Today 54 % of the world's population resides in urban areas and in 2050 the projections are for 66 %. Therefore, the issue of city sustainability becomes increasingly important. This paper analyzes city energy sustainability with consideration to the complex built environment, high population densities, anthropogenic activities, energy demands, environmental impacts, as well as limits on both space availability and renewable energy sources. The evaluation considers models of thermal energy consumption for both residential and non-residential buildings based on a GIS tool. The thermal energy-use models consider established statistical methods as well as the introduction of energy-dependent urban-scale variables.