The interesting parameter for a mains transformer is the no-load current. For our mains transformers LL1648 — 51 the no load current is less than 100mA, indicating a no load impedance above 2kohms at 50 Hz. (corresponding to 6H) The no-load current should be compared to the full-load current, which for a 250VA transformer is around 1A. A 1:10 factor between no-load current and full load current is in my opinion quite acceptable.