In this paper, an analytical procedure for carbon tax evaluation is presented. The aim is the assignment to energy plants of a charge linked to their CO2 emissions. The problem is faced through the use of an environomic approach, including evaluation of the cost of the exergy destroyed inside the system and the cost of exergy rejected to the biosphere with the plant wastes (Eficiency penalty) coupled with evaluation of the Index of CO2 emission herein defined. In this way, the procedure allows a cost of the emitted CO2 to be obtained based not on political considerations but only on efficiency and exergy analysis. The aim is to reward the efficient use of energy resources and to penalize the inefficient plants. The procedure is applied for the analysis of three typical Italian energy plants burning fossil fuels: a 320 MW coal steam plant; a 700 MW natural gas combined plant; and a 30 MW gas turbine cogeneration plant. The plants are analysed using an environomic optimization and taking into account all their pollutant emissions (CO, NOx, SOx and CO2). The values of the CO2 emissions charges obtained with the proposed procedure are presented and discussed in depth and a comparison with the possible costs of CO2 sequestration activities is presented too.