Carbon mitigation options have been measured neglecting the role that fossil fuels play as energy sources. For instance, producing wood or storing carbon would not have a cyclical effect on energy supply. This study suggests an approach to measure the carbon offset of biofuels, regarding the concept of exergy on carbon mitigation and the area required to provide the equivalent amount of exergy from fossil fuels. Sugarcane and eucalyptus were considered as mitigation alternatives within five distinct scenarios for biofuel production in a broad technological range. All scenarios were compared with four fossil fuels: diesel oil, pure gasoline, and two gasoline-ethanol blends. In general, sugarcane presented smaller specific exergy, but higher exergy output than eucalyptus, on average for all scenarios, mainly due to its higher yearly crop yield. On average, both crops require from 20% to 30% more area to synthesize the same amount of exergy than the required area to mitigate the mass of carbon dioxide emitted from commercial fuels. Higher efficiency can be reached with higher crop yields, such as collecting remaining biomass after harvest rather than leaving it in the field. Another option is to reduce moisture content on solid fuels, such as wood and straw. On average, each Mg ha−1 increased in yield would result in an additional output of 4.5 GJ ha−1. Besides, for each percentage of moisture content reduced in solid fuels, there would be an increase of exergy output of 1 GJ ha−1.