To calculate a home's HERS Index Score,

a certified RESNET Home Energy Rater will do a home energy rating and compare the data against a 'reference home' – a design modeled home of the same size and shape as the actual home, so the HERS Index Score is always relative to the size, shape and type of house you live in. The lower the number, the more energy efficient the home.
The U.S. Department of Energy has determined that a typical resale home represents 130 on the HERS Index while the reference home (a standard new home built in 2006) is rated at 100.
• A home with a HERS Index Score of 70 is 30% more energy efficient than a standard new home.
• A home with a HERS Index Score of 130 is 30% less energy efficient than a standard new home.
To calculate a home's HERS Index Score, a certified RESNET HERS Rater does an energy rating on your home and compares the data against a 'reference home'– a designed-model home of the same size and shape as the actual home, so your score is always relative to the size, shape and type of house you live in.

A lower score means a more affordable, comfortable and energy efficient home.