Negawatt is a theoretical unit of power representing an amount of energy (measured in watts) saved. The energy saved is a direct result of energy conservation, increased efficiency, or demand response. Unlocking the negawatt rests on our ability to quantify and improve the performance of the building stock over time. However, building stock energy models, particularly those accounting for both physical formulation and social behaviors of the underlying buildings, are still in their infancy. This book was written as a Ph.D. dissertation that strives to more thoroughly examine how buildings perform aggregately in energy usage by focusing on three major challenges: quantifying building energy performance in an objective and scalable manner, mapping the building stock model space to the real-world data space, and quantifying and evaluating energy intervention behaviors of a building stock. The proposed methodology can be used by energy policy makers and utilities to evaluate energy retrofit incentives and demand-response program economics.