I am surprised so many people are recommending the kill-a-watt when in testing they prove to be inaccurate. The numbers can be skewed by +-10% with a kill-a-watt. Specifically with PC power supplies there are issues of it reading wrong and showing 100% efficiency. The proper way to measure your power is with an amp clamp on your power cord. You test the amperage then multiply it by the voltage to get wattage.
Say for example your rig is pulling 7.3 amps at 116 volts. With those numbers your rig is drawing 847 watts.
Test with 1 card, then 2, etc to see what your cards are actually pulling.
Can you really read amp with an amp clamp on power cord? Have you did that cause as far as I know Amp clamp are meant to be used on a single phase conductor ?