[Mondial]

To get a really accurate reading of power consumption, you need a wattmeter which takes into consideration the power factor of the load. A reactive load like a power transformer will draw an inductive current out of phase with the applied voltage. So an additional current will flow, but no additional power will be dissipated because the voltage and current are out of phase. Therefore the indicated current measured by an ammeter will be higher than what you would expect from the wattage rating.

I bought one of these a while back and found it very useful. It measures Volts, Amps and actual Watts, along with Volt-Amperes which is what you get when multiplying AC volts times amps without taking the reactive load power factor into account.

http://www.ebay.com/itm/P3-P4400-Kill-A-...460f2577c9