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.
> over 170,000 terawatts of solar energy every day
i'd definitely rewrite it myself, but it's also a correct way to specify that there are no days of the week, year, or whatever (solar cycle) in which the terawattage is below 170k. Not very intermittent, is it!
I think we may be talking at cross purposes. I specifically used the number one because it behaves like a unit here. unlike 2 or any other number, 1 is also the standard 1D unit vector, so 1apples is indeed the same quantity as apples/1, but because it is a unit we usually imply its presence rather than express it explicitly as above.
watts/unit thus seems fine to me, whatever the unit may be, even if it itself is derived from time. watts per day would just work out to joules/second/1/24*60*60, making 1 watts per day a derived unit that expresses joules/84600 seconds, or an instantaneous rate of one 84600th of a joule.
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.