Does a power supply draw only as much power as it needs?

Does a power supply draw only as much power as it needs? I recently built a small mini-itx Intel ATOM-based Ubuntu home server. The case I choose is a small case but only a full size ATX power supply would fit it. I choose a mid-grade OCZ 500 watt modular power supply for it and it works great. I could not find a modular ATX power supply out there that was less than 450W.
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Total answers/comments: 3

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Guest [Entry]

When you buy a 500W power supply that means this PSU can deliver a maximum of 500W ! So if yours motherboard + HDDs consume 100W then your power supply will get 100W (+ a negligible cost of transformation ~10%) from the power plug !

Guest [Entry]

Yes, a power supply draws power relative to how much is being used. So if your PC hardware is only using 200W, your 500W power supply won't draw 500W. How much it draws will vary from power supply to power supply.

Guest [Entry]

"I want to put the focus on a certain aspect when it comes to price. Therefore I start with a small ""correction"": a ""500W power supply"" doesn't refere to the power supply ""drawing"" 500W from the outlet but that it's able to ""provide"" up to 500W to the device connected to the power supply.

That being said the OPs goal was to build use inexpensive power supply in terms of initial cost (the price) and running costs (energy consumption). As a general rule of thumb the power supply should provide much more power then the device needs. Simply speaking the higher the watt output the higher the loss due to ""overhead"". This especially applies to cheaply built power supplies.

So as a conclusion: there is a trade off between cheap and expensive power supplies. The cheap ones are usually more expensive in operation especially if they are oversized.

For the technical details I would refer you to the other answers, they are quite through."