Cities are responsible for 60-70% of energy-related CO2 emissions. As the world is increasingly urbanized, it is crucial to identify cost-effective pathways to decarbonize. Here, we propose a 'SolarEV City' concept, in which integrated systems of cities' roof-top PVs with EVs as batteries can supply affordable and dispatchable CO2-free electricity for citie's dwellers, which can reduce CO2 emission by 54-95% with 26-41% of potential cost savings by 2030.