The HERS Index Made Easy

No, a HERS Index number is not a reflection of how much of a home’s closet space is occupied by women’s clothes. Nor is it a gauge of femininity.

“HERS” is short for Home Energy Rating System. As explained in Wikipedia, a HERS Index number is a relative energy use index making possible energy consumption comparisons among homes built during different years, with different building components and methods and governed by different, ever-evolving building codes. Just as EPA Estimated Mileage Ratings for all types of vehicles facilitate fuel efficiency comparisons, HERS Index numbers enable the relative energy efficiency of different homes to be compared.



Unlike the EPA gas mileage rating, where higher is better; lower is better under the HERS Index system. For the HERS system, the index number of “100” is assigned to a home built to the standards of the 2006 International Energy Conservation Code (IECC). For pre-2006 homes, which typically would have been built to significantly less strict energy efficiency standards than those of the 2006 IECC, the HERS Index number will be greater than 100. For homes built to standards that will produce more energy savings than a home built to the building code standards of the 2006 IECC, the HERS Index will be lower than 100.

For example, for a home to be Energy Star certified, the home must be at least 15% more energy efficient than a home built to the standards of the 2006 IECC. In the next paragraph, you will see that a home having a HERS Index of 85 or lower will confirm compliance with the Energy Star energy efficiency criteria. For a home that employs renewable energy resources and produces as much energy as it consumes, such a home is referred to as a “Net Zero” home, reflective of it having a HERS Index of “zero” and requiring no net consumption of energy in its operation.

Knowing the HERS Index number for comparison homes, you determine the percentage by which one home is more energy efficient than another by subtracting the lower HERS Index number from the higher HERS Index number. The difference between two HERS Index numbers is the percentage by which the home having the lower index is more energy efficient than the home having the higher index.

By way of example, if you compared a 31-year-old home built to 1980 building code standards against a 5-year-old home built to 2006 IECC standards, the older home likely would have a HERS Index of 130 or greater and the home built in 2006 would have a HERS Index of 100. Subtract the lower index of 100 from the higher index of 130 and the difference of 30 means that the home having the lower HERS Index should consume 30% less fuel. Viewed another way, if the 31-year-old home were to have a $4900 annual energy bill for heating, cooling, lights and hot water, you could expect the home with the lower HERS Index to consume $1470 less per year in energy costs (30% x $4900). Using the HERS Index for home energy consumption comparison purposes and approximating differences in annual energy costs is that easy.

While a verified HERS Index of 85 or lower is mandatory for a home to receive EPA Energy Star certification, the highest HERS rating permitted for a home to be described appropriately as high performance in energy efficiency is 70. Since the average age of all homes in the US is 34.6 years and a home of average age typically would have a HERS Index at or above 130, the HERS Index difference of 60 (130-70) and an energy efficiency advantage of 60% in favor of the high performance home, bears out what any builder of a new, high performance home will attest. “We indeed do not build homes like they use to. We build them better!”

If you contemplate remaining in your present home many more years and you want to know your HERS Index number and how to improve your home’s energy efficiency, retain a BPI-certified or RESNET-certified energy auditor. This energy specialist can provide a full energy audit of your home, with the audit providing a tested and verified HERS Index number and a listing of recommended energy conservation improvements in order of cost effectiveness. If you are planning your next home investment, knowing the HERS Index number of the home you are considering is essential. If obtaining an energy audit and HERS rating for that next home means paying for an energy audit yourself, do not hesitate to do so. And, having a contract contingent upon obtaining satisfactory energy audit results is at least as important as having a home inspection contingency. If you plan to acquire a new home that will be third-party tested and verified as being high performance in energy efficiency, the initially greater expense of high performance will not be an unrecovered expense. Rather, that cost will more than pay for itself. Making that type of home investment, though, takes us back to the question first posed, “What’s your HERS Index?”