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psu calculator question

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January 28, 2008 10:31:59 AM

Ok so say a system has these specs:

a core 2 duo @ 3.0ghz
2gb ddr2 ram
8800GT
2 Hard disks
2 dvd-rw
3x120mm fans

draws 281w with psu calculator below.
http://www.extreme.outervision.com/psucalculatorlite.jsp

Even if you put a overclocked 8800GTX it only reaches 336w.

So why do we need a psu with 500w and over recommended for such systems? I do understand though just having a 500w psu doesn't mean all of that power is useable but wouldn't a solid 400-450w psu be enough?

Like if you were to have an SLI system and depending on other components then it would draw a lot more power.

I'm not trying to saying that we don't need powerful psu's or anything I'm just trying to understand how things work.
I'd appreciate if someone could explain this to me, don't need to go in-depth.

Also could anyone care to just briefly list how much power each component takes like

processor, ram, motherboard, hard disks, optical disks and so on - how many watts each of them take or are there any good guides or articles out their worth reading to gain more knowledge.

I have found these two below which I found useful
http://www.jonnyguru.com/forums/showthread.php?t=1036
http://www.jonnyguru.com/forums/showthread.php?t=3264

Thanks
January 28, 2008 1:41:47 PM

Some PSU calculators have links to PSU manufacturers! :sarcastic: 
They work from spec sheets which may not be exact. They play safe and give a safety factor. My setup, A64 4200+ with FX7800GT etc comes to nearly 400W, but when I connect a power meter to the mains socket, running 3DMark06 gives peak draw of 260W! Go figure.

Mike.
January 28, 2008 2:11:42 PM

500 Watts is the sum of all the rails:

3.3v
5v
12v
-12v
+5vsb

You are mainly concerned about the 12v rail in a modern system.
Related resources
January 28, 2008 2:18:33 PM

Power supply calculators approximate the power usage of components.
The better way is to look at the actual label of the parts and calculate actual usage at each voltage level and then add them together.
You never buy a power supply to meet the exact usage of your components. You alway leave a safety margin, like 2-4 amps.
January 28, 2008 2:22:44 PM

You are right in the sense that people exaggerate on the size of power supplies they buy. They could get by with a lot less.
January 28, 2008 2:25:01 PM

noexpert said:
Ok so say a system has these specs:

a core 2 duo @ 3.0ghz
2gb ddr2 ram
8800GT
2 Hard disks
2 dvd-rw
3x120mm fans

draws 281w with psu calculator below.
http://www.extreme.outervision.com/psucalculatorlite.jsp

Even if you put a overclocked 8800GTX it only reaches 336w.

So why do we need a psu with 500w and over recommended for such systems? I do understand though just having a 500w psu doesn't mean all of that power is useable but wouldn't a solid 400-450w psu be enough?

Like if you were to have an SLI system and depending on other components then it would draw a lot more power.

I'm not trying to saying that we don't need powerful psu's or anything I'm just trying to understand how things work.
I'd appreciate if someone could explain this to me, don't need to go in-depth.

Also could anyone care to just briefly list how much power each component takes like

processor, ram, motherboard, hard disks, optical disks and so on - how many watts each of them take or are there any good guides or articles out their worth reading to gain more knowledge.

I have found these two below which I found useful
http://www.jonnyguru.com/forums/showthread.php?t=1036
http://www.jonnyguru.com/forums/showthread.php?t=3264

Thanks


Yes, a 450w PSU would be fine.
That is why when you read reviews of the Corsair 450w PSU, they all say that all you need is 450w.

Don't forget the CPU can be a big difference.
Dual Core vs Quad Core. 90nm vs 65nm vs 45nm.
The CPU may use 65w or it could use 130w or maybe 150w or more if OC'd.
a b ) Power supply
January 28, 2008 2:26:06 PM

PSU efficiency is best between, say, 40% load and 80% load. To minimize heat generation and electricity bills and noise you want a PSU with some serious margin of safety, so you never use it over 80%. That also leaves more room for future upgrades, of course.

Also, PSU maximum capacity drops in time (capacitor aging). Even if a PSU is good enough to start with, it's smart to allow a 15% margin for that, so it can still work at max efficiency in 3 or 5 years.

Third, PSU capability drops as temperature rises. The best PSUs (e.g. PC Power & Cooling Silencer) are rated at 50 degrees Celsius. If you buy something that doesn't mention the temperature you should allow another margin of safety for this.

So, anyway, are you actually looking for a PSU for that build, or just trying to learn? If you actually need a PSU I recommend the 550VX from www.buy.com, it's a very good price IMO.
January 28, 2008 2:51:01 PM

Personally i'd say it would require more than a 450W unit. I've not done the maths but that's a pretty powerful system. My graphics card alone 7800GS+ AGP requires 400W on it's own. An 8800GT would blow my card out the water! It's not uncommon to see some systems now coming with over 1KW!!!!!, although that's rather extreme.
January 28, 2008 2:51:05 PM

I'm just trying to learn as much as I can to do with the insides of a computer. Thank you guys thats helped clarify things.
January 28, 2008 4:36:07 PM

Quote:
My graphics card alone 7800GS+ AGP requires 400W on it's own.
:lol:  :pt1cable:  :lol: 
Pardon? Read that again! The box may say, Requires 400W PSU, that is for complete system, not just video card.
look at http://www.tomshardware.com/2007/08/21/energy-efficient...
Your card should be about 100W!

Mike.
January 28, 2008 5:12:31 PM

Also efficiency, PSUs are most effecient at ~50% load.
!