Regulating Power Sector Carbon Emissions


  • Twenty-eight percent of U.S. greenhouse gas emissions come from generating electricity.
  • EPA has authority under the Clean Air Act to monitor and regulate greenhouse gas emissions. The U.S. Supreme Court first confirmed this interpretation of the law in 2007.
  • Power plant greenhouse gas emission regulations are being reviewed by EPA. Regulations on other pollutants from burning fossil fuels remain in effect.

Electric power generation is responsible for 28 percent of U.S. greenhouse gas emissions. These emissions come from burning coal or natural gas to make electricity. Greenhouse gas emissions from the power sector have fallen dramatically since their peak in 2005, but nonetheless represent a major source in the economy. Despite growing renewable generation, fossil fuels still account for about 63 percent of U.S. electricity.

Multiple laws and court cases give the U.S. Environmental Protection Agency (EPA) the legal authority to monitor and regulate greenhouse gas emissions.

Regulations were proposed (and some were finalized) after EPA’s authority to directly regulate greenhouse gas emissions under the Clean Air Act was clarified in the U.S. Supreme Court decision in Massachusetts v. EPA (2007). In response to an Executive Order issued by President Trump, EPA has issued a replacement to the Obama Administration’s Clean Power Plan.

Annual share of U.S. net electricity generation by source (1990 – 2018)

Existing Power Plants

EPA regulates greenhouse gas emissions from power plants under Section 111 of the Clean Air Act, which allows EPA and the states greater flexibility in crafting and implementing a standard. Emissions from new and modified power plants have been finalized under Section 111(b), as described below. Emissions for existing power plants are still being developed under Section 111(d).

In June 2019, EPA issued GHG emission regulations for existing fossil fuel-fired power plants in the Affordable Clean Energy (ACE) Rule. The rule would direct states to require coal-fired power plants to use prescribed technologies to improve their heat rate (i.e., increase efficiency), as a best system of emission reduction for carbon dioxide.

The ACE Rule is not expected to reduce power sector emissions from a business-as-usual scenario. It would replace the Clean Power Plan, finalized in August 2015, that had set state-specific emission rates and provided various market-based flexible compliance provisions.

New Power Plants

In December 2018, EPA proposed GHG emission regulations for new, modified, and reconstructed power plants. The proposed rule would replace EPA’s 2015 “Carbon Pollution Standard for New Power Plants” which established New Performance Source Performance Standards (NSPS) to limit carbon dioxide emissions from fossil fuel-fueled power plants. The 2015 rule determined:

  • New natural gas power plants can emit no more than 1,000 pounds of carbon dioxide per megawatt-hour (MWh) of electricity produced, which is achievable with the latest combined cycle technology.
  • New coal power plants can emit no more than 1,400 pounds CO2/MWh, which almost certainly requires the use of carbon capture and storage (CCS) technology.

The proposed 2018 rule would:

  • Set the best system of emissions reduction for newly constructed large units equivalent to a super-critical coal plant, which has an emissions rate of 1,900 lbs CO2/MWh and would set the best system of emission reductions for small units to 2,000 lbs CO2/MWh.
  • Set separate performance standards for newly constructed and reconstructed coal refuse-fired units at an emissions rate at 2,200 lbs CO2/MWh.
  • Revise the standards of performance for reconstructed power plants to be consistent with the emission rates of newly constructed units.

This standard was adopted under Section 111(b) of the Clean Air Act, which applies to new, modified, and reconstructed power plants, and requires EPA to set a numerical performance standard based on the best available technology that has been adequately demonstrated. States have little flexibility in applying the standard.

Other Federal Regulations

Fossil fuel combustion at power plants emits other pollutants regulated by the Clean Air Act. Environmental controls on these other pollutants like SO2, NOx, and mercury often (though not always) result in greenhouse gas emission reductions. Other EPA rules on power plant air emissions include:

  • Cross-state Air Pollution Rule (CSAPR, pronounced “Casper”) – This rule covers the oxides from nitrogen (NOx) and sulfur dioxide (SO2) that create ozone and fine particulate matter (PM). Emissions of these pollutants from upwind states prevent downwind states from achieving air quality standards. CSAPR covers a few thousand sources in the Eastern part of the United States and employs tradable air permits to reduce emissions. After several years of litigation, the rule came into effect in 2015.

  • Mercury and Air Toxic Standards (MATS) – This rule limits emissions of mercury from coal- and oil-fired power plants (natural gas does not contain mercury). Since the rule took effect in 2015, some coal-fired power plants have retired, but most operators chose instead to install control technology to reduce these emissions. In April 2020, EPA issued a revised supplemental cost finding, which removes co-benefits (e.g., soot reduction) from consideration when setting limits on toxic air pollution. While MATS remains in place, this revision could weaken the legal underpinning for this and future rulings. In addition, EPA revised the rule to allow four power plants to burn eastern bituminous coal refuse and emit higher levels of acid gas hazardous air pollutants (e.g., sulfur dioxide and hydrochloric acid).

EPA has separate authority under environmental laws to regulate power plant emissions and activities that have other harmful impacts, such as cooling water intakes and coal ash disposal.