http://twistedmods.com/article.php?name=Sections&op=viewarticle&artid=136 Looks interesting but QUITE DANGEROUS!! (I dont know if i would attempt it)
Or you could just buy a standard plug-in wattmeter, which will give you a far more accurate result and without the risk of electrocution and untimely death. The method you linked totally ignores the power factor issue and so will likely give you a major overestimation of the power consumption.