Comma questions

Solution 1:

So for sentence 3, here is my suggestion:

3. [Throughout the whole of Europe], nuclear energy was the main source for generating electricity, having contributed to 25% of the [generated energy supply].
   (Note: my personal edits are in the brackets.) You would not wish to say "generation" at the end, because that word is much more frequently used by English speakers to refer to an age range of people (as in Generation X) or the grandfather-to-father-to-grandson type of "generation."

Okay, if you wish to say that nuclear energy was NOT the main source for all of Europe, here's what I would recommend:

"Nuclear energy was not the main source for generating electricity in all of Europe, having contributed to only 25% of the total generated energy supply."

Solution 2:

1 is correct, “thus” is what is known as a fronting adverb – these are followed by a comma. But the commas at “, like dishwashers or electric water heaters, are parenthetical commas because they separate an example from the main clause. The main clause is “Thus, flexible devices can provide significant flexibility to the system."

2 This comma is the “Oxford comma”: it is useful but not always necessary or correct. It is used in a list of separate items.

3 “In entire Europe” is not idiomatic. “Throughout the whole of Europe” is correct. Like 1., it is a fronting adverbial (phrase).

4 While in the current system, and “in future," are also fronting adverbials.

5 seems to be missing a verb