Energy intensity in the United States has been declining steadily since the early 1970s and is expected to decline through 2040, the U.S. Energy Information Administration said Friday.

A country's energy intensity is usually defined as energy consumption per unit of gross domestic product (GDP).

"Greater efficiency and structural changes in the economy have reduced energy intensity," the agency said in a report Friday.

From 1950 to 2011, energy intensity in the United States decreased by 58 percent per real dollar of GDP, according to the EIA.

The EIA expects residential energy intensity, measured as delivered energy used per household, to decline about 27 percent from 2005 to 2040.

Commercial energy intensity, measured as delivered energy used per square foot of commercial floorspace, is projected to decline about 17 percent from 2005 to 2040.

Industrial sector energy intensity, measured as delivered energy per dollar of industrial sector shipments, rises above its 2005 level initially owing to the 2007-09 recession but ultimately decreases 25 percent below its 2005 level in 2040, according to the EIA projections.

Transportation sector energy intensity is more difficult to measure because of the multiple modes of transportation, the agency said.

Light-duty vehicle energy intensity, which is measured as their consumption divided by the number of vehicle-miles traveled, is projected to decline by more than 47 percent from the 2005 value, the EIA said. Endi