The main objective of most of power plants is to inject power as much as possible into the grid from the power plants. However, the transmission system may restrain the output capability of the power plant, especially, if the power plant is located in an area of non-expandable transmission system. In this situation, any disturbance in the nearby transmission system may force the plant to generate lower than its rated value, which disables it from selling the remaining available generated power to the costumers and also increases the cost of energy not supplied. In this paper, an efficient method is proposed to determine the maximum output capacity of the power plant, by which the avoidable cost of energy not supplied, is not burdened on the plant. The effectiveness of the proposed method has been evaluated on a three machine test system and also on the actual large Mashhad power plant in Iran.