This has really surprised me.
I have a GPB350450500WD00, which ostensibly, if I'm reading the label correctly, should convert AC to "SEC: 4.5V DC 500mA"
However my multimeter reads a steady 7.45V!
I had no DC adaptors could be so far off spec?!
What's the point labelling them? How can one rely on what one has bought?
Unregulated power adapters will put-out approximately the rated voltage at the full-rated current load (500mA in your case). The detailed manufacturer's specs should give you the no-load voltage (if you can find the manufacturer's specs). You might not find that information in a resellers catalog. Also, with an unregulated adapter the output voltage is proportional to the input voltage... So, if your 120/240VAC line is 5% high (or low) the output will be 5% high (or low).
A regulated supply/adapter will be much more controlled.
I'm running my Arduino Uno from a "12V" adapter, and it's putting-out almost 18V with the Arduino connected.