If you are like most people, you probably have many electronic devices in your home using AC/DC power adapters.  Older power adapters are not very efficient, anywhere between 20% - 60%.  Newer power adapters that meet the voluntary Energy Star standards have power efficiency as high as 85% minimum.

Energy Star [1] mandates the International Efficiency Marking Protocol [2].  If a power adapter meets the protocol then it will have a Roman numeral with a circle around as per the figure in the international protocol [2].

If the Roman numeral is III or higher the power supply will have at least 84% efficiency (>50W load) and consume less than 0.5W - 0.75W (no load).

Consult the International Efficiency Marking Protocol documentation for an exact summary for each Roman numeral.

[1] Energy Star - External Power Supplies
[2] International Efficiency Marking Protocol