There are two types.
Firstly, the plug in monitors. Our hosts sell them
http://www.navitron.org.uk/product_detail.php?proID=339&catID=67Some of them are not good at measuring low power eg under 100 watts. But these are as good as any at low power, as far as I know. I have one.
Secondly, there is the type that measures power consumption on a larger circuit. They do this by clipping onto one of the cables, not both. That is they cannot clip over a twin+earth cable as that has both the live and neutral in it. They are best suited for use near a meter or consumer unit. Again our hosts sell them
http://www.navitron.org.uk/product_detail.php?proID=17&catID=67Again, there are problems with low power. Mine never measures under 180 watts even if there is less power being consumed.
Another similar one is
http://smartenergykit.com/One of the regulars on this forum helped with the design. I believe that they are more accurate.
Hope this helps.
John