Performance evaluation is used to gain an understanding of how to make the best use of scarce resources. Storage, memory, processing, and communications bandwidth are all relatively plentiful and inexpensive. What is the next frontier for communications networks and performance evaluation? I will argue that it is power management to achieve cost-effective operation. In the past few years, entirely new network protocols have been developed for battery-hungry sensor networks. But, what about the existing Internet? Estimates place the Internet as consuming from 2% to 8% of the total electricity produced in the USA - much of this power consumption is unnecessary. Do our “always on” desktop computers really need to be fully powered-up all the time? What can be done to achieve power-savings in these computers? The goal is to eliminate unnecessary energy usage by desktop computers in the near future and by networked embedded systems in the longer term. Traffic characterization is the first step towards this goal. Traffic characterization at inter-flow, intra-flow, and protocol levels is being done to investigate power management. The resulting savings achievable from relatively simple power management schemes are measured in TWh per year - or roughly equivalent to the electricity generated by one nuclear power plant. This is cost-effectiveness on a large scale!