The performance of a transformer should be judged by all-day efficiency and this efficiency is always less than the commercial efficiency
The ordinary or commercial efficiency of a transformer is given by the ratio –(output in watts)/(Input in watts). But there are certain type of transformer whose performance can not judged by this efficiency. Distribution transformers have their primaries energized all the 24 hours,although their secondaries supply little or no load most of the time during the day except the housing lighting period.It means that whereas core loss occurs only when the transformers are loaded.This is why the transformer’s all day efficiency is always less than the commercial efficiency.