Carbon dioxide capture and storage has been presented as a necessary component of energy plans, because it is presumed to deliver significant environmental benefits. In this study, we systematically evaluate the environmental impact of electricity generated by natural gas and coal power plants with selected CO2 capture technologies with and without CO2 storage. To examine uncertainties that could arise from the provided environmental impacts of the applied methodology of life cycle assessment, we perform sensitivity analyses of important parameters over a large range of values. In addition, a sensitivity analysis of the thermodynamic efficiencies allows evaluation of power plants with different thermodynamic performances. We find that, in plants using either natural gas or coal, post-combustion capture results in a higher environmental impact per MWh than that of business-as-usual (i.e., no CO2 capture). Furthermore, chemical looping combustion only marginally decreases the environmental impact of electricity generation in natural gas plants, while in coal plants it can decrease the impact by up to 17%. In addition, CO2 transportation and storage result in a net negative environmental impact, making an improvement in comparison to the business-as-usual environmental footprint of a plant more difficult. Overall, the most decisive factor affecting the environmental impact of electricity is the energy penalty associated with CO2 capture; because of this factor, CO2 capture and storage does not necessarily result in a reduction of the overall environmental impact.