The problem with specifying a transformer for that pump is that the locked-rotor = instantaneous start current inrush is about 10X the running current, so the transformer must be able to supply ~30A for a couple of cycles of the AC waveform.
1 HP = 746 Watts that is a given. So logically 0.5 HP should be 1/2 of 746 Watts or 373 Watts. Now if someone could show me a motor with those numbers on the name plate that would be really cool. Never thought I would see a perfect motor in my lifetime. Most 1/2 HP motors I have seen draw considerably more, to deliver 0.5 HP many draw in excess of 0.5 KW.