Flexible-grid elastic optical networks (EONs) are widely considered as the future of next generation optical transport networks. Depleting spectral resources and increasing power consumption are two critical bottlenecks of EONs. In this paper, we propose a novel impairment-aware resource allocation scheme that jointly minimizes bandwidth waste and power consumption of EONs. The proposed scheme is presented in the form of an analytical optimization for small-sized networks. A low-complexity heuristic is then proposed for large networks. Numerical results show that by appropriately assigning the route, modulation format, and carrier frequency to each connection request, the proposed scheme significantly increases spectral efficiency and reduces power consumption in networks that handle varying traffic volumes.