Does the charger consume power after charged device is disconnected?
These kind of AC to DC converter will always consumme a small amout of power to maintain the control IC alive. I do not see a way around that unless your USB port has some sort of mechanical switch to flip on the converter when you connect a device. I recommend you to look around as I found this wall plug (for US 120V unfortunatly for you) that only turn on the USB charger when you lift the small window above it (mechanical switch as I said). https://www.amazon.com/Newer-Technologies-NWTPWR2U15ATRW-Nwtpwr2u15atrw-15-amp/dp/B00BGF00TO

Usually, external ac to dc power supply must have an efficiency rating from level 1 to 6 now. 

CUI Inc has a nice page that explains the regulation and this tells you how much "no load" power consumption you can expect for each efficiency level : http://www.cui.com/efficiency-standards

The problem I can see is that I am not sure if your AC to DC USB power supply count as an "external" power supply. Most USB charger from a wall plug doesn't specify an efficiency level... I do not know any regulations for internal power supply...