I see that there is occasional disagreement here over the use of a comma before "and," "but," etc.
Take, for example, the following sentence:
By2030, for example, 80 per cent of buildings here will be energy efficient, and energy consumption will be cut by one third.
I would consider a comma before "and" as acceptable -- in fact, slightly preferable. Others will undoubtedly call for its removal.
Is this a matter of preference or is it a matter of grammatical correctness? You can read one take on it here. The author argues that a comma should be used when "and" leads into an independent clause. Another source can be found here.
My own tendency is to use a comma when the rhythm and/or meaning of a sentence calls for it. In the above sentence I would use it primarily use it so that the reader doesn't mistakenly group "energy efficient and energy consumption" in her mind. The "and" makes it clear that we are moving on to another independent clause.
I would be interested in other responses. It continually in student essays by native speakers, so I can imagine that it can be very confusing for non-native speakers.
In the "By2030, for example, 80 per cent of buildings here will be energy efficient, and energy consumption will be cut by one third." the comma is needed before "and" because we have two clauses with two different subjects, the first one is 80 per cent of buildings and the second - energy consumption.