Electricity trading across borders is shaped not only by demand, generation and weather, but also by the availability of cross-border transmission capacity. With flow-based coupling and more detailed capacity calculations, the border itself can affect price spreads as strongly as fundamentals in each market. The amount of capacity available between bidding zones can therefore influence whether prices converge or separate.
When a border is unconstrained, arbitrage tends to push prices toward convergence. When capacity is limited, neighbouring markets can separate sharply. Transmission capacity availability can thus be treated as an economic driver of price outcomes rather than a fixed background input.
A megawatt of transmission capacity may have limited value when two markets clear at similar prices. If prices diverge by €50/MWh, the same megawatt can become more valuable for traders seeking to transfer power. For Southeast European desks, this shifts forecasting requirements beyond national fundamentals alone.
ACER highlights cross-zonal capacity in Southeast European price formation
ACER’s 2026 analysis of Southeast European electricity markets points to cross-zonal capacity as an important factor in regional price formation. The agency also calls for greater availability of transmission capacity for cross-border trading and improved utilisation of the network. The focus extends to how network constraints influence spreads across the region.
For example, a trader may forecast a Bulgarian surplus alongside expectations of a Greek shortage. The size of the eventual BG-GR spread depends partly on how much Bulgarian electricity can be transferred south. Similar dependencies apply across other interconnected interfaces including Hungary-Romania and Hungary-Croatia.
This interaction creates capacity surprise risk. A desk may start with an expectation of 1,000 MW of commercial capacity across a relevant interface, only for later calculations to reduce it to 600 MW. In that scenario, national demand, solar output and fuel prices may remain unchanged while the expected spread widens due to reduced arbitrage capability.
Flow-based coupling changes how trades consume network capacity
The challenge becomes more complex under flow-based market coupling. Instead of treating each border independently, flow-based calculations account for how transactions affect critical network elements across a wider interconnected system. As a result, trades between two bidding zones can consume capacity on network elements that are not physically located on their direct border.
The approach is intended to support more efficient grid use because market coupling reflects physical behaviour across the broader network more accurately. For traders, however, it reduces the usefulness of traditional bilateral analysis based on ATC-style capacity assumptions. Capacity constraints must be assessed in a system context rather than solely by interface-level figures.
Core Advanced Hybrid Coupling expands the regional framework from June 2026
The implementation of Core Advanced Hybrid Coupling in June 2026 adds another layer to this evolution. The method incorporates selected Core external borders into a more coordinated flow-based framework. It strengthens the link between market coupling outcomes and regional network conditions.
For SEE trading desks interacting with Core markets, border behaviour is increasingly determined by regional network optimisation rather than a simple bilateral capacity figure. This requires an evolution in trading models that incorporate multiple layers beyond fundamentals alone. Fundamentals remain one layer, covering load, renewable generation, hydro conditions, thermal availability and fuel economics.
A second layer focuses on expected price formation in each bidding zone, while a third layer covers network-state forecasting. Network-state forecasting includes identifying which critical network elements are likely to become constrained and which borders could receive additional capacity. It also covers where remedial actions might alter available margins and how much capacity is likely to remain after wider European optimisation.
Intraday reassessment after day-ahead auctions affects cross-border optionality
An intraday dimension also applies because day-ahead capacity is calculated using a specific set of system assumptions. As delivery approaches, network conditions become clearer and available capacity can be reassessed using more recent information. Cross-border optionality can therefore change significantly after the day-ahead auction.
A spread that initially appears trapped may become arbitrageable if transmission conditions improve intraday. Conversely, a spread that looks highly attractive may become impossible to access when available capacity falls. The economic value of the border can therefore change throughout the trading day.
This supports short-term strategy focused on capacity-event trading. In this context, the key signal is not only whether Market A is cheaper than Market B. The signal can instead be that new information about transmission capacity changes the probability that the price difference will persist .
If such probability shifts materialise, they can distinguish next-generation regional power desks from traditional Southeast European electricity trading approaches . Fundamentals still explain why two markets should diverge, while transmission capacity determines whether that divergence can survive .
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