ISLA MABAD (APP) The total generation capacity of the Tarbela Hydropower Station would rise to 4,888 megawatt (MW) after the installation of 1410MW units as part of 4th Extension Project. Sources told APP here on Sunday that on completion, Tarbela's 4th Extension Hydropower Project would provide about 3.84 billion units of low-cost electricity to the national grid per annum. They said the project would be constructed with an estimated cost of $ 935 million and the WAPDA had already arranged the finances required for it through the World Bank. Annual benefits of the project have been estimated at about Rs.30.7 billion. The project would pay back its cost in just three years, they said. The sources said that the project would also be instrumental in saving foreign exchange on import of almost one million tons of furnace oil annually, required for the equivalent generation of electricity from thermal sources. The project would also provide a cushion to undertake rehabilitation and upgradation of the existing Tarbela Power House during the lean period, they added. They said three turbines of over 450-MW each would be installed instead of two turbines of 480-MW each, as envisaged in a study conducted earlier in 90s. It is pertinent to mention that Tarbela 4th Extension Hydropower Project is a part of least-cost energy generation plan, being implemented by WAPDA on priority to harness indigenous hydropower resources of the country with a view to improving the ratio of hydel electricity in the National Grid. Most importantly, the project will also provide a cushion to undertake rehabilitation and up-gradation of the existing Tarbela Power House during the lean period. It is also important to note that hydel is the cheapest source of electricity with an average cost of Rs.1.05 per unit.