An ideal transformer with open secondary would approximate an inifinite inductance
ie its primary would draw no magnetizing current.
and any load connected to the secondary could be represented as a load in parallel with primary .
A real transformer has imperfect core hence finite inductance so draws a small magnetizing current .
At any instant the energy stored in an inductor is [itex]LI^{2}/2[/itex]
and phyzguy's explanation above is 'spot on' .
Does that help?
