Accounting for such system costs can make significant differences to the social and private investor costs of different power generation technologies. In particular, the integration of variable renewables is a complex issue that profoundly affects the structure, financing and operational mode of electricity systems in general and nuclear in particular. The present study, overseen by the Working Party on Nuclear Energy Economics (WPNE) of the OECD Nuclear Energy Agency (NEA), presents an overview of the most important system effects, proposes methodologies to assess them and provides systematic empirical cost estimates. The deployment of electricity from variable renewables is also significantly affecting the economics of dispatchable power generation technologies, in particular those of nuclear power, both in the short and the long run.