Does a larger Power Supply use less electricity?

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Ran across this thread at Tom's Hardware Forum.
Is the following advice true?

"Get the 750w PC P&C its the same price but a better supply, after
the mail in rebate its cheaper. There is no reason not to do this.
It won't use more power just because its bigger, in fact it will
probably use power more efficiently and give you lower power bills."

Re: Does a larger Power Supply use less electricity?

Steve wrote:

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The first part, yes. The second part, maybe. It depends on the design
of the two supplies.

Re: Does a larger Power Supply use less electricity?

Steve wrote:
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A more efficient supply will waste less power for a given power output but
this has nothing to do with the "size". There is generally a range of
efficiencies that a given design will provide and the efficiency will vary
wildly depending upon how much power the supply is putting out -- it has a
preferred power output and will be less efficient when putting out more or
less. In fact running a power supply far under its rated power is _very_
inefficient. Running it at about half of its rated power seems to be best.
Here is an example which shows some curves from a real modern supply:

John McGaw
[Knoxville, TN, USA]

Re: Does a larger Power Supply use less electricity?

Steve wrote:
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The number quoted (the 750W) is the maximum capacity. With
modern low end computers, the consumption is 100W or less.
So we start with that consumption number.

A really inefficient supply (industry standard not that many
years ago), would be 68% efficient. When you draw 100W from
the DC outputs, the AC power drawn from the wall is

   ------  =  147W  ,     147-100 = 47W heat dumped by PSU

If I switch to an 80% efficient supply, that changes to

   ------  =  125W  ,     125-100 = 25W heat dumped by PSU

And if I managed to find an 85% supply, that actually
worked at 85% when only 100W was drawn

   ------  =  118W  ,     118-100 = 18W heat dumped by PSU

The same rules would apply, as the supply gradually has
more load applied to the DC outputs. You do the math,
and work out the heat kicked out. It will be in proportion.
It becomes really important at high load. If you had a
750W supply which was 68% efficient, the thing would be
a sauna inside, require a really high fan speed, and the
supply probably wouldn't last that long.

The 750W is the maximum load that can be connected. The
efficiency determines the extent of the waste heat.
In the first example above, 147W comes from the power
company, 100W goes to the computer guts, and 47W pours
out of the power supply itself. When you buy a more
efficient PSU, the PSU runs cooler, and doesn't need
aggressive internal cooling.


Re: Does a larger Power Supply use less electricity?

Steve wrote:
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If you're running one system, I'd not worry about it.
Variations in individual designs will swamp any differences due
to max capacity only.

If you're running a Google server farm, you'd do well to make
measurements yourself on the EXACT supplies you're considering.

Re: Does a larger Power Supply use less electricity?

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To a degree, that's true.  Let's say your PC's total power
requirement is 450 watts.  You have a choice of two PSUs from
the same model line.  One is a 550 watt unit, and the other is
rated at 700 watts.  The 500 watt unit would be running close
to saturation, but the 700 watt one would working in its most
efficient range.  The end effect is that the larger PSU would
run a bit more efficiently, and definitely cooler, which would
put less stress on its components, resulting in higher
reliability, assuming a quality brand.

Re power savings, you probably wouldn't see much of a
difference in power bills between the two, unless your power
rates are very high.

Re: Does a larger Power Supply use less electricity?


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You're best off totally ignoring Tom's Hardware forum, about
(maybe 18?) months ago, the moderators got in petty quarrels
and started banning people, most of the better informed
participants left and the rest leave too high a bad:good
information ratio.

Quoted text here. Click to load it

Whether it's a "better supply" depends on what it's being
compared to, and the actual needs of the system.  Suppose a
typical consumer PC, not some monster overclocked SLI'd
gaming system.  It might use about 175W if not less.  In
that case, a 750W PC P&C PSU would be overkill, the question
wouldn't be one of whether it's the same price as something
else, but why one would pay the premium for either when such
a system will work fine off of one costing less.

