But idle still would run much more than 15w
This isn't true.
- HP Prodesk 400 G5 i5 9500T > idles at 4.5W
- Optiplex Micro 3080 > idles at 7W
- Unbranded Mini Atom C3758 > idles at 3.5W
Either way, quick math, on a 7W range were talking about less than 10$/year to run the device.