Power system restructuring has led to a significant change on how to design, operate, and decision making in power systems. This major change has brought with itself lots of concerns and challenges in power system studies concerning the generation adequacy issue. Lower amount of costs and investment risk associated with the distributed generation (DG) units has made them a promising opportunity for the power system operators to expand the system capacity. This has made them attractive enough justifying their growing penetration levels in the electrical industry. According to this, effect analysis of the DG units on improving the technical and economical performance of the power distribution systems has to be well surveyed. In this regard, this paper proposes a comprehensive decision making approach in order to reach some optimal and also practical DG investment schemes. In each level of the decision making process and for the sake of planning on the DG capacity expansion mechanisms, critical cost-based indices are defined from the perspective of different involved investors, i.e., distribution utility, customers, and private investors to identify the optimal size and location of DG units. The customer interruption costs and reliability worth criteria are among the most critical ones which have been taken care of in this analysis. An optimization algorithm, namely particle swarm optimization (PSO) technique, is employed to effectively deal with the extracted mathematical optimization problem. 2-point estimation method (2-PEM) is also giving the authors a hand in dealing with the probabilistic factors involved.