Electrical energy demand is continuously increasing due to the global population growth and improved living standards. This proposes a growth in the potential for the use of tri-generation systems in residential sector, due to their ability to produce both useful thermal energy and electrical power from a single source. In Lebanon, the electrical capacity shortage has created an informal back-up self-generation systems, mostly are diesel power generators, estimated to represent about 30% of all electricity generated. This paper presents a simulation study, using TRNSYS software, of tri-generation systems working on diesel engines to supply the different residential energy demands. The recovered percentage of the exhaust gas wasted energy is determined and an optimizing study is carried out to find the optimum storage tanks sizes of the recovered thermal energy. Results show that tri-generation systems are found to make diesel power generators more energy-efficient and thus reduce greenhouse gases emissions.