Question

Efficiency of power transformer is of the order of

Answer 1

An ideal transformer would have no losses, and would therefore be 100% efficient. 
In practice energy is dissipated due both to the resistance of the windings (known as load loss), and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient, and large power transformers (around 100 MVA and larger) may attain an efficiency as high as 99.75%. Small transformers such as a plug-in used to power small consumer electronics may be less than 85% efficient.