I don't mean to be rude, but anyone who thinks watts/day and watt days are ever interchangeable will have severe problems reasoning about anything electricity-related or energy-related.
It is akin to thinking that "2 apples" and "an apple divided by 2" are interchangeable because both expressions involve the concept of an apple and the number 2.
It is akin to thinking that "2 apples" and "an apple divided by 2" are interchangeable because both expressions involve the concept of an apple and the number 2.