Hydropower remains a key flexibility source for Southeast Europe, but Week 21 highlighted how uneven hydrological conditions can affect electricity prices, cross-border flows and system-balancing requirements. Regional hydro generation rose only 0.6% week-on-week to 3.95 TWh. The regional figure, however, masked large differences between national markets.
Week 21 hydro changes across Balkan markets
Croatia recorded the strongest weekly rebound in hydro output, with generation rising by nearly 86%. Türkiye increased hydro generation by 4.8% to 2.85 TWh. In contrast, Serbia’s hydro generation fell by 41.2%, while Bulgaria declined by 34.2%.
Other countries also reported declines: Italy’s hydro output dropped by 9.7%, and Greece fell by 7.7%. Greece’s weekly hydro generation totaled 86.9 GWh. Across the region, the direction and magnitude of weekly changes varied substantially by country.
Hydro’s role in balancing and import needs
In Southeast Europe, hydro is not treated as a typical generation source because it functions as a natural balancing instrument. When hydro output is higher, it can suppress prices, reduce thermal dispatch and ease import requirements. When it weakens, systems can shift toward coal, gas, imports or storage.
Serbia’s situation was highlighted by both generation and price movements. Even with a 41.2% decline in hydro generation, Serbian prices fell to €81.24/MWh, down 16.7% week-on-week. The price drop coincided with factors including regional solar availability, weaker demand and lower thermal costs.
The same hydro shortfall could have produced different market outcomes in tighter conditions. Higher balancing costs, stronger import needs and wider evening price spikes were identified as possible effects of reduced hydro availability during such periods. This variability increases the importance of hydrology for forecasting.
Impacts on trading models and cross-border flows
Renewable developers often use annual average prices when modeling Southeast Europe power markets, but hydrological conditions can create sharp weekly and seasonal deviations. The captured price for solar, balancing exposure for wind and arbitrage value for BESS can change depending on reservoir conditions and how hydro is dispatched.
Hydro volatility also influences cross-border electricity flows. During Week 21, regional net electricity imports fell by 34.6% to 1.03 TWh, partly linked to improved renewable and hydro availability across several markets. At the same time, uneven hydro distribution meant some countries strengthened export potential while others faced greater reliance on imports or thermal backup.
Croatia’s rebound was associated with improved local adequacy and reduced pressure on imports. Serbia and Bulgaria were described as needing more support from broader regional market softness and renewable availability instead of domestic hydro strength. The role of interconnections was emphasized as hydrological surplus can offset weakness elsewhere only where transmission capacity allows.
Interaction with solar flexibility and operational planning
Hydro output also interacts directly with solar generation patterns as solar expands across Southeast Europe. Hydro plants can shift production away from low-price midday periods toward higher-value evening hours when solar output fades. This operational flexibility was described as giving flexible hydro characteristics similar to a natural battery.
The value of this flexibility is tied to the type of hydro system in each country. Reservoir-based systems can preserve water during solar-heavy hours and dispatch during evening ramps when prices recover. Countries with less flexible run-of-river production have fewer commercial options for shifting output across time periods.
The broader generation mix moved further in that direction during Week 21, with solar output across Southeast Europe increasing by 8.1%. Wind output fell by 4%. As solar becomes more dominant, the role of hydro flexibility was presented as increasing rather than decreasing.
Policy and investment considerations alongside gas costs
The operational role of hydro extends beyond seasonal planning into daily market operations including balancing, congestion management, renewable integration and system-security strategy. For investors, hydrological variability increases the relevance of portfolio structures that combine multiple revenue streams rather than relying on average market prices alone.
A hybrid approach combining solar, wind, BESS and contracted industrial offtake was described as potentially reducing exposure to hydrological swings compared with a merchant asset dependent primarily on average prices. Gas-fired balancing costs were also cited as a constraint: with TTF close to €50/MWh, balancing using gas remains costly when renewable output is weak or evening demand rises.
The Week 21 pattern reinforced that assessing hydropower conditions only at a regional aggregate level can miss major national differences. While the regional number appeared stable overall, outcomes diverged sharply between countries in ways that can influence price spreads, import needs, storage economics and balancing costs.
Hydropower remains one of the region’s strongest assets, but its volatility is becoming one of the most important risks for power market participants across Southeast Europe.