Well, that depends on how you look at it.
Efficiency =
(amount of power that does what you want) / (total power supplied).
Power to charge battery . . . 1.31 watts
Power changed to heat . . . 3.52 watts
Total power supplied . . . . . . 4.83 watts
Using the cell-phone as a heater,
efficiency of the charger in supplying heat = 3.52 / 4.83 = 72.9 % .
Using the cell-phone as cell-phone,
efficiency of the charger in charging the battery = 1.31 / 4.83 = 27.1 % .
The charger is more efficient as a heater than it is as a battery charger.
