If baseload is declining, what ensures reliability in Bulgaria’s grid?

Reliability increasingly comes from flexibility: hydropower, cross-border trading, demand response, and storage. Bulgaria benefits from pumped hydro storage at Chaira and its interconnections with Greece and Romania.

It is a misconception that a high share of variable renewables reduces system reliability. In fact, evidence shows the opposite. Countries with large shares of solar and wind power – such as Germany, Denmark, and Australia – consistently demonstrate higher levels of reliability. By contrast, baseload-dominated systems like those of France, Bulgaria, and Romania often report lower reliability compared to many of their European peers.

As baseload plants in Bulgaria are used less often, reliability increasingly comes from flexibility. One of the most important assets in this regard is the Chaira Pumped Storage Hydropower Plant (PSHPP), part of the Belmeken–Sestrimo–Chaira cascade. With 864 MW of generating capacity and 788 MW of pumping capacity, Chaira can supply electricity at full load for more than eight hours, then pump water back for over ten hours to prepare for the next cycle. This makes it invaluable for balancing large amounts of solar and wind power, which rise and fall quickly. However, the plant has faced setbacks: one of its four turbines (Unit 4) suffered a serious failure, and repairs are still ongoing, with Toshiba involved in restoring Units 1 and 4. The return of Chaira to full operation is considered critical for system reliability.

Beyond Chaira, the hydro cascade itself provides flexibility. Its reservoirs and hydropower stations can quickly adjust output – reducing generation when there is excess solar or wind, or increasing it when demand rises. This ability to “buffer” the system is one of the traditional strengths of Bulgaria’s electricity sector.

Other forms of flexibility are now emerging as well. Demand response and smart controls allow households and businesses to adjust their electricity use – for example, shifting heating, cooling, or charging schedules when electricity is more expensive or scarce. Battery storage is also growing. Bulgaria already has about 220 MW of installed storage capacity, mainly batteries linked to new solar projects. While this is an important start, it is still modest compared to the 2,600 MW of storage capacity targeted in the National Recovery and Resilience Plan (NRRP) by 2030. In other words, Bulgaria has reached less than 10% of its goal, meaning significant investment and deployment will be required in the coming years.

Finally, cross-border electricity trade is another pillar of reliability. Bulgaria’s interconnections with Greece and Romania make it possible to import power during shortages or export surplus solar generation. This European integration helps smooth variability and provides a safety net against local imbalances.

Net-Zero Lab analysis demonstrates that achieving a high penetration of variable renewable energy in the Bulgarian power grid is feasible, but only with the implementation of a robust flexibility strategy. The modeling highlights the critical role of a diversified renewable mix, where wind’s higher capacity factors and its complementarity with solar help reduce inefficiencies and enhance overall system performance. To ensure grid stability and economic viability, adopting advanced technologies and flexibility measures is no longer optional – it is essential for a sustainable and resilient energy transition.