- Report outages and maintenance to your trader as soon as you know about them.
- Monitor output down to inverter and string level, so faults that make you short are caught in hours, not weeks.
- Follow ESO’s operational instructions. In April 2024 ESO ordered PV plants not to exceed their trading schedules and, at times, to stay at or below 80% of them (Smart Energy Trade).
For the scheduling side in detail, see Day-ahead production schedules.
Questions to ask your coordinator
- Where are your published imbalance allocation principles, and which version applies to my contract?
- Do I pay a fixed balancing fee per MWh or a share of actual imbalance costs?
- Is my plant a direct or an indirect member?
- By what time do you need my day-ahead schedule, and how do I send intraday updates?
- How do I report curtailment, outages and ESO limitations?
- What imbalance data do I receive per 15-minute period, and when – after the weekly or only after the monthly settlement?
TokPirate sends each plant’s day-ahead schedule, with planned curtailment included, to the trader the day before and alerts you in real time when a plant stops producing – which covers two common causes of avoidable imbalance: unreported curtailment and unnoticed outages.