Briefly, HERS stands for “Home Energy Rating Systems.” A “HERS Rating” provides a numerical evaluation of the energy features contained in a residence or residential building, also known as an index score. A HERS Rating Index number is similar to the Miles-per-Gallon or “MPG” rating of a new vehicle. Cars that have higher MPG ratings are more efficient and go further per gallon of gas or diesel fuel. Homes that have lower HERS index scores use less energy for heating, cooling, water heating, etc. The HERS Index number gets lower as the  efficiency of a home gets better.

A home that gets a HERS Rating Index score of 100 exactly meets the energy reference; roughly equal to the 2006 International Energy Conservation Code (IECC). A home that gets a HERS Rating Index score of 0 (zero) means that the home either requires no outside energy input to stay warm or cool or provide power, or that it generates as much energy as it is consuming. It is considered to be a “Zero Energy Home”. The U.S. Department of Energy has determined that the typical resale home scores 130 on the HERS Index, which is 30% higher energy usage than the energy reference. In contrast, while today’s building energy conservation codes vary based on where a home is being built, new construction ranges from 15% to almost 50% more efficient than the energy reference. This means that their HERS Index numbers are typically between 51 and 85. It is possible to have a negative HERS Rating Score, which means that the home produces more energy than it needs over the course of a year. A negative HERS Rating Score is usually produced by using solar and/or wind energy systems.