Operating at the intersection of multiple sensing and control systems designed for occupant comfort, performability and operational efficiency, modern buildings represent a prototypical cyber-physical system with deeply coupled embedded sensing and networked information processing that has increasingly become part of our daily lives. In this paper, we look at modern buildings entirely as a cyber-physical energy system and examine the opportunities presented by the joint optimization of energy use by its occupants and information processing equipment. This paper makes two contributions: one, a careful examination of different types of buildings and their energy use; two, opportunities available to improve energy efficient operation through various strategies from lighting to computing. Using a modern 150,000 sq feet office building as a closed system, we detail different strategies to reduce energy use from LEED certification to zero net energy use.