I’ll try to explain the 40%, sorry for the parts that you already know.
Electric energy is always produced at the same time (and »place« roughly) as it is consumed. (You can’t pump electricity into some reservoir to be consumed later, you always need a different energy form for storage.)
The problem with volatile sources is that they mostly (more than half) produce energy at the wrong time and/or the wrong place, and at other times produce nothing.
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⇒ Aside: the »place« problem is that you can’t build solar panels and wind turbines just anywhere, and they need a lot of space. E. g. Germany has now the problem that the wind blows much better in the north, but the industry is more in the south. So, you need a lot more/stronger transmission lines. Same for offshore wind: more wind at sea, but you need a lot of cables.
The more wind and solar you already have, the more the good places are already taken.
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⇒ (But at least we already have transmission tech, it is now just a question of materials and effort.)
So, assume that we have enough wind and solar that we can regularly produce 100% of demand from them. You can imagine peaks just touching the demand line at top demand.
(You could imagine more than that, but that would mean overbuilding, which hurts the economics quite badly while not making the end result much better.)
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