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Why do PSU calculator lie? Help!

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  • Power Supplies
  • Components
  • Product
Last response: in Components
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January 15, 2010 3:31:41 PM

I've had a lot of problems with computers over the years, but I never imagined anything quite as hard as picking a correct PSU!

I ordered some parts for a new computer. I was hoping to make it a cheap and very energy efficient machine with a killer GPU. Here it is:

AMD Phenom II X4 945 Deneb 3.0GHz 95W Quad (socket AM3)
ASUS M4A78T-E AM3 AMD 790GX HDMI ATX AMD Motherboard
G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666)
Sound Blaster X-Fi Xtreme (PCI sound card)
Run of the mill DVD-RW (SATA) and 750GB HD (also SATA)
and a VisionTek 900297 Radeon HD 5850 1GB 256-bit GDDR5 PCI Express 2.0 with one 80mm case fan and 4 USB devices (mouse,keyboard, scanner, and webcam)

So, idiot that was, I went to Thermaltake's PSU calculator to find out what I should get for a psu. I typed in everything above, and it said the max wattage draw should be around 314w



Well, excellent, I think to myself, I'll go to newegg and get something. I get a 380 Watt EarthWatts unit. I look on Wikipedia and see that the ATI 5850 uses as max of 151 Watts, with a 95 Watt CPU, that makes sense right? And I think to myself, there's over 70 watts of wiggle room, so I should be good!

Well, after I order the PSU I learn that you have to know about 'rails'. I look at the back of my PSU and it says this:

I read that the 12 V rail is the most important for graphics cards. If this is true, then PSU gives 17 Amps on one of the 12 rail, and that's 17 x 12= 204 Watts. That should more than cover the ATI 5850 which needs a max of 151 W right?

I need help! Can this PSU run my computer setup as is??? What do all those numbers on the back of my PSU mean???

Thanks!!

More about : psu calculator lie

a b ) Power supply
January 15, 2010 3:39:44 PM

did you compensate for 25% capacitor agin,g add 5 fans, and set load to 100%?
on the calculator it gives me 550 watts with those changes
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January 15, 2010 3:55:07 PM

Hi Obsidian86

Thanks for the quick reply!

In response to your questions:

(1) I used the recommended 90% load which is what Thermaltake recommended.

(2) I also used the default capacitor aging, which was 10%

(3) Like I said, I only have one fan, and I don't think I need 5!

Even when I do all that you suggested (minus the fans), I get a recommended power of 375W. Here's another snapshot:

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a c 243 ) Power supply
January 15, 2010 3:58:30 PM

iloveToms said:

(3) Like I said, I only have one fan, and I don't think I need 5!

You may not need 5 , but 1 x 80mm isn't going to provide much airflow to help cool your components.

PS, 5850 only uses around 156 watts when stressed with something like Furmark, under normal use it's more like 105w.
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a c 131 ) Power supply
January 15, 2010 4:02:29 PM

Using http://www.extreme.outervision.com/PSUEngine, at recommended settings on aging and load, and adding just a couple of 120mm fans, I get 382W. I'd want a little wiggle room, so 500W seems reasonable. BUT...
There are a lot of crappy PSUs out there, and these calculators somewhat account for that. It is entirely possible that the EA-380D, a quality PSU, would run your rig. You'd have little headroom, but in this case it would probably work. If the option is available to you, it would still probably be a good idea to get something like the 500W Neo Eco.
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January 15, 2010 4:02:43 PM

Quote:
You may not need 5 , but 1 x 80mm isn't going to provide much airflow to help cool your components.


I'm using an old 8600 GT I had lying around and my CPU/MB temps never get above 35C. So far, I see no need to buy an extra 4 fans!

