If it is a 500W PSU and it is 90% efficient, then at full power it has 50W to get rid of.
Stick your hand on a 50W light bulb if you can't imagine what 50W of heat is like...
||iirc it is about 1250w and it can warm a well insulated room if rendering raw video or 32 track techno|
I would estimate that the efficiency would only be 75-80% on the standard PC PSU and this would decrease as the load is reduced (I sell Power Supplies for a living). They are usually based around old technology due to cost and production life is 20+ years.
We now have PSUs that are 94% efficient (with and without fans) so they are available but not in the PC PSU field.