How many watts needed in a power-supply?

Do you have a question? Post it now! No Registration Necessary.  Now with pictures!

Threaded View

The other day, I built a new machine.  I happened to notice that the
case (a mid-sized/standard-size case) came with a 450-watt power
supply.  (Probably a 'cheap' one.)

When I looked on the shelf (idly curious), most all the separate
replacement power-supplies were mostly in the 360 watt range.

Lastly, the other day, I took a 'tech exam', and one of the questions
listed an exact config (e.g. P4, certain make/model of disk, etc, etc)
and then asked as a multiple choice, which size power-supply
should you select (when building that system).

Ok.  So, now I'm quite curious!

I carry 'spare' replacement 360-watt power supplies around as a
backup repair technician.  (I've installed 2 or 3 of these 360-watt
supplies into mid-sized machines that had bad power-supplies and
they 'work'.)

My questions are:

(1) Am I relatively safe using these 360-watt supplies (or should
be learning how to 'calculate' a more precise replacement supply
for each individual machine?

(2) Can someone point me to a formula/website or whatever that
gives guidelines on choosing wattage for a power supply?



Re: How many watts needed in a power-supply?

On Thu, 7 Apr 2005 11:48:59 -0400, "David Cook"

Quoted text here. Click to load it

If only wattage ratings on generics were accurate these
figures could be used in some useful way... since generics
typically aren't accurately rated, you may as well ignore
the numbers except "maybe" to compare to another unit in the
same manufacturer's model line... and sadly enough, even
then some may merely have a larger fuse.

Quoted text here. Click to load it

I would seriously question the competence of whoever wrote
the exam.  Multiple choice where only the wattage of entire
unit are listed is not a reasonable way to determine a
right/wrong answer... the choice cannot be intelligently
made without more info.  The correct answer would be to
write onto the test space that "It is negligent to choose
any of them based only on the info provided".

Quoted text here. Click to load it

"calculate" "precise" is not a realistic goal, since you
won't even know the actual power useage of a particular
component unless you measure it yourself.  Manufacturers
provide maximum ratings, ratings that are not observed by
anyone when building because it's not possible to create a
sustained maximum current condition on a majority of parts

As important may be what I briefly mentioned above, that you
must first know the true output capabilities of that "360W"
power supply.  360W are somewhat rare too, are they
Enlight/Sirtec/Highpower/etc?  I ask because those have a
power switch in the back, IIRC, and "random, most-compatible
spares" shouldn't have a power switch on them as many OEM
cases aren't compatible with (rear switch).

Quoted text here. Click to load it

You can Google for 'em, and be no better off than you
started.  Regardless, this search does link the most popular

The actual needs are more of an estimation plus some reserve
power, even more reserve for a larger case with more
expansion capabilities.  The goal is not to perfectly fit a
system that uses 187.3W with a ~ 190W PSU, but to determine
where the power rail distribution is such that you know the
current needed on the most heavily loaded rails, which in
the case of a P4 based system is usually the 12V rail, but
often still a fair amount on 5V for motherboard and other
uses.  Contrast that with a P3, older or median aged Athlon
system which needs more 5V amps.   Add more amps as
appropriate for a high-powered video card.  

So primarily you need a PSU with true rating higher than
hungriest system voltage rail.  Then consider the 2nd most
hungry rail.  Rarely are any other rail considerations
necessary, except maybe with a very low-powered system that
has a ton of USB devices and needs them powered in standby
mode.  Online calculators might give you figures like 12A
12V for CPU, drives and fans, 20A 5V for motherboard and
misc... and still the primary issue is whether the PSU is
accurately rated.

Sadly unless one has experience with a particular PSU and
where it's limitations are, the quickest thing to do is
overspec the power supply...  and it's not a bad idea if the
alternative might be one marginal for the system, since the
next-higher wattage typically costs only a little bit more.
If you plan to use one line or a few models you might
consider doing some output and load testing, or have someone
who you feel is qualified to assess them for you.  Once you
have your preferred brand at a good price, stick with it
else repeat whole process to qualify new/alternatives.

Re: How many watts needed in a power-supply?


Quoted text here. Click to load it

Ive seen a few but I saw this link to one that was pretty easy. Some
of them can be sort of vague about some components /

Mine came out close to 360-370 so its close to my 400 watt PS.
My 12 volt rail is kind of low too 11.65-11.71.   And Im suddenly
plagued with random reboots every 1-2 days.  The intervals  are long
enough that Ive done memtest 3 times already for a day or overnight
but I dont really feel like doing it for several days. Im hoping its a
bios problem as all my problems happened after upgrading to the
latest. I thought they were driver problems and they still might be
but Ive changed them and the problem is still there though its

Anyway Ive seen emachines with 2000 athlon XP , dvd burner and cd
drive  and a 80 meg HD - ECS built in video, sound --- one of those
dinky microatx cheapie boards with a 250 watt. Many emachines and
super cheapo small bargain systems with everhthing builtin seem to
come with low powered PSes.

Around here I think the standard now at stores is 400watts. They may
be crappy PSes but they tend to all be 400 watts nowadays and the
lesser ones are AT PSes for older systems. The  few exceptions are a
few at Compusa , a low end Maddog 350 and I think they have an Antec
for 380 watts or so.  People tend to like that round number 400 watts
I think so its generally 400 or over.

Re: How many watts needed in a power-supply?

Quoted text here. Click to load it

Thanks, that link to the 'power supply calculator' really puts things in

Based on running a few numbers,  my practice of using a 360-watt
supply seems to be plenty.



Site Timeline