Marginal Curtailment Under Network Constraints: Evidence from the 2030 GB Power System

High Variable Renewable Electricity (VRE) penetration inevitably leads to curtailment (shedding), typically measured by average curtailment. Marginal curtailment (mc, the fraction of potential output curtailed by the last MW) can be many times higher, raising the long-run marginal cost of investment, proportional to 1/(1-mc). A unit commitment and economic dispatch model of Britain, divided into seven zones by transmission constraints in 2030, demonstrates that these constraints considerably increase mc compared to no congestion, despite the considerable planned expansion of transmission, interconnectors, and storage that mitigates curtailment. The current auction design favours levelised costs, ignoring curtailment, but long-run marginal costs may be 90% higher, suggesting the need for careful locational planning and VRE support design.