The demand of cloud computing grows rapidly with the rapid increase in IT infrastructure. Cloud computing now a day, are widely used by industry, organization and society to deliver IT services. This rapid growth leads to the creation of large data centers. These data centers requires enormous amount of electrical power for its operation and thus, result in high operational cost and carbon-dioxide emission. In this work the key idea is to reduce the requirement of power consumption by efficient task allocation and its cost. Datacenters are connected to conventional power grid as well as to renewable energy source. We performed this work in two phases: first used goal programming optimization for energy efficient task allocation to reduce power consumption and then analyzed the reduction in cost if we use RES for power supply. We used solar power panel as renewable energy source and analyzed the significant reduction in cost.