I think it should be radiated RF power.
That way, when someone wants to regulate the power down for "health reasons", you would pay less tax.
If your AP is on 24/7 then you pay more, than if it's on for a couple of hours in the evening.
What about Bluetooth? Taxing Mobile phones is easy (already done in the RF spectrum license charges), but what about all those people walking around transmitting Bluetooth traffic? They need taxing too (but you'd need a taxman on every lamppost to detect them).