New results from the U.S. Energy Information Administration’s Residential Energy Consumption Survey finds that homes built after the turn of the millennium use roughly the same amount of energy as those built before the year 2000 - despite being 30 percent larger, on average (link). Here is a chart that shows total household energy use in homes built before and after 2000:

Newer homes are inherently more energy efficient than older homes because of stricter building and energy codes, which require higher levels of insulation, reduced duct leakage in HVAC systems, and lower leakage of air outside of the building envelope. Whereas energy codes before the turn of the century were little more than “anything goes”, modern energy codes incorporate the science of building design, materials, and airflow to reduce energy usage.

These efforts have been led by the U.S. Department of Energy’s Building Technologies Program, local governments, and industry standard setting bodies like ASHRAE and the International Code Council. The City of Austin, for example, has adopted both the 2009 International Energy Conservation Code (2009 IECC), and also collaborates on the development of energy codes and efficiency programs. Strict energy codes are a major component of the City’s efforts to provide electricity to its citizens while balancing staggering population growth.

The EIA results also illustrate the population growth in warmer parts of the country, namely the West and South, as seen in the reduced energy consumption in space heating and increased air conditioning. Energy for space heating decreased by 18 percent, while air conditioning increased 56 percent.

According to the 2010 U.S. Census results, the South and West regions account for over 84 percent of the population growth between 2000 and 2010 (PDF):

Also interesting is that newer, larger homes also use more energy for appliances and electronic devices - approximately 18 percent more than a home built prior to 2000. Keep in mind that new household appliances and HVAC systems are more energy efficient than their predecessors, due in part to goals outlined by ENERGY STAR. Excluding behavioral variables, we would expect household energy consumption in older homes (pre 2000) to decrease as appliances and HVAC units are replaced with more efficient units as they reach the end of their useful life, and homeowners make repairs and upgrades (usually in the form of adding insulation) to their homes.