A power supply only draws what power is being consumed right?

[karma4jake]

So if I have a 450w PS but only have a 300w draw in the computer then the PS will only draw 300w from the outlet right.

I'm just getting kinda pushing it with my load in a room in my apartment, I have the PC and a large saltwater reef tank that has 610w of lighting and another 500w or so of stuff. All this is on a 15A breaker.

 

[DaveSimmons]

Yes, just what you're using, including waste heat. The more efficient the power supply is, the less waste heat is generated and the lower the overall load.

www.SilentPCReview.com lists the efficiency of the PSUs it reviews, though the difference is only going to be 10-30 watts so it might not be worth the effort.

Don't use any more fans than you actually need, since they use a fair amount of power.

Be sure to go with an A64 instead of an intel P4. The CPU uses less power, and generates less heat so less power for cooling is needed.

 

[Zepper]

The efficiency of switching PSUs is 60-70% (a few are a bit higher). IOW, if your PSU is actually putting out 100 watts, then the PSU will be drawing between (approx.) 143 and 166 Watts from the wall. Switching regulators also get more effecient as they near the high end of their output range. So if you are interested in saving energy, get an efficient PSU that is not oversized for the task. Getting units with active power factor correction will also help with energy savings - but not by much. The PSU comparison tests run by Tom's Hardware usually have tested efficiency numbers but IDK how accurate they are. Both P3 (Kill A Watt) (see my review in the Power Supply section of http://www.techimo.com/reviews/ ) and the Seasonic Power Angel meters can give you an accurate number re. power usage. Or you can use a DMM, an O-Scope and a calculator.
.bh.