In recent years climate change has emerged as a global issue directly related to quality of life. In this context, one of the key goals in the next few decades will be to transition the global economy to a sustainable system. The nature of the energy planning process dictates the application of complex models. There is no universal solution to the energy planning problem. Each territory requires a bespoke strategy aimed at utilising its specific potential. The research presented in this paper explores reaching a zero-carbon energy system at the city level. It establishes a step-by-step decarbonisation method and proposes an energy transition index (ETI). The index presented is used to evaluate different renewable energy sources (RES) deployment scenarios in the context of affordability, self-reliance, and sustainability. The main aspects and barriers of deploying sustainable energy solutions are also explored. Some of the key challenges of RES deployment are identified as capital intensity, output variability, and the regulatory framework. The approach applied in the paper focuses on a city-level strategy in line with the goal of satisfying demand through local energy sources. The presented analysis offers two basic conclusions: (1) each territory requires a bespoke strategy that can optimally utilise its energy potential and (2) building a local zero-carbon system can be feasible only by implementing energy storage technologies.