Pembahasan pokok masalah dilakukan guna mengetahui efisiensi transformator dan unsur-unsur yang mempengaruhinya. Transformator daya merupakan perangkat listrik yang penting karena berkaitan langsung dengan transmisi dan distribusi tenaga listrik. Dalam praktek selalu muncul masalah yaitu terdapat rugi-rugi pada transformator berupa rugi tembaga dan rugi inti sehingga nilai efisiensi transformator di bawah 100 %. Kedua unsur rugi-rugi itu berpengaruh dominan terhadap efisiensi transformator. Rugi inti itu diakibatkan oleh histeresis dan arus eddy yang bergantung pada frekuensi sinyal sumber tegangan dan jenis bahan inti. Rugi tembaga itu dihasilkan oleh dan bergantung pada arus beban dan resistansi belitan. Distorsi harmonisa yang muncul ketika transformator melayani beban juga merupakan unsur rugi daya pada transformator yang dapat menurunkan kinerja serta kapasitas kerja transformator. Dampak pengaruh semua unsur tersebut dapat dilihat dari nilai persentase (%) efisiensi transformator.Kata kunci : efisiensi, transformator, rugi inti, rugi tembaga, harmonisa.The main problem is discussed in order to determine the efficiency of transformer and elements that influence it. Power transformers are important electrical devices because they are directly related to the transmission and distribution of electric power. In practice, problems always arise, namely that there are losses in the transformer in the form of copper losses and core losses so that the transformer efficiency value is below 100%. These two elements of losses have a dominant influence on transformer efficiency. The core loss is caused by hysteresis and eddy currents which depend on the signal frequency of the voltage source and type of core material. The copper loss is generated by and depends on the load current and winding resistance. Harmonic distortion that appears when a transformer serves a load is also an element of power loss in the transformer which can reduce the performance and work capacity of the transformer. The impact of the influence of all these elements can be seen from the percentage value (%) of transformer efficiency.Keywords: efficiency, transformer, core loss, copper loss, harmonics.