By their nature, system effects cannot be defined nor observed by looking at a single system, but can be understood and quantified only by comparing two or more systems, and their quantitative estimation depends on the choice of the reference system (“benchmark”). As an example, the introduction of a given amount of VRE capacity in a given system, let us say 10 GW of onshore wind, would cause in the long term a change in the electricity system: more transmission would be needed (or a different structure of the transmission grid) as well as a different generation mix, more adapted to accommodate the characteristics of wind production. The quantification of system effects due to the introduction of these 10 GW is possible only by comparing the new generation system with the reference system, in which the 10 GW of wind energy has not been exogenously imposed. How the reference system is defined and how the system with wind energy is constructed and optimised are key aspects in any quantitative evaluation of system effects.