Core losses in electrical transformer remains same through out the operation of the transformer irrespective of the load on the transformer. However copper losses which occur due to the resistance of the winding (I2R) vary with the load on the transformer.
In some types of transformers (Distribution Transformers) performance or efficiency cannot be determined by the ordinary efficiency calculations. For instance consider the distribution transformers supplying power to the house and industrial loads, these transformers will not have constant load and load varies throughout the day. During night times most of the load demand comes down results in the less load on the transformer. The performance of such transformers are determined through it all day efficiency which means efficiency is computed on the basis of the energy consumed during the whole day (24 hours ).
All day input = All day output + Iron losses + copper losses (calculated for 24 hours)
Distribution transformers will not supply the rated load for the full day. Therefore all day efficiency of distribution transformers will be less than ordinary efficiency or commercial efficiency. Most of the time load on distribution transformer will be between (50% to 75%). Therefore distribution transformers are designed for maximum efficiency at half load. On the other hand power transformers are designed to have maximum efficiency at near the full load.