Southeastern Europe's Summer Stress Tests Revealed a Fast-Response Capacity Gap
Five heatwaves this summer exposed southeastern European grids' inability to source fast-response power when solar output fades and cooling demand does not.
Montel reported on Friday (2026-08-28) that southeastern Europe's power systems had been exposed by this summer's run of heat events, with the region's grids unable to source fast-response capacity quickly enough when cooling demand stayed elevated after solar output fell. A companion Montel analysis, published on Wednesday (2026-08-26) by market expert Noemie Baud, examined five heatwaves across four European countries. Flexibility, or its absence, determined which systems coped and which did not.7,6
Southeastern Europe's grid connections with neighbouring systems mean its balancing gaps do not stay local. Shortfalls in one zone can draw on — or deplete — flexibility reserves across borders, making a regional capacity gap a shared problem during simultaneous heat events.6,2
The price of sourcing balancing capacity under real scarcity has a recent reference point. A solar eclipse crossed the UK on Wednesday (2026-08-12), and the electricity system operator cleared the event; analysts told Montel on Thursday (2026-08-13) that the grid had passed a key stress test. But the operator paid up to EUR 750/MWh for balancing power, a figure Montel reported is several multiples of normal intraday settlement levels.4
Solar performed above expectations while the sun was up. Ember data showed European panels generated 17% more power during the heat events than they do in a normal summer, as sustained irradiance amplified output at precisely the hours when afternoon cooling demand peaked. German baseload power settled at EUR 137.88/MWh on Friday (2026-08-28), down 1.16% on the session as late-August heat premiums began to unwind.5
After sunset the advantage reversed. Walburga Hemetsberger, CEO of SolarPower Europe, told Canary Media that temperatures often stay high after dark, keeping air-conditioning load elevated even as solar output drops. Battery systems helped shift stored generation into the evening hours this summer. But installed capacity across southeastern Europe is well below what consistent post-sunset balancing at scale would require.5
Europe's storage buildout is accelerating. The continent added 36 gigawatt-hours of solar-plus-battery capacity in 2025, up 48% over 2024 additions, according to Canary Media. Germany, Spain, and Italy absorbed most of that growth. Southeastern European grids have moved more slowly on both storage installations and the interconnection upgrades that allow cross-border flexibility trade to function at volume.5
Thermal generation faced its own constraints. European heatwaves raise river temperatures, cutting licensed output at nuclear stations that rely on river cooling at the moment when heat demand peaks. France's state-owned utility has committed to more than $10 billion over the next 15 years to adapt its nuclear and hydropower fleet to warmer and lower-water conditions, according to Canary Media. The scale of that commitment signals that intense summer stress is now a design condition for European utilities, not a low-frequency outlier.5
ENTSO-E's summer outlook, published on June 3 (2026-06-03), flagged elevated adequacy risks in markets with limited interconnection, specifically citing Moldova's gas supply constraints. That concern runs along the southeastern edge of the European grid, where restricted cross-border transfer capacity limits how quickly surplus generation in one country can reach an adjacent market under load.2
Growing electricity demand is pulling against the flexibility improvements underway. ENTSO-E has warned that transmission operators may be forced to curtail renewable penetration if data-centre demand is not properly managed. Energy Voice reported that Google's planned 1-gigawatt data centre in Belgium includes flexibility commitments, a structure that could serve as a template for large-load integration. Whether southeastern European TSOs can require comparable terms from large new consumers signing onto their grids will shape how quickly the flexibility deficit shrinks.3,1
The immediate signal to watch is what southeastern European TSOs include in autumn capacity procurement. If this summer's stress events produce formal fast-response tenders before spring 2027, the gap Montel identified is moving toward resolution. If they do not, the same configuration that fell short this summer enters the next one unchanged.7