I do not know about "15w" per se.
Plus I tend to be skeptical of any manufacturer provided specs and likewise cautious about other tests and comparisons.
The TechPowerup testing appears to be fair and professional and I am comfortable with the information/results presented. E.g., the testing details are well described and explained.
Overall then I agee with
@Roland Of Gilead - "13-18 watts is nothing really" and in my mind neglible with respect to potential power requirements exceeding 650 or more watts.
The purpose of your build is to play games - correct?
Then base your decisions on power requirements for/during gaming. Not "idle times".
And consider that if you are not going to be gaming or otherwise using the build for lengthyTBD amounts of time then just shut the system down.
Makes the "idle wattage" issue moot.
Also: Read some PSU Reviews - not sure about that PC Picker PSU.....