The Home Energy Rating System (HERS) Index is the industry standard by which a home's energy efficiency is measured.  

HERS scores are designed for new homes (or homes after a complete renovation). When builders need to meet efficiency standards, they use the HERS Index, which has become the accepted industry standard for documenting the energy performance of a building.

To get a HERS score, a property needs to be inspected by a certified RESNET rater. The process is extensive. The rater comes prepared with equipment. The inspection and assessment typically cost between $1,500 and $3,000, so it’s an investment. But energy improvements toward a good HERS score save homeowners more in the long run.

Here are some of the energy related factors a rater evaluates:

• Air leaks in the building envelope
• Heating, ventilating, and air conditioning (HVAC) distribution duct leaks
• Any combustion risks
• Air infiltration rates
• Wall, ceiling, and floor insulation
• Water heating systems
• Thermostats
• Foundations
• Attics and crawl spaces

After inspecting the home and running the data through specialized software, the rater assigns a HERS score.  The lower the number, the better the score. Each one-point change in a score, up or down, represents a one percent shift in energy efficiency.

To get an accurate score the rater compares the home to a standard Reference Home. The Reference Home isn’t an actual house, just an analysis tool, but it resembles the rated home as much as possible—same size, shape, environment, and climate. This means your score is relative to the type of house you live in.  A score of 100 indicates a new house is built to code. It’s even better to score below the average.  A HERS score of 0 or a “net-zero” home produces or conserves as much energy as it uses.    Some of our homes actually create more energy than they consume, resulting in a negative HERS score.

The HERS report for a new home estimates the cost of utility bills and efficiency upgrades, giving you an accurate picture of how much you’ll be spending. The lower the score, the fewer upgrades you need. A lower score also lowers your projected utility bills significantly.  After mortgages, the biggest expenses homeowners face are usually climate control-related: heating, cooling, and water heating. Naturally homeowners are on the lookout for ways to bring these costs down.

ENERGY STAR certified homes (a slightly different process than the HERS index, but measuring the same factors) have saved their owners a combined $20 million in energy costs, according to the Department of Energy (DOE). A good HERS score is the first step to getting a home eligible for ENERGY STAR certification.

The Department of Energy has a site full of guidelines for energy-conscious homeowners - there are plenty of ways to reduce your energy consumption. 

(The official overview from RESNET: What is the HERS Index?)