Dumb question about power supplies

[ka]

Is power output varied with the load on the power supply - or does the output power stay at rated level?
By this I mean for example a 400w supply, does it always output 400w or does the output vary according to the demand on it? E.g if the components in the PC only drain 200w does the supply provide this wattage or 400w all the time?

 

[Gemini1706]

It just supplies what the system needs. A 200W watt system will make the PSU to output 200W only.
But the rating of the PSU is the MAXIMUM it can do.
Also, many PSU rated as an example 400W are not "actually" able to deliver the 400W. It depends on the quality of the design.