To determine transformer power factor under load, you measure real power P in kW and apparent power S in kVA at the terminals, then compute power factor as PF = P / S. This can be done using power quality meters or three phase wattmeters with voltage and current transformers. For insulation power factor tests, a different concept is used, where the ratio of resistive to capacitive current in insulation is measured at test voltage. That diagnostic “power factor” indicates dielectric condition rather than load power factor.