To calculate the load power factor of a transformer, you measure real power P (kW) and apparent power S (kVA) at its terminals and then compute PF = P / S. Real power is obtained from wattmeters or power quality analyzers, and apparent power from voltage and current measurements. For three phase, S = ?3 × V × I. If the transformer is lightly loaded, power factor can be very low because magnetizing current dominates. It is important to distinguish this load PF from insulation power factor used in dielectric testing.