Various sources of flexibility are available. Often, wind and solar production is complementary on a daily and season scale: in many places there is more wind during the night and in winter, when solar energy production is low. Linking different geographical regions through long-distance transmission lines allows for further cancelling out of variability. Energy demand can be shifted in time through energy demand management and the use of smart grids, matching the times when variable energy production is highest. With storage, energy produced in excess can be released when needed. The final mismatch may be covered via overproduction of variable energy (and curtailing further excess), and by using dispatchable energy, such as hydro, bioenergy, natural gas, or nuclear plants.