What is Efficiency of a Transformer? - QS Study
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Efficiency of a transformer is defined as the ratio of output power to the input power.

η = [output power / input power] = [ES IS / EP IP]

The efficiency η = 1 (ie. 100%), only for an ideal transformer where there is no power loss. But practically there are numerous factors leading to energy loss in a transformer and hence the efficiency is always less than one.