Energy efficiency with all its various aspects has been taken into consideration as the resources are diminishing at various rates. Improving the equipment efficiency is one of the major steps to get closer to ideal efficient use of energy in electric power systems. In this regard, transformer efficiency in distribution networks has been crucially studied over past decade. Advantages and disadvantages of various types of lamps including incandescent, fluorescent, Compact Fluorescent Lamps (CFL) and Light Emitted Diode (LED) from energy point of view have been debated quite effectively in the literature. Since the power utilities and their customers are showing an increased concern about the energy efficiency and power quality aspects, there are some mixed feelings towards recently introduced lamps. In this regard, harmonic distortion level initiated by extensive use of harmonic-generating lamps is considered. Vital electric characteristics of four lamp types will be explored. Then, transformer de-rating under non-sinusoidal condition from standard point of view is investigated. By using power quality analyzer, current harmonic levels generated by the abovementioned lamps are measured. Calculation and simulation of transformer loading while using each light as transformer load is performed, and finally effects of lamps on transformer de-rating is compared.