Power Supply Size vs My Electric Bill

  • Thread starter Thread starter Doug Kanter
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Doug Kanter

Yet another PS question.

I'm about to replace a dead power supply. The machine in question will run
nothing but a hard disk all day, maybe with a music CD playing from time to
time. BUT....occasionally, it will be asked to burn CDs, and it'll run a
tape backup drive once a week, but that'll take a couple of hours, if past
experience is any indication. I'd like to put in a 500w PS, but (here's the
question): Is the size of the power supply directly proportional to its use
of electricity, all things being equal? In other words, if I had a machine
running nothing but a hard disk & motherboard all day, and I tried 3
different size power supplies in that machine, would they all use roughly
the same amount of power unless some other accessory demanded more?
 
Doug said:
Yet another PS question.

I'm about to replace a dead power supply. The machine in question will run
nothing but a hard disk all day, maybe with a music CD playing from time to
time. BUT....occasionally, it will be asked to burn CDs, and it'll run a
tape backup drive once a week, but that'll take a couple of hours, if past
experience is any indication. I'd like to put in a 500w PS, but (here's the
question): Is the size of the power supply directly proportional to its use
of electricity, all things being equal? In other words, if I had a machine
running nothing but a hard disk & motherboard all day, and I tried 3
different size power supplies in that machine, would they all use roughly
the same amount of power unless some other accessory demanded more?
Click to expand...

the rating of the supply is simply the maximum.

if you use a 200 watt supply or a 500 watt supply

(on the same machine performing the same tasks)

your total power consumed will be approx. the same

i doubt you will actually need a 500 watts supply
....but if you go with it , you will see NO difference on your electric bill
 
Doug said:
Yet another PS question.

I'm about to replace a dead power supply. The machine in question will run
nothing but a hard disk all day, maybe with a music CD playing from time to
time. BUT....occasionally, it will be asked to burn CDs, and it'll run a
tape backup drive once a week, but that'll take a couple of hours, if past
experience is any indication. I'd like to put in a 500w PS, but (here's the
question): Is the size of the power supply directly proportional to its use
of electricity, all things being equal? In other words, if I had a machine
running nothing but a hard disk & motherboard all day, and I tried 3
different size power supplies in that machine, would they all use roughly
the same amount of power unless some other accessory demanded more?
Click to expand...

Given a range of power supplies with the same efficiency the power
consumption will be the same and is entirely dependent upon the power
being consumed by the computer's components. The power rating of the PS
has no effect on the power that will be consumed but only represents
what the maker says the PS is capable of putting out. But even that is
far from a given because it is well known that some makers put out PSs
which either will not handle the load, become unstable when loaded, or
simply self-destruct (sometimes with interesting pyrotechnics).

A while back Tom's Hardware did a test of a range of "500W" PSs from
different makers by running them at full rated power for 24 hours
straight. More than a few failed to put out their rated power. Some
others couldn't maintain the proper voltages. Some others self
destructed. But there were a number of others went through the test with
never a whimper.
 
A bigger PSU does in fact use more than a smaller one in the same machine. I
have proven to myself without a doubt.
Heck, even my westell modem with a 1 watt difference in two power supplies
gave a different connect and of course that would be consumption given and
taken, same modem.The use of my modem as an example is just that it uses the
dc converter like a pc.
I've had systems sit dead with the power consumption *not* staying the same
with a different power supply. The power supply *owns* the system, without a
doubt. It does change the electric bill too.....
 
Yet another PS question.

I'm about to replace a dead power supply. The machine in question will run
nothing but a hard disk all day, maybe with a music CD playing from time to
time. BUT....occasionally, it will be asked to burn CDs, and it'll run a
tape backup drive once a week, but that'll take a couple of hours, if past
experience is any indication.
Click to expand...

You have not described the primary power consumers- video,
motherboard, and CPU. # of hard drives or misc other cards
are but a minor % of system power usage except for a moment
when the system is turned on.
I'd like to put in a 500w PS,
Click to expand...

I have to wonder, if the 500W is a generic? Most people
don't overspend if they think the system doesn't need it,
but when a larger wattage PSU seems cheap (like with a
generic) it can be a bad choice because of corner-cutting in
important areas like the fan or safety shutdown limits.
Your best bet is to simply buy a unit with sufficient margin
over expected needs, a quality name-brand unit. From what
you have described (thus far, except no enumeration of the
other items I mentioned above) a better target might be
around 300-400W, if that.
but (here's the
question): Is the size of the power supply directly proportional to its use
of electricity, all things being equal?
Click to expand...

No, the cost is from what the system uses, not the PSU
rating. There is a very minor efficiency loss running one
lightly loaded but not enough to worry about.
In other words, if I had a machine
running nothing but a hard disk & motherboard all day, and I tried 3
different size power supplies in that machine, would they all use roughly
the same amount of power unless some other accessory demanded more?
Click to expand...

Yes
In general the best choice would be the highest quality PSU
the budget allows with a little reserve power, not just the
most labeled watts per $ if it means getting an off-brand or
generic.
 
bgd said:
A bigger PSU does in fact use more than a smaller one in the same machine. I
have proven to myself without a doubt.
Click to expand...

Really? A 75% efficient 500 watt PSU uses more power than a 75%
efficient 400w PSU in the same computer? A 80% efficient 500w
PSU uses more power than a 75% efficient 300w PSU in the same
computer? Ya sure about that?
 
"Doug Kanter" said:
Yet another PS question.

I'm about to replace a dead power supply. The machine in question will run
nothing but a hard disk all day, maybe with a music CD playing from time to
time. BUT....occasionally, it will be asked to burn CDs, and it'll run a
tape backup drive once a week, but that'll take a couple of hours, if past
experience is any indication. I'd like to put in a 500w PS, but (here's the
question): Is the size of the power supply directly proportional to its use
of electricity, all things being equal? In other words, if I had a machine
running nothing but a hard disk & motherboard all day, and I tried 3
different size power supplies in that machine, would they all use roughly
the same amount of power unless some other accessory demanded more?
Click to expand...

Power supplies come in efficiency ratings of 68% up to about 85%.
(68% for the ones where they don't give you an efficiency rating.)
The efficiency tells you how much power is wasted to create the
DC power used by the computer.

To give you some example, say your old computer draws 120W from
this 500W supply. The efficiency can vary a bit, depending on
whether the supply is in the low, medium, or high end of its
operating range, but we'll ignore the curve for a moment.

Low efficiency supply 68% - 120W/0.68 = 176.5W ("wall power")
High efficiency supply 85% - 120W/0.85 = 141.2W ("wall power")

The high efficiency supply saved 35W, and at 0.06 per KWH, this
is 0.035*0.06*24*365 = $18.40 per year. Such a supply might
pay for itself a bit faster, if the computer was using more
of the 500W rating.

This Seasonic is an example of a high efficiency supply.
http://www.tomshardware.com/2005/07/11/stress_test/page24.html

Paul
 
Really? A 75% efficient 500 watt PSU uses more power than a 75%
efficient 400w PSU in the same computer? A 80% efficient 500w
PSU uses more power than a 75% efficient 300w PSU in the same
computer? Ya sure about that?
Click to expand...


Not quite, any PSU with a given efficiency rating, sees that
efficiency change with varing load.

Yes, even your personal fav, PC Power & Cooling, has an
efficiency change with load change.

Yes, I'm sure about that.

However, the difference is minor, typically a half-dozen %
or less... but nevertheless different.
 
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