If the charger burns 1 watt at idle (and we think it burns much less than this) then for a full year (8,760 hours or 8.76 khrs) that's 8.76 kilowatt-hours. At 13 cents/kilowatthour in Los Angeles USA that results in (8.76*.13)--Wait for it:

$1.14/year

Your mileage may vary.