A Tesla model S seems to be able to store 100kWh. France seems to have ~30M cars. So, 100e330e63600~=1e16 J storage assuming all those cars switch to being Tesl

A Tesla model S seems to be able to store 100kWh. France seems to have ~30M cars. So, 100e330e63600~=1e16 J storage assuming all those cars switch to being Tesla S, and are somehow kept unused. France consumed 529e6/1.17 MHw according to https://www.worldometers.info/electricity/france-electricity/ https://www.worldometers.info/electricity/france-electricity... (in 2016). So, average consumption is 529e12/1.17*3600/365/86400 ~= 50GW. 1e16J / 50GW is 200,000 seconds or 2.3 days. So, not completely unreasonable, but it still seems on the low side (only two days), and I am not sure what the impact would be on car batteries to be continuously charged/discharged. This also assumes cars are left at home to serve the grid. This also assumes electricity consumption does not increase, which seems unlikely (since transportation and heating must switch to electricity at some point).