The transformation of the European Union’s power sector over the last decade has been driven by two interconnected processes: the expansion of renewable energy sources (RES) and the reduction of carbon-intensive electricity generation. While these developments are often discussed together, an important question remains: to what extent can the decline in carbon intensity be attributed to renewable growth alone, and how much is the result of broader structural changes in national energy systems?
The EU-Wide Relationship: Correlation and Dispersion
Data from the period 2015–2024 reveals a clear EU-wide negative correlation between the change in renewable energy share and the carbon intensity of the power sector. The fitted regression line in the provided scatter plot illustrates that as the share of RES increases, CO₂ intensity generally declines.
The scatterplot visualises the relationship between the absolute percentage-point change in the RES share and the corresponding one in the CO₂ intensity of the power sector across EU member states between 2015 and 2024.
However, the degree of dispersion among member states is significant, indicating that renewable gains are often insufficient on their own to explain the total carbon improvement. While countries like Denmark and the Netherlands have seen massive RES increases (near 15%), their carbon intensity reductions differ significantly.
A central cluster of countries, including Germany, Ireland, and Latvia, sits near the EU average, which saw around 7.5% increase in RES and almost 200-point drop in CO₂ intensity. Estonia stands out as a major outlier, achieving the most dramatic reduction in carbon intensity (nearly -600) despite an RES increase that was lower than Denmark’s. This outcome reflects the rapid decline of oil-shale generation – historically the dominant and highly carbon-intensive source in the country’s electricity mix. Rising EU ETS carbon prices and changing market conditions made oil-shale generation increasingly uneconomic, leading to a substantial reduction in power-sector emissions. Conversely, countries like Sweden and France, which already had low-carbon mixes, show very little change in intensity despite moderate RES growth, illustrating the concept of “low-hanging fruit” being already harvested.
Maximizing Carbon Returns on Renewable Expansion
Bulgaria occupies a unique position on this map. While its RES increase rank is below the EU average (at approximately +5% compared to the EU’s +7.5%), its carbon-intensity improvement rank is significantly higher than the average. Bulgaria is positioned well below the regression line. In statistical terms, this means Bulgaria is outperforming the general trend: it has achieved a much greater reduction in CO₂ intensity (-300) than would be expected from its 5% renewable expansion alone (which the trend line suggests should result in a reduction of roughly -180). Compared to its Central and Eastern European peers, Bulgaria is highly efficient in its decarbonization. While Poland and Czechia show similar or slightly better RES growth, they have not matched Bulgaria’s level of carbon intensity reduction relative to that growth. Slovakia and Slovenia, meanwhile, have seen much smaller intensity improvements despite similar or higher RES shifts.
The Solar Surge: A New Phase of Renewable Expansion
Between 2015 and 2024, Bulgaria commissioned a total of 3,043.28 MW of renewable capacity. However, the distribution of this growth is extremely concentrated:
- Solar energy dominates the expansion: around 2,973 MW – more than 97% of all new renewable capacity – came from photovoltaic installations.
- Most of the growth occurred in the last two years: from 2015 to 2020, annual additions remained relatively modest, but in 2023 and 2024 alone Bulgaria installed nearly 2,400 MW of solar capacity.
- Other renewable technologies remained largely stagnant: wind capacity increased by less than 9 MW over the decade, while hydro additions were only 30.76 MW.
The fact that Bulgaria achieved a larger reduction in carbon intensity than expected from renewable growth alone points to two complementary explanations:
- Reduced Reliance on Lignite Generation
In the previous analysis of Net-Zero Lab was shown a strong relationship between coal-based electricity generation and power-sector CO₂ emissions in Bulgaria. The substantial decline in carbon intensity is therefore likely linked to lower utilization of the country’s most carbon-intensive lignite-fired power plants.
- The Timing of the Solar Boom
Because most new solar capacity was installed in 2023–2024, its impact on emissions reductions appears concentrated toward the end of the period. The rapid expansion of photovoltaics has likely amplified the overall decline in carbon intensity observed in the 2015–2024 dataset.
Bulgaria’s recent decarbonization performance is stronger than the EU average would suggest when measured against renewable expansion alone. This reflects not only the growth of solar energy, but also the decline of highly carbon-intensive lignite generation. The challenge ahead is different from the challenge of the past decade. Future progress will increasingly depend on deeper structural changes: diversifying the renewable mix beyond solar, expanding storage and balancing technologies, strengthening the electricity grid, and integrating higher shares of variable renewable generation. In this sense, Bulgaria’s next phase of decarbonization will be less about replacing the dirtiest generation and more about building a resilient, flexible, and low-carbon power system capable of sustaining further reductions in emissions.
Authors: PhD Candidate Lyubimka Georgieva & Dr. Mariya Trifonova