[en] France is aiming to reduce the share of nuclear power in the country's electricity mix, and increase the share of wind and solar power. However, given the intermittency of these renewable energy sources, massive storage systems are required when their share exceeds a certain threshold. That generates additional costs on top of the subsidies already granted to these energies. This article attempts to quantify that additional cost, which varies between 6.3 Euros and 31.6 billion Euros depending on various scenarios. In fact, the cost of renewables storage varies according to our calculations between 108.3 Euros and 251.4 Euros per MWh. That additional cost could be reduced if storage yields were improved. One of the lessons to be drawn from this analysis is that it would be preferable to finance R and D in storage technologies rather than using costly feed-in tariffs or feed-in premiums to finance an increase in renewables. The greatest challenge lies with respect to inter-seasonal storage rather than very short-term battery-based storage. The prospects for power-to-gas and gas-to-power are also promising, but have yet to be proven in practice. (authors)