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).