Facilitating megacity electricity decarbonization via grid-interactive demand-side resource management

City power systems face intensified mismatches between generation and dynamic demand under rising zero-carbon energy penetration levels. Although demand-side resource management offers mismatch mitigation potential by regulating load patterns, its city-level capability remains unclear. This study develops a community-to-grid framework to assess how demand-side resources could mitigate generation-demand mismatches at the megacity scale. A case study is performed in Shenzhen, China, covering 4,549 community units with energy profiles and rooftop photovoltaics of 479,259 buildings, and travel-charge profiles of 1,080,000 private plug-in electric vehicles. Under an assumed 80% zero-carbon electricity penetration level, the proposed framework could reduce weekly power supply deficits by 54.5%, or 266.2 gigawatt-hours, corresponding to an estimated reduction of 83.6 kilotonnes of carbon dioxide. Different governing paradigms may affect grid infrastructure upgrade paths: prioritizing administrative equity supports temporally consistent planning, whereas prioritizing mismatch-mitigation efficiency promotes spatially clustered retrofitting. This study may inform city-specific demand-side resource excavation under national electricity decarbonization planning.