"INTRODUCTION Buildings consume nearly 40% of the primary energy resources and 74% of the electricity generated each year in the United States (DOE 2008). Homes and commercial businesses have been growing contributors to CO2 emissions for the last 15 years??a trend that is projected to continue for the next two decades. As shown in Figures 1 and 2@ the increasing CO2 emissions of residential and commercial buildings is being driven primarily by growing consumption of electricity@ including generation losses (DOE 2008@ 2009). Much of the increased carbon impact from residential and commercial electricity use (52% since 1990) comes from power plants and the relatively inefficient ""full fuel cycle"" of production and delivery of electricity to the buildings. Aggregate CO2 emissions from natural gas consumption in U.S. buildings have increased by 13% since 1990. Emissions due to natural gas consumption in residential buildings are projected to remain flat through 2030 despite growth in the number of gas customers@ while emissions from commercial buildings are projected to increase by 15%. During the same period@ emissions from electricity use in residential buildings are projected to increase by 9%@ while emissions from commercial buildings are projected to increase by 26% (DOE 2009). Natural gas is used predominantly for heating@ water heating@ and cooking. Each of these markets has seen significant improvements in energy efficiency in the past two decades@ with gas use per building continuing a steady decline that started in the 1980's (Joutz 2007). For instance@ in the residential market@ households decreased consumption of natural gas by 1% annually between 1980 and 2000 and by 2.2% annually between 2000 and 2006. On the other hand@ electricity consumption per building has increased as a result of higher market penetration of cooling and significant increases in plug loads such as computers@ TVs@ and electronics. Future gains in electric appliance efficiency are projected to be offset by new applications for electricity as well as market growth. Overall@ electricity use per building is projected to grow by 1.1% per year for residential buildings and 1.9% per year for commercial buildings through 2030 (EPRI 2009). CO2 emissions attributed to the electricity sector are caused by fossil fuel and biomass generation (DOE 2008). Hydropower@ wind@ and solar generation do not add materially to the nation's CO2 emissions once construction is completed. Biomas is considered a renewable resource with net CO2 emissions a function of energy returned on energy invested to obtain the biomass ?C factors that are beyond the scope of this paper. Coal is the most significant contributor to buildings CO2 emissions due to its high carbon content compared to oil@ propane@ and natural gas (EIA 2009a). In 2007@ coal provided 49% of total electricity generation in the U.S.@ while the electric power sector accounted for 93% of all coal consumption (DOE 2008). Natural gas use in power plants has also been increasing significantly since 1990@ and accounted for 20% of electricity generation in 2007. Natural gas use in power plants is now higher than residential@ commercial@ or industrial natural gas consumption. Emissions associated with the extraction@ processing@ and transportation of fossil fuels@ nuclear fuel@ and biomass prior to combustion are also important to consider as they can add anywhere from 3 to 20% to the overall CO2 emissions from buildings depending on energy source and application (Deru 2007). These trends indicate an opportunity for future energy efficiency and CO2 emission reduction initiatives that leverage improvements in gas appliance efficiency with cost-effective direct gas use options that reduce resistance heat electricity consumption in space heating@ water heating@ cooking@ and clothes drying. In this paper@ calculation methodologies and sample calculations for site energy efficiency@ full fuel cycle energy efficiency@ and CO2 emissions associated with different technology options in buildings are discussed. First@ site energy and full fuel cycle efficiency methods are described@ with an emphasis on their uses and limitations. Then@ approaches to calculating full fuel cycle CO2 emissions due to buildings are identified. Finally@ sample calculations of CO2 emissions are presented for a residential water heater comparison showing the impact of different calculation approaches on results."