Question

A cell phone charger uses 4.83 joules per second when plugged into an outlet, but only 1.31 joules per

Answer 1

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.