Efficiency is one of the performance indices to judge whether the performance of a machine is good or not.

In general Efficiency of a system means that how much input is being converted into output. In case of Transformer, efficiency indicates that how much primary input (or input power)  is converted into secondary output (or output power). 


So Efficiency of Transformer is the ratio of useful output power to the input power. 

Notation of Efficiency : Generally Efficiency of a machine or a system is denoted by η.

Efficiency of different types of transformers :

Efficiency of a transformer directly affects its aging and its performance also. In any machine we should try to optimize the maximum efficiency that means we try to convert input power to output power as much possible.

In case of power transformer, the losses are very low. That's why Power Transformer works on 99% to 100% efficiency.

In case of distribution transformer,  maximum efficiency is 60% to 70%.

Transformer Efficiency Formula :

                                           η = Output Power / Input Power

Suppose a transformer is loaded x% of full load.


then output power,  P。= x Pfl  : where x always less than 1 (x<1). 

                                P。= x S cosφ  [As, Real Power(P) = Apparent Power(Q) * cosφ]

and the copper loss Pcu = x² Pcufl. : Pcufl=full load copper loss.   Iron loss Pi = Constant.