For a power generator, Carbon Intensity (CI) refers to how much carbon dioxide (CO2) is released to produce a megawatt hour (MWh) of electricity.
Utilities play a significant role in either contributing to or mitigating greenhouse gas emissions. For a utility, Scope 1 emissions are direct greenhouse gas emissions from sources owned or controlled by the utility — those typically associated with the combustion of fossil fuels in power plants, stationary equipment, and other on-site activities. According to the U.S. Energy Information Administration, in 2021, the CI from U.S. power generation was about 855 pounds (or 388 kg) of CO2 emissions per MWh.
In this blog post, I’ll show you how to calculate a carbon intensity score from power generation.
Carbon intensity formula
The carbon intensity formula is relatively simple:
Carbon Intensity (CI) = Total CO2 Emissions/Total Electricity Generated
Let’s work through the calculations for a natural gas turbine with a 6.8 heat rate. To calculate the carbon intensity, we’ll use our heat rate to estimate the amount of natural gas consumed and then use the carbon emissions factor for natural gas combustion.
Here are the calculations step by step:
Calculate natural gas consumption:
The heat rate of 6.8 means that for every megawatt-hour (MWh) of electricity generated, the natural gas turbine consumes 6.8 million British thermal units (MMBtu) of natural gas. To simplify later calculations, let’s assume total power generated is 1 MW.
Determine CO2 emissions factor
The emissions factor for natural gas is the amount of CO2 emitted per MMBtu of natural gas burned. Let’s use the EIA CO2 emissions factor of 52.91 kg of CO2 per MMBtu, which is a typical value for natural gas. Another good source is the GREET (Greenhouse gases, Regulated Emissions, and Energy use in Technologies) model published by Argonne National Lab.
Calculate total CO2 emissions
Use the emissions factor to calculate the total emissions associated with the natural gas consumption to produce our 1 MWh:
Total Emissions (kg) = NG Consumption (MMBtu) × CO2 Emissions Factor (kg/MMBtu)
= 6.8 MMBtu x 52.91 kg/MMBtu = 360 kg
Compute carbon intensity score
Finally, compute the carbon intensity by dividing the total CO2 emissions by the total electricity generated. Since we previously assumed total CO2 emissions for one MWh, the carbon intensity is 360 kg/MWh. Again, this value represents the amount of CO2 emissions associated with each MWh of electricity produced by the natural gas turbine.
To compare across multiple fuel types, the official units for a CI score is grams of CO2 per Megajoule (g/MJ). Here’s the math to convert 360 kg/MWh into a CI score:
360 kg/MWh * 1000 g/kg * 1 MWh / 3600 MJ = 100 g/MJ
Weighted-average carbon intensity score
Weighted-Average Carbon Intensity measures the average carbon dioxide (CO2) emissions intensity of electricity generation across a portfolio of power-producing assets or within an electricity grid, weighted by the amount of electricity each source contributes. As renewables comprise an increasing portion of energy generation, the weighted average score will come down.
In addition to CO2, power utilities may emit other greenhouse gases (GHGs) like methane (CH4) and nitrous oxide (N2O). To account for these gases’ global warming potentials relative to CO2, we use CO2 equivalents (CO2e). The concept of CO2e allows us to express the total impact of all GHGs in terms of CO2, making it easier to assess the overall carbon footprint.
The formula to calculate CO2e is:
CO2e= CO2 + CH4 × GWPCH4 + N2O × GWPN2O
CO2e is the total CO2 equivalent emissions.
CH4 is the emissions of methane.
N2O is the emissions of nitrous oxide.
GWPCH4 and GWPN2O are the global warming potentials of methane and nitrous oxide, respectively. The GWPCO2 is 1. According to the EPA, CH4 has about 27 times and N2O has roughly 270 times the warming potential of CO2.
Utility scale leadership
As society moves toward greater electrification not only in their homes but also for mobility, utilities will play a crucial, outsized role in mitigating climate change. Tools like the carbon intensity score are instrumental to not only inform better decision-making among utility companies but also empower consumers and regulators to advocate for and support cleaner, more sustainable energy sources. The journey toward a lower-carbon future depends heavily on our ability to accurately measure, understand, and ultimately reduce the greenhouse gas emissions associated with electricity production.
Visit our Carbon Intensity page to learn more about how you can forecast, track, and optimize your carbon emission intensity.