Making smart policy decisions on any issue requires sound objective analysis. To contribute to the public debate on a range of regional energy and environmental decisions, including evaluation of proposals for electric ratepayers to finance new regional natural gas pipelines, Acadia Center recently released a fact sheet that takes a comprehensive look at several different regional trends for greenhouse gas (GHG) emissions, electricity generation, and fuel consumption across all sectors.

Recently, Concentric Energy Advisors (Concentric) produced a report that includes an emissions analysis of this past winter’s electricity generation on behalf of a coalition of advocates for ratepayer-funded expansions in natural gas pipelines. Acadia Center’s Climate and Energy Analysis Center (CLEAN Center) reviewed this analysis, and it seems clear that the Concentric analysis significantly overestimated winter greenhouse gas (GHG) emissions from oil and coal generators in New England this past winter, likely between 15–20% higher than actual emissions from these sources.

This is because the Concentric report, like most analyses, makes assumptions as a part of its calculations. Assumptions are often used when data is incomplete or to avoid additional analysis that will only provide small improvements in accuracy. A frequently used assumption in electricity generation calculations is called a “heat rate,” which measures how efficiently a power plant turns fuel into electricity. A lower heat rate means a plant uses less fuel to generate the same amount of electricity. This information can be used to estimate the amount of fuel that a power plant consumes based on the amount of electricity it has generated. The estimated level of fuel consumption can then be used to estimate greenhouse gas emissions. However, fuel consumption data is often reported directly and can be used to more accurately estimate greenhouse gas emissions. Acadia Center’s recent analysis used fuel consumption data from the U.S. Energy Information Administration (EIA) to estimate GHG emissions for recent months where emissions data isn’t directly available.

The Concentric analysis begins with electricity generation data from ISO New England and then uses assumed heat rates to calculate fuel consumption. The calculated fuel consumption is then combined with an assumed emissions rate to estimate GHG emissions. In this case, Concentric used assumed heat rates based on S&P Global Market Intelligence data to represent the heat rates of power plants in New England. The Concentric analysis does not state if this assumed heat rate data is based on regional or national figures, nor does it compare it to other published data for regional heat rates as a check on its accuracy.

The Acadia CLEAN Center examined recently reported EIA data on fuel consumption and generation for individual power plants in New England from December 2017 through February 2018. This data can be used to directly calculate actual heat rates. Based on Acadia Center’s analysis of this EIA data, it appears that the assumed heat rates in the Concentric analysis for oil and coal generation in New England are significant overestimates. In other words, the Concentric analysis assumed those types of power plants used more fuel to generate electricity than they actually did.

Table 1 – Concentric Assumed Heat Rates and Actual Heat Rates (MMBTU/MWh)

Concentric assumed heat rate versus actual heat rate

As shown in Table 1, the assumed heat rate in the Concentric analysis for oil generation is 25% higher than the actual heat rate for oil generation in New England this past winter. Similarly, the assumed heat rate in the Concentric analysis for coal generation is 9% higher than the actual heat rate for coal generation in New England this past winter.

There can be many reasons that assumptions do not accurately represent reality. Assumptions based on historical data or national data do not necessarily reflect recent local conditions in New England. The assumptions in the Concentric analysis about the relative inefficiency of oil generation could also be reflecting a more general misconception about oil generation in New England. The older oil generating units in New England are more inefficient. However, newer units, including combined cycle and combustion turbines that can run on both natural gas and oil, are much more efficient.

Table 2 – Oil Heat Rates for Electric Generation Sites with Significant Oil Combustion from Dec. 2017-Feb. 2018 (MMBTU/MWh)

As this data shows, many of the electric generation sites in New England that used a significant amount of oil this past winter were much more efficient than assumed in the Concentric analysis and only one site, New Haven Harbor, was less efficient than the overall average assumed in the Concentric analysis.

The impact of this inaccurate assumption by Concentric flows through to subsequent calculations. This means that the estimates in the Concentric analysis for combined GHG emissions from coal and oil generation would be much higher than actual GHG emissions from coal and oil generation, likely between 15–20% higher. As a result, calculations of incremental GHG emissions during the winter cold snap are likely significant overestimates, as well as the other calculations in the Concentric analysis based on the estimate of incremental GHG emissions.

No one who is concerned with climate change wants fossil fuel plants to continue releasing greenhouse gases. But using emissions data to justify any policy case requires accuracy. To better inform the broader energy and environmental debate in New England, Concentric should update its analysis based on actual heat rates in New England this past winter. More broadly, all of the tools available to meet our energy needs—market reforms, energy efficiency, energy storage, solar, wind, and fixing gas leaks among them—need to be fairly evaluated in order for the right conclusions to be drawn.


  • Mark LeBel is Staff Attorney at Acadia Center, based in the Boston office. He works on electricity rate design, distributed energy resource compensation, grid modernization, and transportation issues, notably including electric vehicles.