I bought my components with energy efficiency in mind. The 95 W CPU does really well and doesn't give off a lot of heat

Let me know what you guys think about my current PSU! Thanks
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Best solution

a b ) Power supply
January 15, 2010 4:02:45 PM

The Calculator didn't lie, you just have to make sure you input the correct values and understand what you are reading. As for now, the PSU might run your system, but it will be pushing it, and I wouldn't do it myself. I prefer to have a little more room than that. In the rare case that you actually have everything on your PC running at 100%, you'll want it to run without worrying about the PSU. I would RMA the PSU and get something in the 500-600W PSU. When I set it to 100% use on everything with 25% cap aging and all the things you listed, I get around ~450W (not sure how obsidian86 gets 550...). I even added extra fans and an extra HD and didn't get over 500W, so you should be safe with something in that range. You might even be able to go 450W if you really wanted, but I wouldn't feel comfortable with that personally.
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January 15, 2010 4:04:21 PM

You also should consider that PSUs are more reliable and stable when you don't push them to their max.  So, as obsidian pointed out, it'd probably be best to use your system's max settings on the calculator, and then add some.  So, for a 550W max calculation, be safe and go with a decent 650W, which will not make the PSU work as hard to supply the system, plus leaves you room for SLI/Crossfire or some other hard to supply the system, plus leaves you room for SLI/Crossfire or some other expansion in the future.

Hope my first post was helpful...been following these forums for a while.  :hello: 
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a b ) Power supply
January 15, 2010 4:04:25 PM

i used the antec calculator
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a c 243 ) Power supply
January 15, 2010 4:06:45 PM

Antec calc., Tt calc., Newegg calc. are all just versions of the one jtt283 posted, they will all give the same result.
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January 15, 2010 4:07:47 PM

flyinfinni said:
The Calculator didn't lie, you just have to make sure you input the correct values and understand what you are reading. As for now, the PSU might run your system, but it will be pushing it, and I wouldn't do it myself. I prefer to have a little more room than that. In the rare case that you actually have everything on your PC running at 100%, you'll want it to run without worrying about the PSU. I would RMA the PSU and get something in the 500-600W PSU.


Thanks for everyone's recommendations!

But what about all this 'rail' stuff? What numbers should be looking at on the back of the PSU to figure out if it'll run my system? Where does the power to the PCI express hook up come from (The 12V, etc)?

Thanks, I'm trying my best to understand this better so I don't have to keep bothering you guys :) 
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a b ) Power supply
January 15, 2010 4:09:13 PM

obsidian86 said:
i used the antec calculator

I haven't used that one, but I generally try to use ones that are not tied to actual PSU manufacturers since I feel like they want you to get the biggest PSU out there and would rather push you a little overkill than letting you get one slightly under-powered (seems to fit the case as its 100W higher than what I'm getting here: http://extreme.outervision.com/PSUEngine)
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a b ) Power supply
January 15, 2010 4:11:23 PM

iloveToms said:
Thanks for everyone's recommendations!

But what about all this 'rail' stuff? What numbers should be looking at on the back of the PSU to figure out if it'll run my system? Where does the power to the PCI express hook up come from (The 12V, etc)?

Thanks, I'm trying my best to understand this better so I don't have to keep bothering you guys :) 


Your PCI-express power will come off of the 12V rails, as will the hard drives, etc. A significant amount of your system comes off the 12V rails.
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a c 243 ) Power supply
January 15, 2010 4:12:44 PM

iloveToms said:
Quote:
You may not need 5 , but 1 x 80mm isn't going to provide much airflow to help cool your components.


I'm using an old 8600 GT I had lying around and my CPU/MB temps never get above 35C. So far, I see no need to buy an extra 4 fans!

I bought my components with energy efficiency in mind. The 95 W CPU does really well and doesn't give off a lot of heat

Let me know what you guys think about my current PSU! Thanks

A 5850 is going to put out more heat than an 8600 GT.
No matter how much heat the cpu give's off without proper airflow you're going to have problem's.
Don't know what to think of your psu, all you have provided is a picture of a label with rail specs, that tells us next to nothing about it.
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January 15, 2010 4:16:54 PM

delluser1 said:
Don't know what to think of your psu, all you have provided is a picture of a label with rail specs, that tells us next to nothing about it.


Hi Delluser,

Hm, I posted that it was an Earth Watts 380 W model, and I posted a link to the product on Newegg, as well as voltage stuff on the back of it. Surely that's enough info, or no? Tell me what's important and I'll post it!

