The German grid does not rely on nuclear power from France. Germany is a net exporter. Of course, with an European electricity network, there is a lot of electricity moved around every day, often reversing directions in a few hours, depending on local needs and production.
German winters are quite windy, especially near the coast.
Solar (photovoltaic) has its maximum around noon, when there is high demand.
The irony is that in summer, it has happened in several warm years now that nuclear power plants in France had to shut down - they are cooled by rivers and when the water level becomes too low, they need to shut off to maintain safety and avoid ecological damage. These are the times when Germany exports most electricity to France.
> Solar (photovoltaic) has its maximum around noon, when there is high demand.
I think it's exactly the opposite: see [1]. It's a increasingly big problem as more renewable sources are deployed. The peak demand happens at mid-evening and morning hours while, as you said, peak production from renewables is around noon. A possible solution is energy storage but, as far as I now, it's not yet common and not always available depending on the location and type of plant.
Yes, they shut down to not kill the fish in the river, not because the water is not cold enough.
But this problem was/will be solved by adding more air cooling.
One also has to carefully observe, that while France very often exports to Germany, most of the time, the elektricity is passing through Germany towards the south and eastern neighbour states.
