Carbon and emission metrics

The construction and operation of buildings results in significant carbon (and other greenhouse gas) emissions impacts. Emissions may be categorized as either embodied carbon or operational carbon.

Operational carbon refers to the greenhouse gas emissions that are generated during the day-to-day operation of a building. Reducing operational carbon is typically the focus of BEM analysis and this article will focus primarily on operational carbon.

Embodied carbon refers to the greenhouse gas emissions that are associated with the production, transportation, and construction of building materials and components. This includes the energy and emissions required to extract raw materials, manufacture and transport building products, and construct buildings. Embodied carbon emissions occur before a building is operational and are therefore considered a "upfront" carbon cost. Embodied carbon is generally analyzed via separate tools from BEM software, however BEM practitioners who are involved in other sustainability aspects of a project may participate in embodied carbon analysis as well.

Both embodied carbon and operational carbon contribute to the overall carbon footprint of a building. In fact, embodied carbon can account for a significant portion of a building's total carbon emissions, particularly for low-carbon buildings with efficient operational performance. To achieve significant carbon reductions in the building sector, both embodied carbon and operational carbon must be addressed.

Operational carbon

Operational carbon in buildings refers to the greenhouse gas emissions that are generated during the day-to-day operation of a building. These emissions are caused by the consumption of energy for heating, cooling, lighting, ventilation, and other building services, as well as the use of appliances and equipment.

Operational carbon emissions from buildings may occur on-site via the burning of fossil fuels, such as natural gas and oil, for heating and hot water. These fuels release carbon dioxide (CO₂) and other greenhouse gases into the atmosphere, contributing to climate change. Electricity consumption also generates carbon emissions, typically off-site at the power plant but sometimes on-site via running generators. The amount of operational carbon associated with the energy consumption will vary depending on the source of the electricity.

Reducing operational carbon emissions is a key part of mitigating the impacts of climate change. This can be achieved through a variety of strategies such improving energy efficiency, using renewable energy sources, and optimizing building systems and controls. Building codes and standards may also include requirements for reducing operational carbon emissions, such as minimum energy performance standards and requirements for the use of renewable energy sources.

Average emissions factors

eGRID average CO₂ emissions factors by region. (Source: IBPSA-USA BEM Workshop)

Carbon emissions factors refer to the amount of carbon dioxide (CO₂) or other greenhouse gases (GHGs) that are emitted per unit of energy consumption. These factors are used to estimate the amount of carbon emissions associated with operating a building.

The average carbon emissions factors can vary depending on the type of energy source being used. For example, the carbon emissions factor for coal is higher than that for natural gas because coal has a higher carbon content per unit of energy.

According to the United States Environmental Protection Agency (EPA), the average carbon emissions factor for electricity generation in the US in 2020 was 0.957 pounds of CO2 per kilowatt-hour (kWh) of electricity generated. This includes emissions from all sources of electricity, including coal, natural gas, and renewable sources like wind and solar.

Data sources such as eGRID and others (see the Additional resources section below) report average annual emissions factors for various regions across the United States based on the mix of generation sources in each region.

Hourly emissions factors

Hourly emissions factors attempt to quantify the carbon impacts of energy consumption on an hourly basis based on historical and projected grid production. The Title 24, Part 6 energy code in California uses hourly carbon emissions factors as one of the metrics (along with another metric, TDV) to determine whether a building complies using the performance path. The estimated energy consumption for each hour of the year is multiplied by hourly carbon emissions factors that change based on the time of day and time of year to reflect grid production profiles.

Marginal emissions

A marginal emissions factor refers to rate at which emissions would change with a small change to electricity load. These may vary significantly from average emissions factors in areas with renewable production because extra power generation to cover the marginal increase in demand may often be from natural gas power plants.

Additional resources

• US EPA Emissions & Generation Resource Integrated Database (eGRID) - a comprehensive source of data from EPA's Clean Air Markets Division on the environmental characteristics of almost all electric power generated in the United States.
• US EPA GHG Emission Factors Hub - a regularly updated and easy-to-use set of default emission factors for organizational greenhouse gas reporting.
• NREL Cambium - Cambium data sets contain modeled hourly emission, cost, and operational data for a range of possible futures of the U.S. electricity sector through 2050, with metrics designed to be useful for forward-looking analysis and decision support.
• Electricity Maps - API access to electricity and carbon emissions data.
• WattTime - API access to electricity and carbon emissions data.

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