The power factor of a transformer under load is the same as the power factor of the connected load, since the transformer itself is nearly purely reactive and resistive combination with small losses. At no load, the transformer has a very low power factor because magnetizing current is largely reactive and real power is just core losses. In practice, when people ask about transformer power factor, they usually refer to how the transformer should be sized and how the load’s lagging or leading power factor affects system performance.