"In U.S. regions with competitive electricity markets, the market price of wholesale electricity has fallen in recent years due to decreased demand, and the increased availability of relatively low-priced natural gas as a fuel. The relatively higher cost of operating and maintaining older, less efficient coal and nuclear plants in particular make it difficult for them to compete with lower cost, more efficient natural gas-fired power plants, or with renewable electricity generation with lower operating costs (and in some cases, tax credits and state mandates). These coal and nuclear power plants may be increasingly faced with closure and eventual retirement if they cannot offer their generation at prices that allow them to sell their electricity into the competitive markets. Competitive electricity markets are administered by independent system operators (ISOs) and regional transmission organizations (RTOs), and account for 60% of the electricity supply in the United States. These entities are under the regulatory authority of the Federal Energy Regulatory Commission (FERC)."
CRS Insight, IN10806
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html