PJM Releases Annual Emission Rates Report


PJM has released its annual Emission Rates Report for generators in its footprint, showing both average and marginal emission rates for carbon dioxide, nitrogen oxides and sulfur dioxide.

PJM publishes the yearly report to support the efforts of regulators, stakeholders and other interested parties as they work toward achieving environmental goals. In addition to the annual report, PJM in September began posting 5-minute marginal emission rates for individual load nodes on Data Miner.

The average CO2 emission rate for electric generators in the PJM footprint increased approximately 7% from 2020 to 2021, as COVID-19 precautions relaxed and business and consumer activity – and corresponding energy use – climbed back toward pre-COVID levels.

In longer-term trends, the average CO2 emission rate dropped 1% compared with 2019, continuing a steady 17-year decline.

The PJM system average annual value is a weighted average that accounts for higher loads during the summer and winter months. Emissions are measured in pounds per megawatt hour.

According to the report, from 2020 to 2021:

  • The carbon dioxide emission rate increased 6.6% (down 1% from 2019).
  • The nitrogen oxides emission rate increased 5.6% (down 15.6% from 2019).
  • The sulfur dioxide emission rate increased 11.6% (down 12.7% from 2019).

Between 2005 and 2021, carbon dioxide emission rates fell 35% across PJM’s footprint; emission rates for nitrogen oxides are down 85% and sulfur dioxide 94%, as competitive wholesale electricity markets continue to encourage the entry of more efficient technologies, and cleaner, more efficient resources replace older, less efficient units. (Those figures reaching back to 2005 are not included in this report.)

The report also measures the emission rates of marginal units during peak and off-peak hours. (Peak periods are all non-holiday weekdays from 7 a.m. to 11 p.m., and off-peak periods are all other hours.) A marginal unit is the final generation resource committed during a dispatch interval to maintain system reliability and match energy supply and demand. Variations in dispatch patterns – based on customer demand – may change the set of marginal units for a dispatch interval, which can shift marginal emission rates.

In 2021, on average:

  • Combined-cycle gas-fired generators were the marginal units 59.75% of the time, compared with 64.33% the previous year.
  • Coal units, on average, were the marginal units 14.15% of the time, down from 17.53% in 2020.
  • Wind made up 11.04% of marginal units, on average, up from 6.75% in 2020.

PJM also publishes five-minute marginal emission rates for individual load nodes on the wholesale grid through the PJM Data Miner tool.  

This data can help interested parties better understand the environmental impact of electricity use.

PJM stakeholders have identified potential uses for marginal emission rates to determine the times each day when electricity use can have the least impact on the environment by drawing on lower-emitting resources. General Motors Co., in partnership with PJM member TimberRock, said last fall it plans to use the PJM marginal emission rates data to help the carmaker reach 100% renewable energy to power its operations by 2025 – five years earlier than previously announced.