Midday prices below zero used to be a curiosity on the Bulgarian power exchange. They are now a regular feature of sunny spring and summer days. For a solar plant that sells on the market without a premium, every megawatt-hour exported in such an interval costs money instead of earning it (for plants with a premium the arithmetic is different – see the section on premiums below). This guide sets out how often it happens, what a single negative quarter-hour actually costs and what owners and asset managers can do about it.
Key terms
- IBEX (БНЕБ) – the Independent Bulgarian Energy Exchange. Its day-ahead market sets the reference price for electricity in Bulgaria. Since 1 October 2025 it clears a separate price for every 15-minute interval – see IBEX 15-minute prices.
- Negative price – a day-ahead clearing price below €0/MWh. In that interval the seller pays the buyer for every MWh delivered.
- ESO (ЕСО) – the Electricity System Operator, Bulgaria’s transmission system operator (TSO). It keeps the grid balanced and settles imbalances.
- EWRC (КЕВР) – the Energy and Water Regulatory Commission, the Bulgarian energy regulator. It sets grid fees and the parameters of renewable support.
- Curtailment – deliberately reducing or stopping a plant’s output, usually by limiting or switching off the inverters.
How often prices go negative
An analysis of ENTSO-E day-ahead data by Capalo AI counts the hours with a negative price in Bulgaria:
| Year | Hours with a negative day-ahead price |
|---|---|
| 2023 | 11 |
| 2024 | 55 |
| 2025 | 170 |
| 2026 (just over seven months) | 166 |
A word on definitions: these figures count hours. Some Bulgarian media count trading intervals instead, or include zero-price periods, and arrive at much larger numbers. Now that prices are set per quarter-hour, always check what a statistic counts before you compare it with another.
The dips are also getting deeper. In late April 2026 the daily Sega reported a day on which the price was zero or negative from 8:00 to 17:00, falling to around −€100/MWh in some intervals.
Why it happens at midday
The main driver is the amount of solar capacity that produces at the same time. Bulgaria had 5,910 MW of solar PV at the end of 2025, according to IRENA figures quoted by Capalo AI, and Solarplaza puts it above 6.5 GW in early 2026. On a bright weekend or public holiday, when demand is low, supply at noon simply exceeds what the country can use or export.
The system operator has been flagging this for some time. In April 2024, with system load expected around 3,500 MW against 5,800–6,000 MW of expected production, ESO ordered PV plants not to exceed their trading schedules and, at times, to stay at or below 80% of them (Smart Energy Trade).
The same effect shows up in annual revenue. In its decision Ц-8 of 30 June 2026, EWRC calculates that large PV plants captured an average of €65.68/MWh in 2025, against a base-load average of €106.90/MWh – a ratio of 0.614. The same decision sets the forecast market price for PV for 1 July 2026 – 30 June 2027 at €70.10/MWh.
What a negative quarter-hour costs
For a plant that sells on the exchange through a trader, producing in a negative-price interval has several cost components:
| Cost item | When it applies |
|---|---|
| The negative price itself | On every MWh sold in that interval |
| Grid access fee to ESO: €2.72/MWh excl. VAT | On all metered production fed into the grid, from 1 July 2026 (EWRC decision Ц-13) |
| Trader and balancing fees | As set in your contract |
| Imbalance | Only when actual output differs from your schedule |
The access fee is published by the distribution operators, for example ERP Sever. It means that even a price of exactly zero loses you €2.72 on every MWh you export.
Worked example (sample data)
Take a plant exporting an average of 0.8 MW during one quarter-hour priced at −€100/MWh:
- energy exported: 0.8 MW × 0.25 h = 0.2 MWh
- negative price: 0.2 MWh × €100 = €20.00 paid to the buyer
- access fee: 0.2 MWh × €2.72 = €0.54
- total: about €20.54 for 15 minutes, before any trader fees
Sixteen such quarter-hours – four hours around noon – add up to roughly €329 in a single day. These are sample numbers, not a forecast: your exposure depends on your prices, volumes and contract.
Plants with a premium: check before you curtail
Renewable producers with an installed capacity of 500 kW or more must sell their output on the exchange (ATEB). Many of them also receive a premium from the Electricity System Security Fund (ФСЕС). The premium equals the plant’s preferential price minus the forecast market price set by EWRC (TBK). Because it is paid per MWh, it changes the arithmetic of curtailment.
We have not found a published rule that settles, for every plant, whether the premium applies to energy produced in negative-price intervals, so we make no general claim either way. Confirm it with your trader or legal adviser before you set a curtailment threshold.
Plants below 500 kW whose output is bought by the end supplier sit in that supplier’s special balancing group (article 56a (чл. 56а) of EWRC’s Electricity Trading Rules). Their exposure to negative prices is defined by their contract.
Curtail – but tell your trader
Stopping the inverters removes the negative price and the access fee, but it can create a new cost. If your schedule still shows 0.8 MW for a quarter-hour and you produce nothing, you are short against that schedule. Under article 155 of the Trading Rules, producers pay for imbalances precisely when their registered schedule and actual production differ.
The timing works in your favour. Day-ahead prices are published in the early afternoon of the day before delivery – preliminary SDAC results at 12:55 CET, 13:55 Bulgarian time (Nord Pool). A curtailment plan can therefore be built into the schedule your trader notifies. More on this in Day-ahead production schedules and Imbalance and balancing groups.
How to set a minimum price
A practical rule: curtail when the price falls below what it costs you to export one more MWh. For a plant without a premium, that cost is at least the access fee of €2.72/MWh plus any per-MWh trader fee – so the threshold sits slightly above zero, not at zero. For a plant with a premium, the threshold depends on how the premium treats those intervals.
Set the threshold per plant, review it whenever fees or contracts change, and apply it per quarter-hour rather than per hour.
Checklist for owners and asset managers
- Read what your trader contract says about negative prices: who pays, and whether there is a price floor.
- Confirm your premium status and how it treats negative-price intervals.
- Set a minimum price for each plant.
- Make sure the inverters can be curtailed and restarted remotely, and test it before spring.
- Agree with your trader how curtailment is reflected in the day-ahead and intraday schedules.
- Review curtailed volumes and avoided costs every month.
TokPirate automates the routine part of this list: it follows IBEX prices, stops and restarts plants through the inverter manufacturer’s cloud when the price drops below your minimum, and sends the resulting schedule to your trader the day before. See how it works.