That's an awful lot of conservation, even by European standards. Ideally, just using less power is always the best way to go, if that's possible. The question is how much can you cut while still maintaining a lifestyle that people will accept. I know going LED for lighting is projected to cut something like 10-15% off annual electrical usage. Maybe you can gain another 5%-10% with more efficient appliances, and another 5% if lots of people bike to work instead of taking trains. But on the flip side transportation is going electric, so all those EVs need to plug into the grid to recharge. If they mostly do so at night, it's not an issue. Plenty of surplus power at night. During the day is another story. It might come down to if someone forgets to plug in their car at night, they'll be SOL the next day because they won't be allowed to charge it anywhere.
Heating and cooling should be able to account for a massive change. Most houses have terrible insulation, and even worse passive design. (Germany is much better than Oz in that respect, but so is Mars.) That will take time - you can't replace/refit an entire nation's housing and office stocks overnight - but there are huge savings to be had.
Add some also from a more enlightened approach to temperature regulation. You don't
have to be at 19 degrees all summer long and 29 degrees right through winter. (Some people are friggin' stupid - I have to take a jumper to put on when I go there in high summer, and I want to take my shirt off to cool down when I go there on a cold winter day. What's wrong with (say) 15 in winter and 25 in summer? Friggin' brain dead weaklings, most of 'em. They need to harden the F up a bit.)
As for not being allowed to charge, I should imagine that you will always be allowed to, but you will pay a heap more for it at the wrong time - three times higher, maybe.
But with even the conservatives in Germany going the clean, green route now (the Greens and centre-left have wanted this for years already), it strikes me that they must have plans afoot to go with other technologies. I cannot imagine the Germans ever going without their creature comforts! And they are too well organised to stuff this sort of thing up. I imagine that they will be leading the world in clean, low-wastage technology before long - and getting even richer as a result.