Photovoltaic electricity has the potential to mitigate CO2 emissions from the grid. A methodology to more accurately evaluate CO2 abatement by PV electricity is developed. We develop a capacity factor based dispatching model to evaluate marginal abatement in the load zones of ERCOT and CAISO, and compare it to the abatement using national, regional and state average resource profiles. The average cases over-estimated and under-estimated CO2 abatement in ERCOT and CAISO, respectively. Marginal abatement was lower by 17% than the average cases in ERCOT, due to the predominant displacement of the low carbon natural gas plants at the margin. In CASIO, marginal abatement was higher (1.3-2.4 times) than that of the average cases due to the displacement of highly inefficient gas plants at the margin. We demonstrate that actual CO2 abatement of PV electricity is dependent on both peak load resources and capacity of installations. Subsequently, we develop a CO2 indicator that can be used as a guideline for selecting PV installation sites to derive maximum abatement. Installing photovoltaics in regional areas of MRO, SPP and RFC was determined to be most beneficial. The results of this study can guide energy planning and CO2 mitigation policy-making using photovoltaics in the future. (C) 2010 Elsevier Ltd. All rights reserved.