If you drink a cup of tea (250ml) at 80°C, you have a temperature difference of 80°C-37°C=43°C. If you drink it in 5 minutes (300s) then the power you get from the temperature is

  P = 250ml * 1g/ml * 43°C * 1cal/g°C * 4.18J/1cal / 300s = 150W

If you add one tablespoons of sugar (12g), you get approximately 3.8 kcal/g so in the same 5 minutes you get

  P = 12g * 3.8kcal/g * 4.18J/1cal / 300s = 635W

but this is not absorbed instantly, but you have to average with all the other food you eat during the day (the average is approximately 100W).

So you get much more energy from a cup of tea than using a WiFi router inside your mouth (anyway, I'd not recommend it).

I accidentally upvoted you. :-/

If I lived near overhead power lines (or had them on my land) I'd definitely make use of as much of this kind of charging as I could get away with. It's probably not legal, but it'd make an interesting case. (Assuming it hasn't already been tested in court.)

In the US it is definitely illegal to siphon power from a utility, even without a physical connection.

Detecting it with casual use is probably impossible though.

But living that close to high power transmission lines can't be good for you in the long term.