System costs in this study are defined as the total costs above plant-leuel costs to supply electricity at a given load and given level of security of supply. In principle, this definition could include costs external to the electricity market such as environmental costs or security of supply impacts. This study, however, focuses primarily on the costs accruing inside the electricity system to producers, consumers as well as transport and distribution system operators. A large subset of these costs are the monetisable system costs that are mediated by the electricity grid and are referred to in the following as “grid-level system costs” or “grid costs”.