Everyone knows how many miles per gallon their car is expected to get when they buy it. This reduces the idea of efficiency into a single number, something you don't have to be a mechanic to understand. If you want to buy a vehicle that conserves fuel, it isn't difficult. Having the MPGs prominently displayed helps the consumer.
A home's energy use is arguably more complicated than a car's energy use, and so there is no real equivalent to MPGs - no one number that everyone knows and understands. Buying an efficient home is not as simple as buying an efficient car.
To that end, here is a list of some of the home ratings systems that are available.
The Department of Energy is introducing a new rating system - the Home Energy Score, a simple 10 point scale.
Acting Under-Secretary Cathy Zoi explains:
The program is being tried in a few communities across the nation and will be fully implemented this summer.
More information can be found at the Department of Energy site.
This program is separate from the ENERGY STAR program that is also partially ran through the Department of Energy. Similar to the ENERGY STAR rating for appliances, a certified home will be 20-30% more efficient than an average new home, although the requirements for ENERGY STAR certification are becoming more strict. More information can be found here.
Another existing rating system is the HERS rating system, explained here.
The HERS system has been around since the 1990s, and operates similar to a golf score - lower is better. Your average new home would be scaled at around 100, older homer would be higher, and a net-zero home would have a HERS index of 0.
It has been developed by the Residential Energy Services Network (RESNET). HERS compares a home to the International Energy Conservation Code (IECC) standards for 2006.