Improving the representation of end-use energy efficiency, and of the effects of policies and programs to promote it, is an emergent priority for electricity load forecasting models and methods. This paper describes a “hybrid” load forecasting approach combining econometric and technological elements that is designed to meet this need, in a novel application to long-run electric power transmission planning in the western United States. A twenty-year load forecast incorporating significant increases in energy-efficiency programs and policies across multiple locations was developed in order to assess the potential of efficiency to reduce load growth and requirements for expanded transmission capacity. Load forecasting and transmission planning background is summarized, the theoretical and empirical aspects of the hybrid methodology described, and the assumptions, structure, data development, and results of the aggressive efficiency scenario are presented. The analysis shows that substantial electricity savings are possible in this scenario in most residential and commercial end-uses, and in the industrial sector, with magnitudes depending upon the specific end-use as well as upon the geographic location of the utility or other entity providing the electricity.