Are American homes more energy efficient? Not exactly.

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."

The U.S. Department of Energy estimates that 5 to 10 percent of a home's energy use comes from "vampire losses" -- devices such as cellphone chargers, DVRs and computer power supplies that remain plugged in and draw power even when set to "off."

An average digital cable box with a DVR built in, for instance, draws 43 watts when in "off" mode -- more than a 40-watt light bulb when it's on.

Tanner said he doesn't spend much time trying to dissuade customers from indulging their love of gadgets. He said set-top boxes that receive cable signals or record TV shows often suck power all day long.

"You tell them to turn off their cable box or their video box," he said. But then "they crank it on and they lose five minutes when it's warming up. That's the last time they're going to do that."

Residences account for about 22 percent of all the energy used in the country, a total that includes both electricity use and fuels burned for home heating such as oil, natural gas and coal.

Since the late 1970s, U.S. government statistics show the use of heating fuels declining and electricity use edging up: In 1978, electricity counted for about 23 percent of an average household's energy use; in 2005, the number was 42 percent.

That shift has had serious consequences for the country's energy problem. The process of making electricity at power plants and sending it across long distances to homes is inefficient, and some current is lost along the way.

That makes the amount of electricity required by U.S. homes greater than the amount used inside them, because the electrical grid has to allow this lost current in order to get homes what they need. And so, although the amount of energy used inside homes has declined since the early 1980s, the amount required has held steady. That second number matters most because the lost electricity still requires fuel to produce and accounts for greenhouse gas emissions before it vanishes.

The Department of Energy is now rolling out tougher energy-efficiency standards: 20 have already been issued under the Obama administration, and officials say more are planned. Among the targeted devices are televisions, which are not required to meet minimum efficiency standards the way dishwashers, clothes dryers and other appliances are.

Some electric utilities are even looking for ways to sell less of their product. James Owen of Edison Electric Institute said the goal is to reduce greenhouse gases and peak demand on the hottest summer days.

"We call that 'shaving the peak,' " he said, which lowers the highest amount of electricity that the system must be ready to provide. "That's going to enable you to build fewer power plants, absolutely."

Owen said that, in some places, utilities have done this by providing home-energy audits to customers or with programs that help customers replace incandescent light bulbs with high-efficiency compact fluorescents. But he said that in order to pursue these programs, utilities want states to pass "decoupling" laws, which allow utilities to get paid for kilowatts they save as well as ones they sell.

Some environmental groups want to go further and perhaps even to challenge the conventional wisdom that the best way to make homes more energy efficient is to make the changes invisible -- hidden inside new appliances, for example.

Steve Nadel of the American Council for an Energy-Efficient Economy said the old idea was that customers would never really have to sacrifice to save. That strategy was born at the end of the energy crisis of the 1970s out of unhappy memories of wearing sweaters indoors.

"Energy prices were going up, and people really did want to do things" to save energy, Nadel said. "But then energy prices moderated, and people said, 'We want to go back to the old things.' "

Now, he said, "Do we tell people that 'Maybe you should consume a bit less?' I think we need to do more of that."

The current messages from the government and mainstream groups such as Nadel's do not urge serious gadget purges. Their tips on saving energy start with better insulation and sealed ductwork and proceed to choosing more efficient light bulbs and unplugging "vampire" devices. They usually urge customers to buy Energy Star-rated appliances and electronics rather than to stop buying them altogether.

It is possible to get modern, wired-up Americans to cut back sharply on power use, said Alan Meier, associate director of the Energy Efficiency Center at the University of California at Davis. He cited as evidence a shift in Juneau, Alaska, where in 2008 avalanches severed the city's connection to a hydroelectric dam -- and raised rates more than 400 percent.

Residents saved energy by washing dishes by hand and eating by candlelight, according to news reports. The town cut its power use by 25 percent, Meier said.

But is it possible to replicate that in the rest of the country, where electric power remains fairly cheap?

"That's a good question," Meier said. It remains to be seen, he said, whether people are willing to change their habits and make them permanent.