We look at how much energy efficiency can reduce US energy use by 2040 and what this might mean for savings in 2050. Specifically, we analyze 13 packages of energy efficiency measures and find that together they can reduce US 2040 energy use by 34% and US carbon emissions by 35%, both relative to the most recent US Energy Information Administration forecasts. These reductions put us on a path to 50% savings by 2050. To achieve these savings, we will need to expand energy efficiency efforts beyond business as usual by (1) developing new building codes, equipment efficiency standards, and ENERGY STAR® specifications; (2) substantially improving the efficiency of existing factories, homes, commercial buildings, transmission and distribution systems, and power plants; and (3) better managing freight and aviation energy use, reducing vehicle miles traveled, and spurring changes in how individuals use energy at home, at work, and in transport. A companion fact sheet is available here.