American houses are getting more massive. They’re becoming more plentiful. We’re cramming their outlets with an ever-expanding array of power-hungry electronics — from large flatscreen TVs to multiple smartphones to the occasional iPad.

And yet, surprisingly, the average American home now uses less energy than ever before. That’s according to a new analysis from the Energy Information Administration, which offers up the graph below. As a result, total energy use for all U.S. homes has flatlined since 1980, even as the overall number of houses keeps growing:

Why is that? One reason is that homes are becoming considerably more efficient — the EIA notes that newer houses tend to feature better insulation and things like double-paned windows that help lower the utility bills. (Remember, heating and cooling takes up the biggest chunk of a home’s energy use.) Many appliances, like refrigerators and washing machines, have also become more efficient, thanks to government regulations signed by President Reagan in 1987.

But it seems like there’s another, oft-neglected reason, as well. Demographers have noted that, in the past 20 years, more and more Americans are moving away from the chilly Northeast and to the Sunbelt in the West and Southeast. And, judging from this graph, made by Stuart Staniford using EIA data, that can have a huge impact on energy use. Houses in those booming Sunbelt regions use less energy:

Average annual household energy use, by region. Data from the Energy Information Administration. Graph by Stuart Staniford.

Note that homes in the South and West use a whole lot of energy for air conditioning during the summer — the muggy South uses more than twice as much as anyone else. But they also require considerably less energy for heating in the winter. And that tends to outweigh the A/C effect.

In any case, there are two ways to interpret the graphs above. One is that the United States has figured out how to house and shelter its growing population more efficiently in the past three decades, without energy use exploding. Another possibility, though, is that at least some of our efficiency gains over the years have been negated by bigger homes and by the growing popularity of various appliances and electronics. Indeed, that’s one reason why some energy wonks worry that improved efficiency might not ever make a significant dent in our energy-consumption patterns.

Related: How Americans use energy, in three simple charts.