A primary winding while there is no load on the secondary is an inductor.
Determine the behaviour of an inductor at 50Hz.

 

Over and over it has been said that the minimum load is 20W. Then the minimum power consumption is 20W + 5%= 21W.

 

Some basics points about losses and power consumption in a transformer are mentioned here in this video:

Posting it here in this thread so that any other novice like me can find some enlightenment

 

Over and over it has been said that the minimum load is 20W. Then the minimum power consumption is 20W + 5%= 21W.

And it's also been said, the way he measured that, it's apparent power, not real power.