Power-awareness in networking attracts more attention as the trends in the energy consumption of the Internet raise growing concerns about the environmental impacts and sustainability of the network expansion. Building energy efficient equipment is definitely an integral part of the solution. However, such a strategy should be complemented with appropriate network protocols and routing methods to achieve maximum performance. In this paper, total power consumption of an optical WDM network is modeled in terms of the power consumed by individual lightpaths. The proposed model is then used to develop an ILP (Integer Linear Programming) formulation of the grooming problem. The exact solution of the formulation on a small network indicates that significant energy savings can be achieved with power efficient grooming.