Thanks again
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a c 248 ) Power supply
January 15, 2010 4:23:53 PM

The official ATI recommendation for a system with one ATI HD Radeon 5850 video card is a 500 watt or greater power supply. That is for the entire system.

The general rule of thumb is a high quality 500 to 550 watt power supply with sufficient current (amps) on the +12 volt rail(s) can easily power a system with any single video card made. A high quality 700 to 750 watt power supply with sufficient current (amps) on the +12 volt rail(s) can power a system with two video cards operating in dual mode. There are a few exceptions like the new ATI Radeon HD 5XXX series cards which use less power due to their energy efficiency.

A high quality 500 to 550 watt psu will have a +12 volt rail rated at 40 amps. A high quality 700 to 750 watt psu will have a +12 volt rail rated at 60 amps.

In addition the power supply should be at least 80+ Bronze certified for energy efficiency. There are some models available which have achieved 80+ Silver and 80+ Gold Certifications.

Corsair, PC Power & Cooling, and Seasonic are some of the 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. Lately we've been seeing a few other brands offering some high quality units. One example would be the Antec Earthwatts series which is a major improvement over Antec’s older psu’s like the Basiq models.

For a system with just one HD 5850 video card the Corsair VX550 power supply would be an appropriate choice.

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a c 243 ) Power supply
January 15, 2010 4:25:05 PM

iloveToms said:
Hi Delluser,

Hm, I posted that it was an Earth Watts 380 W model, and I posted a link to the product on Newegg, as well as voltage stuff on the back of it. Surely that's enough info, or no? Tell me what's important and I'll post it!

Thanks again

My bad, sorry.
The EA 380 should be fine for your system.
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January 15, 2010 4:31:47 PM

JohnnyLucky said:
The official ATI recommendation for a system with one ATI HD Radeon 5850 video card is a 500 watt or greater power supply. That is for the entire system.
A high quality 500 to 550 watt psu will have a +12 volt rail rated at 40 amps.


Hi,

Thanks for your comments. I guess I don't understand why I should take ATI recommendations because system to system, there is a LOT of variation! So, I for example have NO interest in ever purchasing two video cards for SLI/Crossfire, whatever name they come up with next. I only want one video card, and I don't see why I could take the recommended minimum (which is already a worst case scenario when everything's at 100% with capacitor aging), and then add 200 Watts. That seems crazy!

I think I'll go up to 430 or 450 watts (80 bronze/silver certified of course) based off what people have been suggesting. As others have said, 500 watts is a little extreme and seems unnecessary with my use (heavy gaming on weekends/breaks, no SLI/LEDs or any fancy mod work).

The information on the back of my PSU is still a big mystery to me. I feel like everyone talks about how important "rails are", that watts really means nothing, but then I never get a good explanation as to what it all means. If anyone can help with that, I'd dying to know

Thanks again guys!

EDIT:

Like here's a quesiton: if the 12 v rails power the PCI express, harddrives, dvdroms, etc, then do the 3.3V and 5V rails only power the CPU? Why does the Earth Watt 380W described above have two 12V rails, what does that mean? Cheers lads
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a c 248 ) Power supply
January 15, 2010 4:52:34 PM

The +12 volt rail supplies power to the cpu, the video card, and other components. The +12 volt rail accounts for about 75% of the power delivered to a pc system.

There is a limit to how much power the +12 volt rail can deliver. A 400 watt power supply cannot normally deliver 400 watts on the +12 volt rail. The middle of the fourth line on a power supply data label will show the maximum wattage for the +12 volt rail.

If you are interested you also might want to factor in energy efficiency. Up until recently power supplies were most efficient at delivering power at about 50 to 60% of total capacity. A 400 watt power supply would operate most efficiently at about 200 to 225 watts. Things have improved so that modern power supplies are now able to operate efficiently over a broader operating range.

Just in case there is a misunderstanding - A power supply will only draw the power that is required to operate. It will not draw excess power. If you have a 400 watt power supply but only require 300 watts, then the psu will only draw 300 watts.

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January 15, 2010 5:01:53 PM

JohnnyLucky said:
The +12 volt rail supplies power to the cpu, the video card, and other components. The +12 volt rail accounts for about 75% of the power delivered to a pc system


Thanks again for your comments!!

