The concept of a sustainable energy system for smart cities focuses on the importance of using renewable energy sources like solar, hydro, wind, and others instead of fossil fuels, which are running out and harming the environment. These renewable sources can be naturally replenished and produce less pollution. The goal is to conserve energy and use it efficiently to protect our natural resources and combat climate change. Switching to clean energy is essential for a sustainable future, as it reduces carbon emissions and benefits both our environment and our wallets.