Recently, to solve the energy and environmental problems caused by the fossil fuel depletion and the global warming, it has been widely studied in the area of power electronics research to increase the energy efficiency and to implement the green energy system. The hybrid energy system for a smart green building or an isolated microgrid system can be installed a diesel generator with energy storage systems to optimize the fuel consumption efficiency of a diesel generator. It is important to reduce the consumption of fossil fuel in the diesel generator. It has been used as an emergency power source in standalone mode with two options for the operation of the diesel generator. One is to change the generating power of diesel generator as the change of load power. The other is to regulate the generating power constantly to meet the highest fuel consumption efficiency. To reduce the fuel consumption of a diesel generator, the generator output power is regulated at the rated power by integrating the hybrid energy system with energy storage devices. Hybrid energy system can be co operation with renewable energy source to reduce power of diesel generator. In this paper, the fuel consumption of a diesel generator according to the operation region of the diesel generator was analyzed. Also added a renewable energy sources for fossil fuel consumption reductions in hybrid energy systems. The P & O MPPT method was used to effectively control the renewable energy sources. For power management of renewable energy sources, energy storage system was used consisting of a battery and supercapacitors. A strategy for controlling the battery and supercapacitor was established. Each system was modeled using Psim. To simulate the hybrid energy system, the load was used as the average load of 20 kW. Renewable energy sources are used for solar power. Solar power has been modeled to represent the average power output of 10 kW. When the hybrid system applied, fossil fuel efficiency of diesel generator was improved, stable power was supplied in response to the load.