From Rapid Expansion to System Saturation: Modelling Prosumer Diffusion, Policy Effectiveness, and Grid Constraints in Romania

The rapid expansion of decentralized photovoltaic (PV) systems is transforming electricity consumers into active market participants across Europe. While prosumer development has been extensively studied in Western Europe, the dynamics, constraints, and transition pathways of emerging Eastern European markets remain insufficiently understood. This study provides a comprehensive assessment of prosumer ecosystem development in Romania as one of the fastest-growing prosumer markets in Europe and a representative case for broader Eastern European energy transitions. A novel integrated analytical framework is proposed, combining modified logistic diffusion modelling, a Policy Support Index (PSI), a Grid Saturation Index (GSI), and qualitative stakeholder-based barrier assessment. Unlike conventional approaches, the framework explicitly incorporates policy responsiveness and infrastructure constraints into prosumer diffusion analysis. Results reveal an exceptional acceleration of prosumer deployment, with cumulative installations increasing from 303 prosumers and 1.52 MW in 2019 to 339,107 prosumers and 5311.96 MW by April 2026. Diffusion modelling identifies three transition phases—market formation, rapid expansion, and saturation-driven moderation—and estimates an effective carrying capacity of approximately 398,000 prosumers under current institutional and grid conditions. The PSI increased from 0.22 to 0.80 between 2019 and 2026, highlighting the GSI deteriorated from 0.96 to −0.59, indicating severe distribution-grid saturation. Institutional and technical barriers emerged as the most critical constraints, surpassing market and behavioral factors. The findings demonstrate that prosumer growth in Eastern Europe establishes a constrained socio-technical transition rather than a purely market-driven process. The proposed framework offers a transferable methodology for evaluating prosumer ecosystems and provides evidence that future expansion will depend increasingly on grid modernization, energy storage deployment, and adaptive regulatory governance.