I'm sure this question is better posed in an electrical engineering or science type forum, but I'm not involved in those communities, so I figure I'd toss it out here...

For quite some time, I've wondered about how a PSU actually draws current, and whether a 250Watt PSU, for example, actually draws (or dissipates?) a constant 250Watts when plugged in, or if the 250Watts is a rating that the unit can't supply higher than that.

I ask because I am very interested in minimizing my electrical impact in this world, and am persuing the via epia chipset to meet those needs, as well as a large, single WD IDE drive instead of multiple smaller drives for my server needs. In my quest to accomplish this, I've run into a brick wall with understanding the PSU notion of "does it draw 250W constantly or just if the need arises?" How much would it draw on average?

Are there any good primers or papers on the matter? Can I acquire some hardware that can test the *actual* AC and DC dissipation for the unit and the computer components?

Thanks all you electrically minded folks!

The function of the PSU is to convert AC current i.e. 120VAC 60Hz in your case to the DC voltages +/- 5VDC, +/- 12VDC and +3.5VDC necessary to power the components in a PC. The total current supplied to the PC is equal to the current required by all the components in the PC plus what the amount that is lost due to the conversion process. Nothing is 100% efficient. Anyway, 250 watts is a max rating. The actual power depends on what stuff is installed.

Primers or papers? How about a good book? Go take a trip down to the local library and spend some time reading up on basic DC electrical circuits.

The type of power supply in a computer is called a switching power supply and is much to complicated to explain here:

http://www.smpstech.com/tutorial/t01int.htm#SMPSDEF
http://www.qsl.net/xq2fod/Electron/PS40/PS40.html

very helpful and quick response. I will read up further on it, but your answer was consise and very helpful, so thank you!