Falling U.S. electricity production in the past two years is frustrating the utility industry and shaking up timetables for some major infrastructure projects.

Electricity output decreased 3.7% last year, the steepest drop since 1938, according to federal statistics, following a nearly 1% decline in 2008.

The recent downward trend is making it trickier for utilities to forecast future power consumption, a critical component of planning investments in new power plants and transmission lines.

The falling electricity demand and production are attributed to a weak economy, conservation efforts and, in 2009, a relatively mild summer in many parts of the country.

The possible completion date for the $1.8 billion Potomac Appalachian Transmission Highline, or PATH project, that Allegheny Energy Inc. and American Electric Power Co. intend to build from West Virginia, through Virginia, to Maryland may be delayed by several years because of weaker electricity demand.