Is it right as xkcd states:
You can use heat flow to come up with simple rule of thumb: If an unused charger isn’t warm to the touch, it’s using less than a penny of electricity a day.
Or, more importantly, because one has usually a lot of devices like that at home:
For a small smartphone charger, if it’s not warm to the touch, it’s using less than a penny a year.
Can one estimate by some joule per time-unit calculation how much electricity is turned into heat to warm up a wall-plug cellphone-charger to, say, a maximum of 25°C?
From there I would will calculate the wasted money by using .25€/kWh -- a calculation which does not involve physics, which I can therefore do myself ;-)