Calculate the electric bill of a phone charger (AC adapter input or output?)
They are correct that you pay for the input. Now the question is, "How do I calculate the input when I only know the output?"

We need one more piece of information: the efficiency of the PSU. For this type of device it should be about 85%. 

$$ P_{OUT} = P_{IN}Eff $$

where Eff is efficiency.

You can rewrite this as $$ P_{IN} = \frac {P_{OUT}} {Eff} $$

From this you can calculate your watts and multiply this by your hours of use per year. Note that your result will be on the high side because the charger will taper off the current as the battery becomes charged and eventually drop to a trickle.