Reducing the power consumption of network interfaces contributes to lowering the overall power needs of the compute and communication infrastructure. Most modern Ethernet interfaces can operate at one of several data rates. In this paper, we present Queue Length Based Rate Adaptation (QLBRA), which can dynamically adapt the link rate for Ethernet interfaces at runtime using existing Ethernet standards. An implementation of the proposed rate adaptation functionality is demonstrated at runtime on a NetFPGA platform. Our results show that the rate adaptation approach can achieve significant energy savings and at the same time improve the throughput of TCP traffic due to the effect of packet pacing.