This paper describes physical image of transformer work, magnetising current which is needed for creating magnetic field in core and losses which occur when transformer is working. An overview of legislation about energy efficiency of transformer is provided. Amorphous alloy core transformer, mode of production and their characteristics are described. Amorphous alloy core transformers have less losses in comparison with conventional grain oriented silicon steel transformer, especially no-load losses. Economic comparison with TOC method was done and it has been found that amorphous alloy core transformers are economically viable in despite of bigger initially price. Analysis of replacement of existing 20/0,4 kV transformers with help of the simulation model of network excerpt Elektra Karlovac in software package DIgSILENT was carried out. Losses in the network are reduced by 23,42 kW. Amorphous alloy core transformers have higher inicially prices but make savings because of lower cost of losses.