The generation of power from coal in the United States fell to the lowest monthly level in 35 years in November 2015 as generators switched to cleaner and cheaper natural gas, according to federal data. Gas surpassed coal as the leading source of power in the United States for a fifth month in a row in November, according to the latest data available from the Energy Information Administration, a federal agency. The first time gas overtook coal was April 2015. With just data for the month of December missing from 2015, some analysts think power companies may have burned more gas than coal for the full year for the first time. Coal has been the primary source of fuel for American power plants for at least a century, but its use has been declining since peaking in 2007 and is expected to decline further as the federal government imposes rules to limit carbon emissions.