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.