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PSU Rails - Crunching the numbers

J

joshg

Golden Member
How accurate are the "estimations" found on the table of this guide ? I'm trying to figure out exactly how much power I need to power the new upgrade that I am contemplating...

The case that I'm looking at getting includes the Antec SmartPower SL300S 300W Power supply.

According to the sticker of this unit, it has the following max:
+3.3V -> 20A
+5V -> 30A
+12V -> 15A

Now, if you use the formulas that they give you for POWER on the website guide that I posted above, you would get this:

3.3*20 = 66
5*30 = 150
12*15 = 180
Total = 396W (but isn't this "rated" as a 300W PSU ??)


OK... well, then if you use the "Estimated" Device usage table that they give you, as an estimate here would be a system that I would be putting together:

nForce2 Motherboard (Leaning heavily towards Soltek 75FRN2-RL)
Barton 2500+ @ 11x200 (or higher 😀 ) cooled by YS Tech 80mm Adjustable
2x512MB PC3200 (will probably use Corsair XMS, but might go with Bufallo Winbound modules if I'm running low on funds! :frown: )
1x40GB WD PATA100 HD [ will move to 2x40GB WD, and hopefully eventually to 2x RAPTOR ]
Lite-on 24x CDRW
Lite-on 16x DVD
ATI Radeon 9500 Pro
Floppy
Probably 2 additional fans 92-120mm range (intake/exhaust)
1xUSB Mouse
1xPS/2 Keyboard
(VERY occasionally - usually it's not even plugged in) 1xLogitech WebCam

Well, with that in mind, and using the chart supplied at the PSU selection guide, here's the "math" that I've come up with:

1x     Motherboard w/ onboard devices 3@+3.3V 2@+5V 0.3@+12V
4x High Performance fans (including power supply fans) 0@+3.3V 0@+5V 0.25@+12V
4x Memory (128MB DDR) 0@+3.3V 2@+5V 0@+12V
1x VGA 3@+3.3V 0@+5V 0@+12V
1x PCI Sound 0.5@+3.3V 0.5@+5V 0@+12V
1x PCI Network Card 0.4@+3.3V 0.4@+5V 0@+12V
2x High performance hard drive 0@+3.3V 0.8@+5V 2@+12V
1x CD-RW 0@+3.3V 1.2@+5V 0.8@+12V
1x DVD 0@+3.3V 1.2@+5V 1.1@+12V
1x Floppy 0@+3.3V 0.8@+5V 0@+12V
2x USB devices 0@+3.3V 0.5@+5V 0@+12V
1x6.9 16.95 7.2
Keyboard 0@+3.3V 0.25@+5V 0@+12V

Giving me a total of...

6.9A on +3.3V
16.95A on +5V
7.2A on +12V
POWER = 193.92W

Then, if you use their "80% more than what you need guideline", that would put me at:

12.42A on +3.3V
30.51A on +5V
12.96A on +12V
POWER = 349.056W

... on a PSU that is once again rated:

20A on +3.3V
30A on +5V
15A on +12V
POWER = 396W



Now... here is my assumption, based on this math:

This Antec SmartPower "300W" SL300S power supply should be more than enough to handle what I need, as it basically supplies enough power to supply 80% MORE power than my system will even need IF all devices are drawing their full power simultaneously (which will probably never happen...).



My question is, is this math correct? Am I missing anything? To me, and according to "common knowledge" that is passed around about hardware, it just doesn't seem "right" that a "300W" power supply would be able to handle this system. However, if my calculations are correct, then this PSU would be more than enough...

Please enlighten me if I have fell too far from the tree about this! It just doesn't seem right (and knowing my brain, it probably isn't! 😉 ).
 
got any banana nutbread..?the SL300S 300W Power supply should suffice but you are going to have naysayers comin' thru this thread...pimpin' Tru-Power supplies only with bigger numbers..🙂.
 
Excellent!

Also, just a note: in my calculation, I assumed that 2x512MB would draw about the same amount of power as 4x128MB. I could be wayyy off, and instead this might just be equal to 2x ... but then again, I could be in err on the other side, in which case I would have to recalculate. Does anyone know the power draw difference between 1x128MB DDR and 1x512MB DDR ?

Also, assuming that everything is correct, I should hopefully have enough room to easily add another HD or a few PCI cards if I so desire to do so...
 
Here's how to think about PSU ratings. A 300W PSU can put out 300W among all lines, all lines having loads. Within that 300 Watts overall limit, no one line can exceed the separate rail rating. IOW, if one rail nears its max, the others must be drawing respectively lower to keep the total within the 300W rating.
. The PSU would (should) shut itself down well before the max separate ratings could be achieved simultaneously. Comprende?
.
bh.
 
Ok, I understand now.

Also...

Giving me an actual total of...

6.9A on +3.3V
16.95A on +5V
7.2A on +12V
POWER = 193.92W

Then, if you use their "80% more than what you need guideline", that would put me at:

12.42A on +3.3V
30.51A on +5V
12.96A on +12V
POWER = 349.056W
Click to expand...

Do you think I'd be ok on that PSU that's rated at "300W" :

20A on +3.3V
30A on +5V
15A on +12V
POWER = 396W (ed: but apparently "caps out" at 300W simultaneous draw)
Click to expand...

... all keeping in mind that I will probably never be in a situation where every piece of my hardware will be drawing full power simultaneously.

??
 
... wow I must be an idiot! I just realized that I completely left out CPU in those calcs. 🙁

Anyway, apparently the new chips take all power from the +12V, this would put "actual if all components were being used" to ~14.32A on the +12V, which is VERY close to the limit of 15A.

This is bad, right? 😉
 
Don't forget that when you turn the computer on, everything is taking power. Also, the hard drives draw more because they have to spin up
 
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