It's probably not what you think. HERS is the acronym for Home Energy Rating System. We scored 45. That doesn't sound so good, but we were quite happy with it.
A score of 100 means that a home meets the requirement of the energy code. Every one point decrease represents a one percent decrease in projected energy usage. Our score of 45 indicates that we are projected to use 55% less energy than a baseline house designed to meet the 2006 IECC energy code.
The HERS rating takes into account many factors including insulation levels, HVAC equipment efficiency, window efficiency, the blower door test and water heater efficiency. This is not a superinsulated house and we didn't take extreme measures. The house, though, is well insulated and the HVAC equipment is efficient, but we couldn't even use the some of the new highly efficient windows because we had to use impact resistant windows. We did have an ace in the hole, though.
The PV solar system provides about half of our electricity. I don't know how much this adds, but seemed to really help with the HERS rating. The HERS rating also factors into our final LEED rating.