Citing data from representative PJM Interconnection, the agency said the fuel mix in the Mid-Atlantic area of the country had been fairly consistent while other US regions saw larger shifts.

Despite the increase in generation from natural gas in 2012 and this year, particularly in Pennsylvania and Ohio, cost-efficient Illinois Basin coal has allowed Mid-Atlantic coal-fired units to produce about half of the electricity in the PJM system, the nation’s largest and especially in Indiana and Illinois.

“Although natural gas-fired units do not make up a majority of the generation units in this region, these units often set the wholesale electricity price,” the EIA said.

“This is because in the PJM market, the electricity price is determined by the last unit selected (the marginal unit) to run at a given time, based on fuel and other operating costs.”

The agency said additionally, because natural gas often fuelled the last unit selected, wholesale power prices for the PJM were generally parallel with natural gas prices on any given day.

Last week, EIA experts reported that production was on the rise in the US as the nation braces for colder, shorter winter days.

For the first full week of November, total US coal production was 19.8 million short tons, 2.8% higher year on year.