But honestly, on second thought real quick, not all the power probably comes from the 12V rail, some might come from the 5V, or even 3.3V. I don't know.
Then there are the motherboard DC regulators which aren't 100% efficient.
There isn't going to be one set formula, only approximate guesses. What are you trying to do, just figure out a power supply for a system, better to list all your system components and let someone here based on their experience and what we've seen in hardware reviews approximate a power supply for you.
Im just trying to figure out the amp rating needed on the 12v rail for specific processors for any number of builds.. so Im thinking there is a formula for doing this..like for low end systems (pentium D or low end amd processors) Im sure 18A would be plenty. But for the higher end systems higher amp single rails or even dual and triple 12v rails will be in need.. Is there a scientific was for finding this./.How do system builders figure all this out ?? (dell,Hp,etc....) the wattage is not the issue its the amps on the 12v rail or rails thats of importance.. if u got a 500 psu with a 10A 12v rail.. well u got a sucky psu in my opinion.. but thats what IM needing basically..
I did find this artcile but it deals for automobile psu's and wondered if it can be used in relation with desktop pc's..
Let's say you want to use an Opus 150w PSU with a P4 2.8C CPU. If you look up that CPU on Chris Hare's page, you can see that the max power usage is either 68.4 or 69.7w, depending on flavor. Let's call it 70w for simple math. ALWAYS ROUND UP!!!!
So you look up the specs for the Opus 150w and it says that the output on the 12v rail is 5A. 12v x 5A = 60w.
You'd get 60w out of the Opus PSU and the CPU requires 70w. It's not going to work, so there's no point in calculating how much power the other components are going to draw from the 12v rail, as the CPU itself uses more than the PSU can supply.
Your example is fine and is what I'd use. Take the peak wattage of the CPU and divide by the voltage.
This will tell you if it won't work. To determine if it will work, factor in some surplus capacity to give yourself a bit of room for unknowns. If you calculate that it needs, say, 8A and the rail delivers 10A, you have 25% extra capacity. Another might output 12A and 50% extra. Both will work most of the time, but the former might be marginal once in a blue moon. Decide what margin you're comfortable with and go with it.
That's the best you can do without fancy measurements of power and CPU usage or a whack of complicated calculations.
The CPU gets power from many different sources, not just the 12v. The P4 connector supplies 12v to the CPU for use by converting with the Vregs and Mosfets on the board. The actual ATX connector supplies power to the CPU as well.
I would think that a 150w PSU would be fine, since with some HD spinup load and motherboard load, you're looking at about 100w of power consumption from multiple rails, which is about 66% load... This is a prime spot to be to draw power from the PSU.
I build machines for a couple of medium sized businesses and a few select private customers. My experience (yours may be completely different so feel free to disregard all of this) is that it never pays to "size" the power supply. For instance my Business machines are modest machines, on-board video etc, I have discovered that it works best to just settle on one, overspeced, high quality, moderate cost supply. Its easier to stock, easier to supply a replacement and in general just simplifies things in general. I now use Corsair VX450's which are way way overkill, and make sure my customers know they are getting a dependable supply with a great warranty. I give the IT manager of the companies I supply a spare one in case of a failure (have not had one with the Corsair yet, cannot say the same with some earlier models I used) and I am not immediately available to service the machine. I have found it just does not pay to save $20 on a supply, it WILL come back and bite you on the ass. A little customer education (and the well known Corsair name helps, I use there value ram exclusively and make sure the customer knows it) goes a long way. Substitute any good well know brand for Corsair if you like but the principle of a standard stock item is what I am trying to get across. Your time in researching costs money too, eliminate that overhead by doing it just once with each big change in technology and work to eliminate the common points of failure if you are expected to provide any kind of warranty or service after the sale.
As a counterpoint to the above, my standard office drone case is a $20 rosewill that is, by most yardsticks, a POS. However 120mm fan mounting front and back, looks decent, does the job, side vents, cases do not "fail", yada. I just tell my customers it is a computer case, don't sit on it and they appreciate the fact I did not soak them for a box that is going to be parked on the floor beside a desk and ignored if all goes well. ( The CEO or IT manager or VP gets a nice sexy looking case and upgraded keyboard and mouse for no extra charge as a "thanks for the business" )
you make very valid points Bill thanks for your input on this matter.. I am starting to get into building alot of computer for people here and there but I want reliablilty as a must but a good price also helps too...