The objectives of the analysis reported in this paper are to evaluate the environmental impacts of the electricity produced in a $17MW$ solar thermal plant with central tower technology and a $50MW$ solar thermal plant with parabolic trough technology, to identify the opportunities to improve the systems in order to reduce their environmental impacts, and to evaluate the environmental impact resulting from compliance with the solar thermal power objectives in Spain. The methodology chosen is the life cycle assessment (LCA), described in the international standard series ISO 14040-43. The functional unit has been defined as the production of $1kWh$ of electricity. Energy use needed to construct, operate, and dismantle the power plants is estimated. These results are used to calculate the “energy payback time” of these technologies. Results were around $1yr$ for both power plants. Environmental impacts analyzed include the global warming impacts along the whole life cycle of the power plants, which were around $200g∕kWh$ generated. Finally, the environmental impacts associated with the compliance of the solar thermal power objectives in Spain were computed. Those figures were then used to estimate the avoided environmental impacts including the potential $CO2$ emission savings that could be accomplished by these promotion policies. These savings amounted for $634kt$ of $CO2$ equiv./yr.