The problem of energy quality has recently gained much recognition, one of the reasons being, that there is an increasing number of devices which require energy that meets high quality standards. An improvement in this respect, can be achieved by effective management of reactive power flow in the power system. Maintaining balance in active and reactive power is of key importance for the flawless functioning of the power system. This paper discusses theoretical issues underpinning calculations of active, reactive power and of energy loss in MV/LV transformers. Based on the parameters of transformers and data from consumer meters on active and reactive power, active and reactive power and energy loss was obtained, with the view to assess the efficiency of active power and energy transfer through MV/LV transformers.