I want to specify something really clear to all about PSU efficiency. When a PSU is rated at 500w total output and has a 75% efficiency factor the PSU does not have an output of 375W(500*75/100 or 500*0.75). The PSU has an output of 500W.
The efficiency factor represents how economical the PSU is. This means that it wastes 25% of the power drawn from the wall, with the remaining 75% used to power the components inside the computer.
If the total power drawn from the wall socket is 500W with 75% efficiency, the PC components (other than the PSU itself) are only consuming 375W.
If a 500W PSU is used at maximum rated capacity, 666.7W will be drawn from the wall.
Most of that 25% loss is transferred into heat.
(Credit also goes to hunter315 for some corrections to this post)