The specs say 10W. I've got it plugged in via USB to my PC.
Makes no difference to me either way... the wireless charging is more of a quick convenience... it isn't going to replace wired charging this year I don't think... and I still have cables around the house for that reason.
Does the 8 even support "fast charging" wirelessly? I thought I read something recently about someone using a 70W wireless charger but don't recall exactly...