In Tanzania, the electricity supplier, Tanesco, works in a similar way. Every house/appartment has a box/meter installed where the amount of kWh left is displayed. To recharge it, one needs to pay at distribution points by giving the number of the meter and the amount of money necessary. A code is generated, that works only for the meter it was meant for, which is typed into the meter. The entire country's electricity runs this way.

Hi, Try looking for Giancoli's Physics Textbook. It explains is and makes it quite easy to understand