Estimating Carbon Dioxide Emissions Factors for the California Electric Power Sector

The California Climate Action Registry ("Registry") was initially established in 2000 under Senate Bill 1771, and clarifying legislation (Senate Bill 527) was passed in September 2001. The Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab) has been asked to provide technical assistance to the California Energy Commission (CEC) in establishing methods for calculating average and marginal electricity emissions factors, both historic and current, as well as statewide and for sub-regions. This study is exploratory in nature. It illustrates the use of three possible approaches and is not a rigorous estimation of actual emissions factors. While the Registry will ultimately cover emissions of all greenhouse gases (GHGs), presently it is focusing on carbon dioxide (CO₂). Thus, this study only considers CO₂, which is by far the largest GHG emitted in the power sector.

Associating CO₂ emissions with electricity consumption encounters three major complications. First, electricity can be generated from a number of different primary energy sources, many of which are large sources of CO₂ emissions (e.g., coal combustion) while others result in virtually no CO₂ emissions (e.g., hydro). Second, the mix of generation resources used to meet loads may vary at different times of day or in different seasons. Third, electrical energy is transported over long distances by complex transmission and distribution systems, so the generation sources related to electricity usage can be difficult to trace and may occur far from the jurisdiction in which that energy is consumed. In other words, the emissions resulting from electricity consumption vary considerably depending on when and where it is used since this affects the generation sources providing the power.

There is no practical way to identify where or how all the electricity used by a certain customer was generated, but by reviewing public sources of data the total emission burden of a customer's electricity supplier can be found and an average emissions factor (AEF) calculated. These are useful for assigning a net emission burden to a facility. In addition, marginal emissions factors (MEFs) for estimating the effect of changing levels of usage can be calculated. MEFs are needed because emission rates at the margin are likely to diverge from the average.