I picked up a 550w power supply for my computer, but there is no rating
on the PSU for individual rail wattage, only combined. I was wondering
if I'm calculate the wattage correctly, given these system specs:

550w total

+5V @ 45A
+3.3V @ 30A

+12V @ 20A
-5V @ 0.5A
-12V @ 0.8A
+5Vsb @ 2A

=(5*45)+(3*30) gives me 315 watts, which is a big leap over my 350w
supply's 175 watts, but I'm not sure if that's the proper way to
calculate it. The K7S5A FAQ says I should have 185 watts for stability.

Just for reference, it's a Real PC Power SilenceBlue PSU with a three
year warranty, and cost $34 CAD. It has two fans and blue LEDs inside!

http://www.keyco.ca/phase2/pages/realpcblue.htm

"Dekaritae" <maur (AT) mdi (DOT) ca> wrote

Since power (in W) = voltage (in V) x current (in A), I think you did your
calculation correctly if you are only computing the power rating for the 5V
and the 3V rails combined. Unless you are not using any 12V devices (desktop
HDs, CD ROM drives, etc), you should also include the 12V rail in your
computation. Why did you leave out the other rails, especially the 12V rail?

Just a reminder, these numbers specify maximums.

Al-U

alpha_uma wrote:

The 175 watts I mentioned regarding my current PSU is only from the 3.3v
and 5.0v rails combined. The K7S5A FAQ on the forums specified that the
185 watts needed for stability is the wattage of just those rails.


Aye, and it's not likely the PSU could put out 315 (actually, 324) watts
at any one time. I'm not sure how to calculate the maximum that it
could put out, though, to compare it with my current PSU. There are no
ratings listed on the PSU, and I can't find a web site for the company.

"Dekaritae" <maur (AT) mdi (DOT) ca> wrote

<snip>
I think I misread what you wrote and computed wrong in my previous post. If your new PSU specifies a combined rated wattage for 3.3V/5V, then it would be a mistake to add in the two SEPARATE rated wattages of the 5V rail and the 3.3V rail.

I re-read your original post more carefully. If your new PSU does not give separate wattage rating for each of the 3.3V and 5V rails, then how did you obtain these two separate (V @ A) pairs that you posted earlier (+5V @ 45A), and (+3.3V @ 30A)?

Also, you said in your previous post that your new PSU only gives you a rated wattage for the 3.3V and 5V rails COMBINED. So, what is that combined rating? Is it something like the following?

3.3V/5V Limit: 225W

What is this number for your NEW PSU?

My understanding is that when computing the maximum total wattage of your PSU, you should use the 3.5V/5V combined rating but ignore the two individual ratings for those two separate rails. Then you also add in the ratings from the remaining rails: -12V, -5V and -5V standby.

To answer your question again, let X denote the combined rating for the 3.3V/5V. Note: X should be printed right on the PSU, not to be computed from other numnbers.

3.3V/5V : X W
+12V @ 20A : 240W
- 5V @ 0.5A: 2.5W
-12V @ 0.8A: 9.6W
+5Vsb @ 2A : 10W

Then, the estimated max total = X + 240 + 2.5 + 9.6 + 10.

Indeed, the so-called "combined" rating should be interpreted as "beware of our fine print". PCGuide has some good tips and examples on how to interpret power supply ratings and specifically how to use the combined rating correctly. A good place to start is here on this page:

http://www.pcguide.com/ref/power/sup/output.htm

The page that is most relevant for your question is probably the "next" page right after the above link. Or start from the beginning of the section on power supply here:

http://www.pcguide.com/ref/power/sup/index.htm

I think I did the math correctly this time.
Al-U

alpha_uma wrote:

The 550w PSU lists the amperage for each of those rails seperately.


That's what I'm trying to figure out, actually. The rated wattage for
those rails on my 350w PSU is 175w, but this new 550w PSU doesn't have
the rating for those listed anywhere on it.


AH, there's the problem then. I was thinking I could compute X based on
the listed numbers, as it is not listed on the PSU itself, or on the
company's "web site".

http://www.realpcpower.com/


Thank you, that looks like it will be useful in the future.

I think that whatever the combined rating on this supply is, it is
enough for what I have as after installing it, Windows is stable at
133/133 and at 147/147. It runs a bit hot at the latter, but I don't
really have heavy CPU needs so 133 is fine.