PC power supply question...

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This is probably a simple question (sorry if it is), but does a PC motherboard have any communication with the power supply unit that it is attached to? For instance, if the computer is placed in sleep mode, does it have any way of digitally signaling the reduced power requirements to the power supply, or does it simply draw less current from the supply and that's it? Reason I ask is I just want to know if a 750w power supply is, for instance, always consuming 750w from the wall socket when its turned on and just dissipating the unneeded amount as heat, or only draws what it and the motherboard, non-self-powered peripherals and expansion cards etc. need? I'm guessing the latter, but if the former, since sleep mode would in theory use less electricity, can the motherboard switch a 750w power supply in to a mode where it would only draw, say, 100w?

Sort of. In a modern computer with an ATX standard motherboard, the power never truly "turns off" unless you either pull the back plug, or flip the switch on the back of the PSU, if you have one. The reason why you don't hear fans going when your computer is "turned off" is because the motherboard only draws enough power in this mode to be able to be "turned on" when you use the front power switch. What the front power switch does, as opposed to the one on the back, is tell the computer to "wake up", and as a result the fans start going, the drives start receiving power, and the rest of the motherboard's components turn on as well.

Even when your computer is turned on, however, usually it doesn't use nearly as much power as what the PSU is rated for, but rather it uses almost just enough to power your components. The reason I say "almost", is because even the best power supplies are only about 80% efficient.

I'm not entirely sure what happens in "sleep mode", but I can assume that a computer in sleep mode will use less power than one running idle, and more power than one just sitting plugged in.

If your computer has a 750w power supply, and you don't have 750w "worth" of components hooked to it, that's actually a good thing. It means the power supply isn't being pushed beyond its design limits, and there is some breathing room in case you decide to run an intensive game or program.

What mr_bigmouth_502 said.

A 750W PS means it has the capability to supply 750W combined on the +12V, +5V, and -12V buses.

Would question why you need a 750W power supply unless you are going to have multiple kick-ass video cards.

At the crib we have one SFF POS in the kitchen then two full towers each with 400W PS and an nVidia GTX 650, with the usual mobo, DvD, 2 HD, etc stuff, no problems.

I bought a used 1000W from a co-worker just to have a back up.