Starting with the 2012 version, the Scoring Tool uses an expanded scoring method which maps a home to its local weather station and a unique 10 point scale for that climate location.
Under the initial Home Energy Score program design for pilot tests, each of 19 geographic zones across the United States had a corresponding 10 point scale with source energy thresholds defined for each point on the scale. However, analysis showed that weather differences within each of the 19 zones are significant enough to skew scoring results. In fact, the scores of identical homes – with different weather but within one geographic zone - could vary by several points. As a result, the Department of Energy recognized the need to generate a larger set of 10 point scales for more than 240 different weather stations across the U.S.
In the final stages of testing the Scoring Tool, in an effort to more accurately account for climate differences the final version of the Scoring Tool was used to estimate the source energy use for a wide range of homes in each of the 240+ weather station locations (See table example below). The DOE then established energy values for the 10 point scale in each location based on the following guidelines and objectives:
With the above methodology, the Home Energy Score label conveys concise information about a home’s absolute energy consumption and ranks the home on a consistent scale which is applicable across all U.S. regions. Since the energy budget is absolute energy consumption, calculated scores are sensitive to home size, meaning larger homes will tend to score lower. The Home Energy Score FAQs clearly acknowledge this fact.
NREl score batch analysis methodology is available here.