Distribution transformers are essential components in power systems, serving to step down medium-voltage networks to levels suitable for end users. Transformer efficiency is significantly influenced by power losses, particularly core loss and copper loss. This study aims to analyze power losses in the GU.02 Feeder distribution transformer at PT. PLN (Persero) ULP Medan Denai, identify factors affecting efficiency, and compare transformer performance using different core materials. The research methodology includes field data collection, manual calculations based on transformer technical parameters, and simulations using ETAP software. Manual calculation results show an average power loss of 14.37 kW (3.01%), while ETAP simulations yield 14.18 kW (2.97%). These differences are due to rounding and calculation approaches, but both results are closely aligned. Factors influencing efficiency include load conditions, core material quality, conductor resistance, operating temperature, connection conditions, and equipment age. Comparative analysis of core materials indicates that amorphous cores exhibit lower core losses than conventional steel cores, offering higher efficiency and reduced energy losses in the distribution network. These findings confirm that selecting the right transformer design and implementing optimal operational strategies can enhance energy efficiency and support a sustainable electrical distribution system.