As part of its 2050 energy strategy, Switzerland plans to phase out nuclear power production and replace most or all of its significant share of national electricity production (40%) by renewables, in particular by photovoltaics (PV) and wind energy. We investigated how the proportion of PV / Wind in the energy mix affects the national energy balance throughout the year and how storage hydropower and pumped-storage hydro interact with the intermittency of those new renewables. We got quantitative results about the amount of import and export required to balance the seasonal mismatch between production and demand and investigated solutions to reduce it.