Cities consume more than 67% of global primary energy, the production of which results in approximately three-quarters of global CO2 emissions, exacerbating the global warming trend and related extreme weather events and natural disasters. Therefore, it is critical for cities to use existing and new sources of energy efficiently and effectively. This paper introduces a methodology that can combine sustainable energy planning with economic analysis, proposing a form of sustainable urban energy planning that could reduce energy consumption with the minimum economic cost. Taking a postindustrial city (Shenzhen, China) as an example, this paper defines four scenarios by which to analyze future projections of energy generation and consumption from 2015 to 2030 based on the Long-range Energy Alternatives Planning System model. Also developed are Sankey maps for the energy flow from the energy supply to demand sectors for different scenarios. The results show that energy efficiency improvement and energy structure upgrade policies implemented in Shenzhen would have a significant impact on its energy system. Energy consumption is projected to increase steadily up to 2030 under each scenario except for the Peak Scenario, but with different growth rates. Electricity generation in all scenarios is supposed to expand by 2030 and sustainable electricity (such as distributed photovoltaic power, waste-to-energy power, and Combined Cooling, Heating, and Power) will play an important role in the Energy structure upgrade and Peak scenarios.