Jul 25, 2011
So, if a PSU CAN draw 800w, and the components need 500w, will the PSU ever pull the extra 300w from the outlet? Or will it always pull 800w from the outlet?
if it can pull various amounts of wattage would it be a good idea to get a bigger PSU than you need?
The other way of looking at it:
If you see your PSU is drawing 500W from the AC socket and your PSU is 85% efficient in 400W-600W range your parts are drawing 425W DC power.

A PSU will only produce the wattage needed by the system, BUT it will always pull at least a little more than that from the wall, due to inefficiency.
That's why it's good to get a 80 Plus certified PSU, since those are rated to be at least 80% efficient (that percentage grows as you go up levels from Bronze to Platinum).

As for the other part of the question, it's never a bad thing to get a little more than you need, for expansion purposes (ie. SLI/CrossFire), but getting one that is WAY more than you'll ever need (like 1kW, when you'll never use more than 2 GPU's) causes the PSU to be much less efficient, since it's running well below its rated output.

Edit: WR2 explained quite well, actually.
PSUs are rated in DC power available to the parts - not in AC power drawn from the socket.
The majority of power used will be +12V, especially for gaming systems. On some systems you might find 650W rating but only 528watts +12V (or 44Amps). Others will have 650W rating and 576W +12V (48Amps). So you want to watch for that if you're loading up the system with graphics cards.