A performance assessment of sizing an auxiliary boiler for a solar driven supercritical double recompression CO2 Brayton cycle was conducted. The Brayton cycle is designed to deliver three different power outputs and the required size of the auxiliary boiler was examined in detail. The heat fraction to be delivered from the solar field and from the auxiliary boiler for each month of the year are reported. Furthermore, the daytime solar multiple and the twenty-four hour solar multiple were examined. Another key parameter that was studied is the effect of the turbine inlet temperature on the net power, energy efficiency, and exergy efficiency. Among the other exergy parameters that were examined are exergy destruction, exergy improvement potential, fuel depletion ratio, relative irreversibility, and productivity lack. The power output for Case 1, Case 2, and Case 3 is about 41.5 MW, 60.0 MW, and 90.0 MW, respectively; and for the month of June, the fraction of the heat from the auxiliary boiler during daytime hours is about 0.25, 0.40, and 0.54, respectively. For the three Cases the overall system energy efficiency during the month of June is 20.7%, 25.0%, 29.6%, and the overall system exergy efficiency is 22.2%, 28.3%, and 35.7%, respectively. The cycle efficiency is about 47% for the baseline conditions. In addition, the lowest thermal heat collected in the receiver is during December and, therefore, during this month, the highest auxiliary heat is required from the boiler. The 24-h average solar multiple for Case 1, Case 2, and Case 3 is 0.437, 0.303, and 0.202, respectively; and the average daytime solar multiple for these cases is 0.858, 0.590, and 0.396, respectively. Moreover, similar results are reported for each month of the year. Furthermore, the findings demonstrate that the heliostat has the highest exergy destruction rate and, thus, it has the highest exergy improvement potential. (C) 2016 Elsevier Ltd. All rights reserved.