Quote Originally Posted by cleancloths
As others have said, the best way is to use the house electric meter. You don't have to turn everything else off, just make sure it stays constant when you run the test. You might want to unplug your refridgerator, as you cannot control when it will cycle. The power factor can vary alot. I have an old fridge that has a power factor of about 0.6 rather than the 0.9 mentioned.

Volts times amps only works for DC not for AC.
While that is technically true for motors, most other devices (e.g. light bulbs, electronic equipment, etc) watts = volts * amps AC or DC. The exact formula for AC is RMS Watts = RMS Volts * RMS Amps * Cosine (theta) where theta is the power factor angle and cosine (theta) is the power factor. For most circuits, theta is 0 so yes watts = volts * amps. However, for most motors, theta is small and the power factor about 90%.