If green energy is implemented on a very large scale, like a one third or more of all energy in the world, then an intelligent system of providing energy is required. This is because in some periods the wind is much stronger or sunshine is more intense than in other periods. If there is an excess of wind energy, the electricity needs to be stored somewhere. This can be done by temporarily stopping hydro plants, so the water level in the lake next to the dam rises, and once the wind lies down again the hydro plant can be turned on to provide the needed energy. This is already being done in Denmark (lot of wind mills) and Sweden (lot of hydro energy).
The countries surrounding the North Sea (UK, Netherlands, Norway, Denmark, Germany) are planning to line up their power grids so they can easier exchange their excess wind, hydro or solar energy. The EU has a goal to make 20% of all energy renewable, so building the North sea full of windmills, installing more solar panels and building this international power grid is essential in reaching this goal.