Power and energy management, and more recently, sustainability are emerging to be critical challenges for future IT systems. While there has been a lot of prior work in this space, a lot more needs to be done. In this talk, I will discuss the challenges and opportunities in rethinking how we study and reason about energy efficiency for future systems. Specifically, I will talk about how confluence of emerging technology and industry trends offer exciting opportunities to systematically rethink the "systems stack" for the next orders of magnitude improvements in energy efficiency.