Wind and solar electricity is produced without direct CO2 emissions. However, the introduction of this electricity in the grid is delicate due to the intermittent character of its sources. Wind and solar production is characterized by multiple, strong variations in the electric power. These variations put stress on the grid where the total production of electricity must always be equal to the consumption. We present a synthesis of five studies conducted for Germany and France with different assumptions of electricity mixes, all with large shares of wind and solar power. These mixes are subjected to the dynamics of wind and solar production as recorded in 2010 (Germany), 2012 and 2013 (Germany and France). Common structural trends are exhibited when the results of simulations (instantaneous power distributions and average annual values) are expressed as a percentage of the annual reduced load to be produced by these intermittent energies. We focus on the evaluation of these trends and the resulting constraints on the grid. The results obtained make it possible to anticipate the problems brought about by a large share of renewable intermittent energies in the production of electricity. They show the need for backup production in order to complement the intermittent sources. This leads to CO2 emissions unless storage systems of large capacity are available.