The current clamp methods do work, but are nowhere near as accurate , mainly because they dont measure the voltage, only the current.
Go look at the site above, they measure the voltage. They also have done a lot of comparisons with various wattmeters and come out pretty darn close. Using the pulses from the power meter is probably the best way to go, if you have pulses. A heck of a lot of us don't have pulses to measure. My setup measures the voltage and the current and compares very, very favorably with the power meter. I don't accumulate the readings, too many power failures, but I can tell you instantaneous usage on a second by second basis, which the power company won't tell me.