This paper presents the theoretical determination of power generating capacities of Kainji, Jebba and Shiroro hydropower stations in Nigeria. The theoretical power output equation was developed from the fundamental energy equations. Hydrological data like gross head (H) in metres (m) and Volumetric flow (Q) in cubic metres per second (m3/s) were obtained from the various efficiency departments of the hydropower stations for a period of 2013 to 2017. The results obtained reveal that the average power generation ranges from 298.03MW to 367.53MW for Kainji, 251.45MW to 337.24MW for Jebba and 176.42MW to 207.37MW for Shiroro. The above generation levels fall short of the installed capacities of 760MW, 578MW and 600MW of Kainji, Jebba and Shiroro respectively. Thus, there is need to increase the efficiency of these hydropower stations in terms of operational improvements and application of new technologies.