can be measured using wattmeter-ammeter-voltmeter method, where the power in watts is divided by the product of measured voltage and current. hmmm, methods of improving it by using a passive or active PFC.
cosine phi meters are doing that job automatically.
P=IxVxcos(phi) is the idea behind the wattmeter-ammeter-voltmeter method.
By the way in wattmeter-ammeter-voltmeter method you cannot find the current is leading or lagging right?