the PSU is a True 380s Antec, I am using at multimeter set at 20dcv, the specs I'm not too sure about because I haven't used the computer in awhile.
The Antec True Power True380S, that is included with the Sonata case, has a self-adjusting feedback mechanism built in (i.e. voltage feedback detection). When connected to a load that PSU should have a voltage regulation within 1% unless the PSU is defective.
When there's no load there's no feedback, hence no proper voltage regulation.