SPCR

Somebody should do a full detailed explanation of the rated figure for a PSU. Basically it means zilch. You have to delve into the specific figures for the unit in question to see what you actually get.

For example, in virtually all PSUs, the key 3.3v and 5v rails are interconnected electrically. This means, that although each line may quote a generous amperage, the COMBINED figure is much lower. Take the QuietPC 300W and 400W units. The total combined power on the smaller unit on these voltage lines is 175W and on the larger, 200W. One of the few units to have separate 3.3v and 5v lines is the Antec Truepower range. When they state a current - that is what you get.

On the 330W Truepower PSU the combined power on these lines is 242 W!! That's 20% more than the bigger QuietPC 400W PSU. And the Enermax 465W only offerred 220W!!

So don't just take the manufacturer's gross figure - do some research.

I'd like to know what the real typical power distribution is in a system. What draws how much from each supply voltage. I might have found something on it somewhere, maybe Anandtech article on PSU's. Maybe it really doesn't matter if 5v and 3.3v are shared total. I think 12v powers all the really big loads anyways. I think it goes like this:

12v : all drive motors, mobo Vcore supply, cooling fans
3.3v : ram, northbridge, GPU, misc other mobo chips
5v : other mobo logic, all drives logic, USB, KB, mouse

Of that, 12v is the vast bulk of the power. The 5 & 3.3 are not that much, and things like chipsets, mobo peripherals would be one or the other, not both. Frankly, I fear for cooling the system if 5 & 3.3 pile up to 175W. The 12v loads are enough heat. Many drives plus externals like USB/KB/mouse could pile up the 5v a bit though, so I'd hope it's got more overhead.

On the other hand the manufacturers could put in two whole low V regs., one each for 5v and 3.3v. Maybe a good idea in extremely loaded systems, but is probably overkill/$$$ in most simple boxes.

If you goto the PSU review at tomshardware, you will find some typical power consumption figures on the various voltage lines as well as a link to AMD for deeper info.

The AMD & Intel typical system power configs can be found in their system builders' guides. Discussed right here in these forums many times. here, for example, http://forums.silentpcreview.com/forums ... php?t=2021
or
http://forums.silentpcreview.com/forums ... php?t=2126