In the case of a more power hungry system, it would depend a
bit on just how close the system is to the actual capacity
of each PSU, momentarily ignoring that two PSU rated for the
same capacity may have differences in build quality that
effect their lifespan.  Generally PC P&C PSU are long lived,
much better than average though many other modern designs
are good in that regard as well.

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"Probably use power more efficiently" again has to do with
what it's being compared against.  If that is important, one
can select something else that is also 80%, or 85%...
whatever it's efficiency rating is, and expect a similar
eough result.

However, if a high capacity PSU is used with a system using
significantly less power than it's rated for, like the
aforementioned < 175W system running off a typical high
efficiency 750W PSU, that PSU will not be as efficient at
such a low load, a lower capacity PSU with the same
efficiency rating will, on average, be more efficient at the
level used by the system.

Oversized PSU have something going for them - ample margins
contribute to longer life, allow upgrading to practically
whatever you might want in the future, and tend to be built
with higher quality parts that lower capacity PSU would've
benefitted from as well.

The choice depends mostly on what the other PSU alternative
was, how much power the system actually uses, and how
important achieving a small difference in efficiency is.
Someone with a high powered system can't be terribly
concerned about efficiency in the first place else they'd
have built something not so high powered.

Re: Does a larger Power Supply use less electricity?

Somewhere on teh intarwebs kony wrote:
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I wish that the PC P&C PSU Silencer 750 Quad was available in New Zealand. I
did extensive research a while back when I needed a decent PSU but, sadly
for me, most of the reviews are Ameri-centric and only feature supplies
available there. We have a whole different product range here, being closer
to Asia.

Check this out:

In particular this page:

Shows that it's very efficient at lower loads. In fact 88.8% at 20% of rated
wattage going up to it's best figure, 89.1% at 40%.

Another thing that I really liked about this PSU is that it's the only one
I've seen tested that has one, single 12V rail. At first that might not seem
so good but when you consider that, with the 750W PSU I'm using, with three
12V-18A rails, I had to trace the wires as only one of the two supplied
graphics card connectors was on the third rail (the other was tacked onto
the rail that served the drives/molex connectors) you'll see the value of
having a single 12V rail.

Also, the same supply reviewed by the other most reliable PSU review site

A couple quick quotes from the last page of the review: "The efficiency is
good ...." and "Over all, the PC Power & Cooling Silencer 750W is a great
power supply." Jonny gave it a 9 out of 10.

As the efficeincy is so good with these supplies, if I could get them I'd
have them in all of my builds. During my research they stood out as pretty
much the gold standard amongst power supplies. It has a three-year warranty
too (but I would expect it to last a lot longer than that, especially if
only lightly stressed).

If people were to bookmark hardwaresecrets and jonnyguru (perhaps along with ) we'd never see
another PSU questions here again.


"Build a man a fire, and he`ll be warm for a day. Set a man on fire, and
he`ll be warm for the rest of his life." Terry Pratchett, Jingo

Re: Does a larger Power Supply use less electricity?

~misfit~ wrote:

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Except when the contract supplier of the power supply changes.

For example, one Antec product changed from CWT to Delta, and
the Antec model number probably didn't change as a result. The
reviews on Jonnyguru are only good for as long as the
original contract supplier is still making the same product.
Antec doesn't make their own supplies, and they can ship
whatever suits them.

I do appreciate the supplies which have one output rail,
because it reduces the amount of research needed to use
them. How many manufacturers provide a wiring diagram, so
you can plan your build on a quad rail supply ? Expecting
each customer to open the PSU, trace the wiring, to figure
out what rail their GTX280 is going to be running from,
is just stupid. If they're going to make true quad
rails, or current limited type designs, they should be
properly documented. Most times, when I do find the
details on the wiring, I'm disappointed with the
choices they've made. So for that reason, the PCPower
approach is a winner.

But consider what happens some day, when a component fails
on the 12V rail, and there is 60A to blow it away. It's
eventually going to happen. If you get a partial short in a PCB,
and the total load is 59A, there could be a glowing portion
of PCB, melted plastic on the wires and so on. And a good
deal of smoke and smoke damage. The same could happen at
the 20A level, on a current limited supply, but there
the power involved is a bit less.


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