This project examined the long‐term role of energy efficiency and renewable energy technologies in reducing energy use and meeting greenhouse gas emissions goals. This work presents an integrated, century‐scale perspective that, for the first time, allows consistent long‐ term (100‐year) scenarios that simultaneously include California building end‐use and renewable energy detail, the rest of the United States, and 13 other world regions. The global and national context for climate mitigation under a variety of climate policy regimes are briefly presented, followed by an analysis of the potential value of end‐use energy and renewable energy technologies. This analysis found that the deployment of a comprehensive suite of more energy efficient technologies has the potential to substantially lower climate mitigation costs. As an exploratory application of these modeling concepts at a state level, century‐scale scenarios of building energy use in California are also presented, along with an analysis of long‐term role of wind and solar power in California. The current divergence between California and the rest of the United States in terms of per‐capita energy use is sustained. Residential building electricity demand in California, however, still increases by a factor of almost three, due to population growth. Solar and wind energy have the potential to supply a large portion of California’s electricity needs, but further examination of the associated transmission and electric system stability requirements is needed.