Given the statement by Transistor this post has been heavily edited for his and others elucidation.
Charger Samsung Galaxy Alpha Network, Note Edge, (EP-TA50EWE). White
Input: 100-240V @ 0.3 A
Output: 5V @ 1550mA
Standard USB jack
1W = 1 j
7.5W = 7.5j
3600 seconds in 1 hour
so 7.5j x 3600 = 27kj/h
Power(Watts) = Energy(Joules) / time(in seconds)
An energy expenditure of 1 Wh represents 3600 joules.
So we now have 27kj per hour divided by 3600seconds = 7.5Wh or 0.0075kWh
W and Wh are interchangeable.
Regardless of the comments below, there is a bit of nit picking going on here with regards to kW and KW, I am sure people understand what is being meant and that temperature is not being talked about.
That over with, just to explain a few points with my earlier post for those who saw it.
PSU's designed for the Eu and USA have strict tolerances they must meet, however that does not mean that every PSU made outside of those areas will comply with those regulations.
Some low current PSU's are half wave rectified and rely on a tuned circuit at mains frequency to provide 5V @ XmA for use in low power circuits. Please see http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf
However I ripped apart a Samsung charger yesterday to find a switch regulator circuit.
The efficiency of these are stated to be between 75% - 90% + any losses within the circuit. Again these losses are so small that its really not worth mentionig, unless you want precise, which is just not worth it in every day life or it is a cumulative circuit where the sum of the losses are important. I tend to look at switch regulator supplies as lower current devices, just to differentiate between switched mode regulator and switched mode power supply(multiple Voltages and higher currents), although strictly speaking you will find a switched mode regulator at the heart of a switched mode PSU.This stems from a work nomenclature where I want a SMPS does not mean I want a switched regulator.
There are a number of factors here with regards to charging and efficiencies. the latter you only need a rough figure for and best to take worse case scenario( and easiest calculation for) of 75% to calculate power loss.
So if we wanted we could just say that the actual W used would be 25% more, than that quoted for the output power, which would be approx 9.375W or if you want to go through the rigmarole of calculating Wh to find it means the same thing go ahead.
I could go on about charging batteries and characteristic curves for each of the devices used, however all you wanted was a rough figure and a way to get to it :)
So saying that I have seen others state things that need modifying a little.
When a flat battery is put on charge it will, given the Voltage stated, want to suck current at its maximum of 3A, however as the PSU cannot provide this it will hit the limits of the supply current and the % efficiency may drop as low as 50%/ As current rises above the stated value, the efficiency of a switched regulator goes down Many Switch mode PSU's however have fold back current limiting to stop this, unlike these little switched regulator supplies. What will happen here, if left unfettered, is that until the battery has enough power to balance out at the figures given, charge rate will be slow and power consumed will be high. As the Battery power levels increase, the stated figures will apply and the efficiency will increase, that is until a low charge current is required and again efficiency will decrease. Most cell phone batteries do have battery management so can turn off and limit the current supplied for each of these conditions. Whether they do or not is solely down to the manufacturer/design team.
Fast chargers work on the principle that the temperature of a battery is used to regulate charge applied, ergo hitting a battery with a large current will charge it fast but temperature rises at the same time. At a given temperature current is turned off, and the battery is left to cool down below a set limit, before charge current is again applied. Doing it this way should show you that calculating the power used is not simple, however fast charging is never very good when used regularly, as the battery material will degenerate faster. Maybe the next generation of fast charge batteries will be better, 12 minutes for a 4000mAh battery is touted.