Actually in almost all cases they DO use power continuously when nothing is plugged in as the circuitry to provide 5V is powered! I.e. "standby power" consumption.
My understanding of the regulations is that, CELEX:32019R1782 from October 2019 applies which has details of "Ecodesign requirements for external power supplies" with "no-load" consumption required to be <=0.1W per device (a double socket counting as a single product/device).
BUT it is very hard to find out what they actually take as most details on web sites and even in specification, etc. documents do not state this!
We had some double mains sockets with USB sockets fitted in our new kitchen/extension 3 years ago and it turns out they take 1.38W (CELEX:32009R0278 from April 2010 specified <= 0.3W)! - So should not have been allowed! Others I have found recentlly quote 5mA which at 230V is 1.15W!
Products with <= 0.1W USB socket "no-load" consumption do exist and we had 2 double mains sockets with USB sockets fitted in our new shed this year.
I know that these wattages sound low, but considering they are on 24/7, each house has potentially many USB sockets and multiplying up across the country, the actual power usage is quite considerable and obviously an issue for climate change, sustainability, etc.
