The Ramai Mall Yogyakarta is one of building which is using the electric source from PLN. However, the electric source from PLN cannot supply continuously without several problems. Therefore, in order to anticipate those problems, we need a backup system as a generator set to keep on the electric energy of the building. Generator set works automatically, when the electric source from PLN cannot be used, the generator set turn on automatically. Then, when the electric source from PLN can be accessed, the generator set turn off automatically. Because of the backup system in The Ramai Mall Yogyakarta had worked for a long time, we need to evaluate and reanalyze the backup system of the building in order to make the system run well. The analyzing of the system applies quantitative approach and it is supported with ETAP I2, 6. 0. Simulation Program which simulates from the result of calculation, the components of the backup system is suitable with PUIL 2000 requirements. However, the efficient degree of the generator set of the building is low 48, 5% because the loading of the generator set is low. According to the result of simulation, the loading which have to be supplied by generator set is 865 kVA and the capacity which is used by generator set is 1000 kVA. In this case, the generator set works in its ability.