How do I calculate the energy (kWh) consumed by my 5V smartphone charger?
Simple maths: Energy Loss  = Input ac energy - Output DC energy.

How to measure input ac energy:

Use something like this:


Plug your mobile charger to this and let it run for an hour or so. It will give you a reading of total energy consumed.

How to measure output DC energy:

You can use a similar power meter if available or use an arduino or some other micro-controller like this:



Take readings every second or so and go on summing up to get total energy used on the DC side. This arrangement is required because current and voltage might change over time. Also, you might have to change 1 Ohm resistor according to current in the circuit. 

Once you have input and output energy values, you can calculate the wastage.