The components of transformer efficiency are primarily no-load {iron} losses and load {copper} losses. Search ieee.org for standards like the C57.12-series of texts.
Else, have you tried contacting the manufacturer of the device?
Transformer losses can be separeted into no load losses (iron) and load losses (winding) with the load losses usually beeing the larger part.
Only the load losses will vary with temperature significantly.
In steady state winding the temperture difference between ambient and winding will be fairly constant for different ambient temperatures. Based on this you can guess the winding temperature and calculate the losses for different ambient temperature using the temperature coeffcient of the winding material. Using this you can calculate efficiency for different ambient temperatures.
