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Yes it was an educated guess. Greater than 30% loss is highly unlikely.
I have removed that statement in my main post.
There is no way to measure the charging wattage on iOS.
I don’t have Android devices with wireless charging capability that I can test.
Idle drain is around 0.4W.
There is probably a way to measure delivered using a Qi receiver loop. But also not accurate for your phone as your phone has more things in the way lowering efficiency.
I’d expect the losses between AC-DC converter, cable, the Qi transmitter to be nearer to 50% loss, and then losses within the phone. So, yes, you’d probably need to take 15W from the wall to deliver 7.5W.