Watts per day is a nonsensical measure.
Watts per day (considering voltage & amperage) is an important calculation, for me. I use watt hours and determine the daily average. When determining how long my battery bank could power which items and for how long in the absence of my charging methods, it was crucial. Assuming that anyone building such a system is familiar enough with electricity knowing that a watt is a joule/second and has the ability to determine kw*hr.