Using firm-level supply chain networks to measure the speed of the energy transition

International climate targets rely on the success of the energy transition, however systematic monitoring on how the economy adopts low-carbon energy remains underdeveloped. Here we use nationwide supply-chain network data to reconstruct energy portfolios for 25,000 Hungarian firms between 2020 and 2024, covering 75% of gas, 70% of electricity, and 50% of oil consumption. This allows us to quantify the speed of the energy transition -the transition towards low-carbon electricity- on the firm level. We find substantial heterogeneity in decarbonization progress: half of firms increase low-carbon energy shares, but 50% reduce it. Energy cost structures are closely associated with transition behavior, indicating technology-related lock-in effects. Extrapolating current trends yields an aggregate low-carbon share of 20% by 2050, highlighting ineffective decarbonization efforts. If firms strictly adopted strategies of decarbonization frontrunners within their industry sectors, a low-carbon share of 70% could be achieved by 2050, putting climate targets within reach.