Your second option is probably the better choice here:
Germany's and Italy's combined electric energy usage was....
since you're probably referring to the total amount of electric energy used by each country, but starting with two individual usage amounts. Individual but compound ownership is easier seen in something like
Mike's and Sam's books are mixed on the shelf. [Each retains ownership of his books, but they're stored together.]
Your first option would indicate joint ownership of the item. This is more clear in
Jack and Jill's wedding was in the evening. [It's their wedding.] Mike and Joe's car is a Honda. [They own the car together.]