Solar-and nuclear-electricity-generation technologies often are deemed ''carbon-free'' because their operation does not generate any carbon dioxide. However, this is not so when considering their entire lifecycle of energy production; carbon dioxide and other gases are emitted during the extraction, processing, and disposal of associated materials. We determined the greenhouse gas (GHG) emissions, namely, CO 2 , CH 4 , N 2 O, and chlorofluorocarbons due to materials and energy flows throughout all stages of the life of commercial technologies for solar-electric-and nuclear-power generation, based on data from 12 photovoltaic (PV) companies, and reviews of nuclear-fuel life cycles in the United States, Europe, and Japan. Previous GHG estimates vary widely, from 40 to 180 CO 2 -eq./kWh for PV, and 3.5-100 CO 2 -eq./kWh for nuclear power. Country-specific parameters account for many of these differences, which are exacerbated by outdated information. We conclude, instead, that lifetime GHG emissions from solar-and nuclear-fuel cycles in the United States are comparable under actual production conditions and average solar irradiation, viz., 22-49 g CO 2 -eq./kWh (average US), 17-39 g CO 2 -eq./kWh (south west) for solar electric, and 16-55 g CO 2 -eq./kWh for nuclear energy. However, several factors may significantly change this picture within the next 5 years, and there are unanswered questions about the nuclear fuel cycle that warrant further analyses.