Germany leads the world in harnessing the benefits of energy efficiency, followed by Italy, the European Union, China and France, according to a new ranking of the world's 16 largest economies. The United States was near the bottom, placing 13th.

America's poor showing is sobering for a nation accustomed to being a world leader, and it could have economic consequences. "How can the United States compete in a global economy if it continues to waste money and energy that other countries save and can reinvest?" said Rachel Young, the principal author of the energy efficiency report card.

Even more sobering is the news that many of the countries are stalled or regressing on energy efficiency, and all them could save significantly more energy by embracing the full range of proven technologies and policies, according to the American Council for an Energy-Efficient Economy (ACEEE), which produced the International Energy Efficiency Scorecard released on Thursday.

"There are substantial opportunities for every country to improve," said Young. The average score of all the ranked economies was just 50 points out of a possible 100—an indication, she said, that "overall, countries are failing to adopt best practices, and if they improve, they do so in small increments."

Energy efficiency must provide more than half of the world's carbon emissions reductions to avoid a catastrophic temperature increase, according to the International Energy Agency.

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Energy efficiency refers to a wide range of measures that allow people, businesses and governments to use less energy but get the same services. The measures include mandating better fuel economy for vehicles, reducing wasted energy at power plants and on transmission lines, and designing buildings and homes so they require less heating and cooling, among other things.

The ACEEE's 2014 scorecard ranked 15 countries and the European Union, which together represent 81 percent of worldwide economic output and 71 percent of global energy consumption. Each economy was rated based on 31 criteria in four categories: national efforts/policies, buildings, industrial and transportation.

Germany's top score in the ACEEE report stems from the country's emphasis on energy efficiency as a big part of its "Energiewende," the nation's ambitious energy shift from fossil fuels and nuclear power to renewable energy and lower energy consumption.

"We see this as a validation that Germany's measures are bearing fruit in its ongoing efforts to transition towards a low-carbon and energy-efficient economy," Dr. Philipp Ackermann, deputy chief of mission at the German embassy in Washington, said in a statement.

"The cheapest energy is the energy you don't have to produce in the first place," he added during a conference call with reporters. "Our economy is growing—and with a satisfactory growth rate—but without using more energy."

Still, Germany got just 65 points in the ACEEE scorecard—so it's only two-thirds of the way to a perfect score.

The United States, meanwhile, was far behind Germany. It captured a mere 42 points and outperformed only three countries—Russia, Brazil and Mexico. The ACEEE said the U.S. performance was unimproved from 2012, when the group last ranked international energy efficiency efforts. In that report, the nation ranked ninth out of 12.

U.S. Congressman Peter Welch (D-Vermont), a proponent of energy efficiency, said "There's really no excuse for the U.S. lagging behind other nations on energy efficiency." Energy-saving policies in his home state—including an energy efficiency utility called Efficiency Vermont –have demonstrated that the effort "saves money, reduces environmental impact and creates jobs."

Rep. Welch introduced energy efficiency legislation that passed in the house this year with bipartisan support. In the senate, though, a companion bill got stalled when lawmakers sought to tack on controversial measures such as approval of the long-delayed Canada-to-Texas Keystone XL tar sands oil pipeline.

Welch believes the bill got caught up in election year politics, and pledged to try again. "This is an extraordinarily important issue that has enormous upside potential for our economy and for our environment," he said.

The ACEEE and Welch are also hopeful that U.S. energy efficiency will get a boost from the recently proposed federal air pollution rules requiring states to meet targets for reducing carbon emissions from power generation. The draft rules were released by the Environmental Protection Agency last month.

"I think energy efficiency will get a lot more attention if EPA finalizes its power plant rule along the lines of what they proposed," said Steven Nadel, ACEEE's executive director. "Energy efficiency is the low-cost compliance path for all the states. Efficiency can often do the majority of what each state will need to do to meet its targets."

Other steps could also help, including pending fuel economy standards for light-duty and higher standards for heavy-duty vehicles.

The ACEEE report included these other notable findings:

The United States performed best in the category for buildings, where it tied for eighth place with the United Kingdom. The country's ranking was substantially boosted by its appliance and equipment standards.

The United States got the maximum points in four out of the 31 criteria, including tax credits and loan programs, appliance and equipment standards, investment in manufacturing research and development and fuel efficiency standards for heavy-duty tractor trucks.

The United States scored worst in the transportation category, landing just eight points out of a possible 25. The nation's performance suffered because the vehicle miles traveled per capita far exceeds that of other countries. It also scored poorly on public transit and fuel economy for light vehicles.

The United States is one of only two countries in the report without a national energy-savings plan or national greenhouse gas reduction plan. The nation also lacks water efficiency or conservation policies, as well as mandatory energy audits or on-site energy managers at manufacturing facilities.

China tied for fourth with France in the overall energy-savings scorecard. It ranked first in building energy efficiency. Its residential buildings consume less energy per square foot than in any of the other countries in the ACEEE report, and its commercial buildings are second in the same metric.

China scored well in transportation efficiency, too. It has mandatory fuel economy standards for light and heavy vehicles, has a low number of miles traveled per person, and a higher percentage of trips on public transit. The country has a target of five million plug-in hybrid and electric vehicles by 2020.

Australia ranked 10th in the ACEEE scorecard, and had been improving. The country was strongest in building efficiency, partly because of comprehensive building codes that include mandatory minimum energy-performance requirements. Australia also got a boost from incentives and voluntary energy performance agreements with manufacturers, and from government investments in energy efficiency.