PSU Watt

[Sklo]

Is it possible that some power supplies deliver less then the labeled power and why?

[Juganog]

Generally they do, my 750 watt actually only gives about 738 or something. they normally just round it up as to why probably something to do with manufacturing or number of outputs 

[Clyne]

well you can tell by looking at the rails

http://linustechtips.com/main/topic/199255-how-many-watts-do-i-need-check-here/

[daemonowner]

And more than looking at the rails, some manufacturers simply lie. Raidmax sent a unit in to be 80Plus tested (achieved Gold), and then sold a different inferior (Bronze) PSU with the Gold label.