It is essential to note the level of efficiency that is associated with a transformer, since it is based on those standards that functioning standards will be determined.
The ratio of input power to output power is taken as total amount of efficiency in regards to an efficiency of a particular transformer. Also, loss in energy of efficiency is noted via volt-amperes, when it comes to maintaining the level of efficiency, it is taken in terms of watts. So, maximum value has to come up at unity power factor level.
The efficiency of a transformer varies with power at different levels. Also to be noted is with a correct designing of transformer, maximum efficiency can be put forth whatever be the level of load.
To find efficiency levels, you can also deduct copper losses for close circuit test and core losses in case of open circuit test. At a certain point, values for both copper and core losses are same.