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PSU - Amps or Watts

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September 24, 2010 3:36:03 AM

Questions for those wiser than I
I have been futzing around building systems for a couple years now. I have learned a lot but am still confused about PSU watts and Amps. I shop carefully and have read the guides on the topic here on this site. I have also read a few articles, some that are way over my head with mathematics that just make me tired.
Question; Why is a 750 watt PSU with 50 amps better than a 550 watt with 69 amps. I realize most people think more watts are better. I wonder if that is true and if it isn't why aren't amps more obviously advertised? On newegg I have to sometimes look at the picture to read the label.
I like an Antec PSU, the 550 basiq modular. It has 69 amps on 3 rails. It's hard to find that many amps on a 550. You need to get into the 750's to see that many amps.
In basic language can anyone tell me
A. should I be shopping Amps or watts?
B. Why?

More about : psu amps watts

September 24, 2010 3:41:49 AM

Watts=Volts*Amps

Since the important stuff is on the 12V rail, you take 12 and mutliply it by the amps on the rail. So 12*50=600Watts which does not equal 750W. Of course the 750W includes the 3V and 5V rails also.

12*69=828Watts. I would like to see that PSU if it's only advertised as a 550W.
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a b ) Power supply
September 24, 2010 3:48:00 AM

The watts are more obviously advertised so people making cheapo PSUs can advertise them at twice what they actually give out as the "Peak Power" rating or something.

You should be shopping for a quality PSU maker like antec, corsair, pc power and cooling, silverstone, xfx, ect first and then looking at the watts.
This will ensure that the PSUs actually put out the advertised watts.
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September 24, 2010 4:18:56 AM

The UL plate on every PSU tells the whole story.

If you install a PSU with multiple 12V rails,
pay attention to the maximum amperage
which each rail can deliver, because each
rail by itself cannot deliver the maximum
wattage rating of the entire PSU.

(This point should be obvious, if you think about it :) 

If you're not sure about the wattage needs
of each component, particularly modern video cards,
then go with a quality PSU that has a single 12V rail,
and shoot for 50% utilization of the PSU's rated wattage.

As a general rule of thumb, you should strive
to hit a stead-state average of 50% of rated capacity,
because that utilization will be at or near the
highest efficiency, on average.

duk3's advice above is very good, because it lists
several of the quality PSU manufacturers.

You would need to examine the efficiency chart
for each separate PSU, in order to determine
the utilization at which its efficiency peaks.


MRFS
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a b ) Power supply
September 24, 2010 4:26:44 AM

g048989h said:
Questions for those wiser than I
I have been futzing around building systems for a couple years now. I have learned a lot but am still confused about PSU watts and Amps. I shop carefully and have read the guides on the topic here on this site. I have also read a few articles, some that are way over my head with mathematics that just make me tired.
Question; Why is a 750 watt PSU with 50 amps better than a 550 watt with 69 amps. I realize most people think more watts are better. I wonder if that is true and if it isn't why aren't amps more obviously advertised? On newegg I have to sometimes look at the picture to read the label.
I like an Antec PSU, the 550 basiq modular. It has 69 amps on 3 rails. It's hard to find that many amps on a 550. You need to get into the 750's to see that many amps.
In basic language can anyone tell me
A. should I be shopping Amps or watts?
B. Why?

when a psu has more Then one rail you do not add up the rails rating to get the total amps you have to actually look on the label how much is available to the 12v rail.because as the formula goes v*a=w or w/a=v and w/v=a.so 69a x 12= 825 watts which is not possible
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Best solution

a c 248 ) Power supply
September 24, 2010 4:39:33 AM

Let's slow down a bit and take a closer look at the data label on the Antec Basiq 550 watt modular power supply. I know it lists three +12 volts rails. Two of them are rated at 22 amps and the third one is rated at 25 amps. That does not mean that the combined total of the three rails is 69 amps. 69 amps x 12 volts = 828 watts and that is not possible with a 550 watt power supply.

Take another look at the information further down on the data label. It clearly states that maximum load for the three +12 volt rails is 354 watts. Divide 354 watts by 12 volts and you get a grand total of 32 amps on the +12 volt rails.

Here is what I usually post when I make a recommendation:

"Corsair and Seasonic are two brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty (in the USA). Lately we've been seeing a few other brands offering some high quality units. One example would be the newer Antec models."

The veterans and regulars in this section of the forum will recognize that paragraph. I bet I've posted it several hundred times over the past couple of years.
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September 24, 2010 6:05:47 AM

alextheawesome said:
Watts=Volts*Amps

Since the important stuff is on the 12V rail, you take 12 and mutliply it by the amps on the rail. So 12*50=600Watts which does not equal 750W. Of course the 750W includes the 3V and 5V rails also.

12*69=828Watts. I would like to see that PSU if it's only advertised as a 550W.


Its listed on newegg and I have had 3 of them. Maybe I just dont know how to read the specs though. It wouldnt be the first time.

Learn more about the Antec BP550 Plus
Model
Brand Antec
Model BP550 Plus Spec
Type ATX12V V2.2
Maximum Power 550W Continuous Power
Fans 1
PFC Active
Main Connector 20+4Pin
+12V Rails 3
PCI-Express Connector 1 x 6-Pin, 1 x 6+2-Pin
SATA Power Connector 6
Modular Yes
Efficiency Up to 85%
Over Voltage Protection Yes
Input Voltage 100 - 240 V
Input Frequency Range 47 - 63 Hz
Input Current 10A @ 115V, 5A @ 230V
Output +3.3V@28A, +5V@26A, +12V1@22A, +12V2@22A, +12V3@25A, -12V@0.5A, +5VSB@2.5A
MTBF >100,000 Hours
Approvals UL, CE, TUV, CB, FCC, CCC, C-TICK
Dimensions 3.4" x 5.9" x 6.1" Features
Connectors 1 x Main connector (20+4Pin)
1 x 12V(P4)
1 x 12V(8Pin)
6 x peripheral
6 x SATA
1 x Floppy
2 x PCI-E Manufacturer Warranty
Parts 3 years limited
Labor 3 years limited
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September 24, 2010 6:17:50 AM

JohnnyLucky go through the fog. Thanks to everyone who replied. I think though that there must be a ton of confusion out there. Even with the "Experts??" writing the articles I have read. Thanks for straightening me out
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September 24, 2010 6:18:47 AM

Best answer selected by g048989h.
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a b ) Power supply
September 24, 2010 7:38:40 AM

This topic has been closed by Mousemonkey
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