The Index—known by its acronym REDTI—is based on population-weighted heating and cooling degree days and is a valuable tool for explaining year-to-year fluctuations in energy demand for residential heating and cooling.

Residential energy consumption is known to be highly correlated with heating and cooling degree days. Energy consumption increases as the number of heating and cooling degree days increases, and falls as the number of heating and cooling degree days falls. Because of this strong relationship, seasonal changes in the REDTI can provide a good indication of the nation's fluctuating energy demands.

Data is provided in tabular form and via an interactive graph for the contiguous United States from June 1895 to the present, and is also available for download in XML, CSV, and JSON formats.