Answer:
Answer #1: If it has no load then it acts as an inductor. Find out what the value of it is(in Henry).
Answer #2: At no load, you have hysteresis loss, eddy current loss, some copper loss, and dielectric loss. These losses are constant, and are losses in real power (no reactive power, as implied by answer #1). When purchasing you can specify how much loss is acceptable, as you have to pay for this power used by the transformer. Ultimately the transformer design determines how much losses you will have, so this can vary. A rough estimate for a small transformer would be in the range of .5 - 1% of the rated current. It's best to consult the manufacturer, or the test report if you have it.
For your case, I'm assuming this is a single phase, 15kVA, with center tapped neutral (so it supplies single phase 115v, or 230 volt across the full transformer). I'd estimate loss of 75-150 watts, or .3 to .6 amps.