Well the other problem is that calculated power assumes the current is consumed continuously as a sine wave where as in reality many loads just 'sip' current when it reaches a certain level. The classic case is a standard linear DC rectifier/filter capacitor configuration. Once the filter caps charge then the rectifier only draws current when the AC voltage is at or higher then the DC voltage charge on the caps, so that power is consumed in 'pulses' rather then in a continuous draw like a lamp or pure resistance load. Anyway a typical solution requires a ''true rms' converter chip much like what the nicer DVM meters have internally so that voltage and current are displayed as their true rms value.
Lefty