This product has a poor sensitivity. It is unable to measure low energy consumptions of waking devices such as my TV or my Hi-Fi system. It was also unable to detect any battery-charge consumption of my mp3-player and merely displayed "0.00". I couldn't find the limit of detection, but I could see nothing lower than 10W.
As I bought it mainly to detect waking device hogs (devices without a true "off" switch, power adapters, as it is known to represent a large part of total power consumption) in order to detect where to add power switches, I can consider it as totally useless.