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Different power supplies "power" differently even if they are 400Watt power supplies. This article goes into Chooing the right power supply. It explained a lot to me. Also there are power supplies that only draw as much as they need. I am not sure that this applies to all though. I agree that if they do produce that power at all times that would definitely be a real waste.
The rating of any power supply is governed by design and components......
For example, a linear power supply running from a 240v ac supply,could be pulling as much as 1 amp(AC current ) giving an output of 12 volts @ 10 amps would be designed to do that.......although if it werent loaded to its full rated output, the consumption of the supply would be a lot less, than if it were fully loaded.
Switch mode power supplies draw the same parallel.......less load, less current draw from the primary supply......be it a computer power supply or whatever..
The main advantage of a switch mode over its linear counterpart is the fact that its more efficient....
A power supply rated at 400 watts means that it is capable (theoretically) of delivering 400 watts of power to the load.
The components themselves actually determine how much power is used.
The power supply itself uses some power even if there is no load connected to it, but not much.