The amount of energy that the average American requires at home has changed little since the early 1970s -- despite advances in technology that have made many home appliances far more energy efficient.

Dishwashers use 45 percent less energy than they did two decades ago, according to industry data. Refrigerators use 51 percent less.

But on a per-capita basis, Americans still require about 70 million British thermal units a year to heat, cool and power their homes, just as they did in 1971. (One BTU is the energy required to heat one pound of water one degree Fahrenheit.)

A key reason, experts say, is that American homes are getting bigger, which means more space to heat and cool. And consumers are buying more and more power-sucking gadgets -- meaning that kilowatts saved by dishwashers and refrigerators are often used up by flat-screen televisions, computers and digital video recorders.

These trends "have balanced each other out. It's been a wash, basically," said Lowell Ungar of the nonprofit Alliance to Save Energy.

Now, environmental groups as well as some in the utilities business are trying to find ways to end this long period of no change. They say that as the country's population grows and we build more homes, the fight against greenhouse gas emissions will require that every home use much less energy than current levels.

The flatlining of at-home energy use "has been a success story," Ungar said, because without increased appliance efficiency, use of power in homes would have shot up. Still, he said, "it's not enough."

Saving energy is easier than making more of it. Increasing energy efficiency is called the low-hanging fruit in the effort to cut emissions as well as imports of foreign fuel without harming the economy.

Last year, the consulting firm McKinsey and Co. estimated that by 2020 the United States could cut its projected energy use 23 percent by implementing efficiency measures and that about one-third of that change could happen in homes.

But success has remained frustratingly out of reach in American homes. In Northern Virginia, Troy Tanner, president of a company called the Home Energy Detective, has an up-close view of why.

Tanner helps clients add insulation or seal leaks in air ducts, which saves energy needed for heating and cooling -- and money spent on bills. But often, Tanner said, neither the money nor the energy stay saved.

"When they start saving money on their bill, and they get that extra $100 per month," they go out and buy a 96-inch TV that's on sale, said Tanner. "The temptation is too much: They buy it."