Many studies have shown the feasibility of fully renewable power systems, in various countries and regions. Yet the future costs of key technologies are highly uncertain and little is known about the robustness of a renewable power system to these uncertainties. We build 315 long-run cost scenarios on the basis of recent prospective studies, varying key technologies costs (inshore and offshore wind by +/- 25%; PV, batteries and power-to-gas by +/-50%). We model the optimal renewable power system for France, optimising simultaneously investment and dispatch.

The optimal energy mix varies a lot across cost scenarios: the installed capacity in PV, onshore wind and power-to-gas varies by a factor of 5, batteries and offshore wind even more. Yet a robust result is that the system cost (including electricity production and storage) is not higher than the cost prevailing today: it reaches 50 €/MWh in average and 65€/MWh in the worst scenario. Even if the energy mix is optimised on an ex ante cost scenario which is not realised ex post, the cost of error is limited.