Understand I work for a power company, and as I said I'm giving the electric companies perspective.
And energy use goes down on cloudy days (less A/C used). You are looking for a problem that doesn't exist. There are plenty of battery technologies out there. They aren't used because they aren't economical, not because they don't exist.
I'd say you live in a warm climate.. Cloudy days means more energy in colder climates. France uses at its peak about 100 GWatts of electricity. Say you'll need at least 10 hours of battery storage, then you are talking about 1 TWh of power storage for 100% replacement by PV. You can't seriously imagine that any current battery technology can supply that type of storage. Hydro can get you part of the way there, but the largeset damn in France has 800MW of generation and can only supply about 2h at that level. As well that dam is not a STEP (Pump water up hill when power is cheap to store energy) and there are only 5 such dams in France for geographical reasons. So as I said there is not storage technologies that are suitable
I never meant storage technologies don't exist just as you say they are not economically viable or available in large enough quantities to make any meanful difference.
There isn't enough solar production to make storage save anything.
Storage is not about "saving" anything, its about making the supplied energy meet exactly the demand. With an intermittent source such as PV you absolutely need another means to ensure demand meets supply, with the ultimate means availble to the power grid being of course a blackout.
And it's what, about 80% of usage during the day? So until that 40% approaches 80%, they obviously don't have over-production of solar that would allow storage. If they had "perfect" storage now, it wouldn't be used. So that's obviously not the problem..
If fact the 40% peak PV is for a Sunny Sunday afternoon, so a lot further away from 80% than you think. .
"Production" needs to be 100% of usage before storage without parallel generation would be feasible. 40% is well below the ability to serve those areas, no matter what the storage was.
Why ? Storage already exists in the grid in reasonable quantities by pumping water up hill. I really think you have no idea how a power grid works.
The supply of electricity must meet the demand at all times, with a little bit of slack taken up in voltage or frequency drops. To meet this simple supply/demand equation, traditionnally there the power production means were split in the two types; "Base" and "Peak". The optimal base energy source is extremely cheap, in Europe on the order of 50€ to 100€ per MWh but with frakking in the US a bit cheaper on the other side of the pond, but difficult to put online, with lead times from hours to days. "Peak" power is optimised for the time it take to put online, of the order of minutes but not cost. A gas turbine might, basically a jet engine with a 200Wh inline generator, costs 1000€ or so for a MWh of prodution. PV and most other renewable energy sources are "Intermittent", so in periods where the base supply is sufficent, they subsitute foe the base supply source, but without storage you can you remove the base source because the source is intermittent. Any amount of storage can allow the removal of some base power source, but to make a meanful reduction in the base source you'd need at least 10h of storage of that base storage. For example to get rid of a 900 MW nuclear reactor at replace it with 900 MW of PV you'd need 9GWh of storage.