In 2010, as one of many energy initiatives within a broader economic stimulus program, the U.S. Department of Energy (DOE) and Lawrence Berkeley National Laboratory (LBNL) initiated development of a new web-based computer tool and method for providing an energy rating of existing single-family homes. The resulting Home Energy Scoring Tool is a key component of the DOE's Home Energy Score Program for residential building energy labeling, a voluntary national asset rating method that employs a simplified and standardized energy assessment process. The tool-development component of the program has been designed to support the energy audit marketplace by providing a substantially lower-cost, entry-level assessment method analogous to the fuel-economy ratings associated with vehicles. Averaged over a well-characterized sample of homes, the Home Energy Scoring tool is accurate to within 1% of mean weather-normalized energy bills (with 82% of homes having an absolute error of 25% or less), significantly better than two other popular methods known as SIMPLE and REM/Rate. This article presents technical details of the Home Energy Scoring Tool, and how it has evolved over time, including the calculation methodology, accuracy validation, and the web services feature that allows any qualified third-party software developer to integrate the methodology into their own web-based applications and market delivery strategy. As of April 2014, approximately 200 individuals had been qualified to deliver the assessments and had rated 10,600 homes in cooperation with 23 partner organizations across the United States.