Transformers are essential machines in electrical energy distribution systems, which is why more efficient technologies for power transformers have been developed for decades. This has not been the case for mall transformers used in low voltage circuits, in literature there is little knowledge about his efficiency. This work seek to determine the efficiency of conventional transformers used in low voltage through a methodology that includes an analytical method and experimental tests. Three transformers in the Dominican Republic market are selected, two of 900 VA and one of 1500 VA, all step-downs of approximately 120 V/25 V. The initial load is a resistive load of 18.4 Ω. The results indicate that the efficiencies are in the range of 47 to 86.5%, considering variations in the load. The highest efficiency of the transformers was in the demand range of 12 to 24% of the nominal demand. In general, losses in the excitation branch in the core represent at least 90% of the losses, while losses in the windings represent the remaining part. Based on the results, a more exhaustive evaluation of the efficiency of small transformers is suggested, as well as economically evaluating the replacement of transformers with more efficient ones.