One of the first problems we are confronted with when dealing with the calculation/ estimation of ERs is the lack of a clear definition of terms. In general, most of us understand that when we use the term ex ante estimation of ERs, we are referring to a calculation of ERs based on fixed parameters estimated today, kept unchanged in the future if the basic assumptions are maintained (“dramatic” changes will trigger a recalculation). On the other hand, an ex post calculation of ERs will use the actual (real) system operational dispatch of the project/plant involved and the emission factors of the actual marginal plants “displaced” by the project.