Yes, I am very excited about the 80+ certification, I got the Earth Watts 380Watts because it is the most efficient PSU I could find in my watt range

So what does the 3.3V and 5V rails do??? It sounds like I should ONLY look at the 12V when buying a PSU. And why are there two 12V rails? Should I only look at one, or both combined?

Regarding your comments about PSUs only taking what they need, I totally understand what you're saying. But, from what I've read, the closer to total maximum that you are with a PSU the better the efficiency! And the lower you go, the less efficient you'll be. So, yeah, I (and everyone else) could go ahead and buy a 80+ certified bronze 1200Watt unit just to be safe, but our efficiency would be horrible! I want to try to get 86-7% efficiency most of the time, and from what I understand I should go lower rather than higher

Does what I'm saying make sense? Again, I get all this stuff by reading review articles from this website and others, but I know you can't trust a lot of what you read on the internet :) 
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January 15, 2010 5:15:58 PM

iloveToms said:
Thanks for everyone's recommendations!

But what about all this 'rail' stuff? What numbers should be looking at on the back of the PSU to figure out if it'll run my system? Where does the power to the PCI express hook up come from (The 12V, etc)?

Thanks, I'm trying my best to understand this better so I don't have to keep bothering you guys :) 



If you're that confused about rails and amperage, I use the eXtreme calculator

For a couple dollars you can use the Pro version which lets you calculate amperage per rail.

And yes, the 12V rail(s) is almost all you should be considering amp-wise.
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a c 144 ) Power supply
January 15, 2010 5:33:50 PM

That PSU looks like it can provide 32 amps at 12 volts. Look at the second line. Total 12 volt current is 28 amps.

In the modern computers the 12 volt output(s) do the heavy lifting. The 5 volt line powers motherboard and drive logic and the 3.3 volt output powers the memory. So any output current over 20 amps there is wasted PSU capacity.

Basically, any PSU powerful enough to provide all the 12 volt current you need will be powerful enough to provide the 3.3 and 5 volt outputs.
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a c 248 ) Power supply
January 15, 2010 6:31:24 PM

In a modern psu the 5.0 volt rail supplies power to USB, PS/2, and PCI devices. It also supplies power to storage devices such as optical drives, cd drives, hard drives, and floppy drives.

The 3.0 volt rail supplies power to some of the motherboard components, logic chipsets, and memory.

The concept of mutiple +12 volt rails is a royal pain because the marketing guru's used the original concept to create an advertising gimmick that fooled a lot of uninformed customers. There was a big stink about it a few years ago. Originally the use of genuine mutiple rails was meant for very high end, high wattage power supplies that delivered power to four video cards in Quad SLI mode. Mutiple +12 volt rails are not intended for low wattage psu's powering a single video card. I'll let it go at that.
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January 15, 2010 7:13:10 PM

Wow, great replies!

Senoroll: I did see that I could pay $3 for the privilege of seeing what 12V 'rails' I needed, but I thought that was a bit of ripoff. Maybe I'm spoiled by the internet, but I feel like that quote should be free....like if I asked Sears what sort of refrigerator I need, they wouldn't charge me for letting me know what would fit my family, right? I'm surprised that Thermaltech, Antec, etc. don't provide that sort of information for free

JSC: Thanks for the info! So I guess I'll ignore the 3.3 and 5.V rails and just look at the 12V rails (combined). That's great information, thanks again

JohnnyLucky: That too is great info, although I'm still not sure I get the point of having multiple rails, but I accept that some of this stuff is just way too technical for my understanding :) 


So, based off what people said, I have formed new ideas about PSUs. Please let me know if any of these points is wrong as I'm really trying to learn here:

(1) only the 12v rails matter. Add both 12V rails to get the total Amps your PSU is capable of
(2) Ignore 3.3 and 5.5 V rails
(3) Capacitor aging is a big deal, use 20% when estimating PSU power
(4) Always buy 80+ certified PSUs

Thanks again everyone for your input!
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January 15, 2010 7:24:38 PM

Teh numbers behind your PSU tell you the AMPs every single line or rail can give out any given moment. And thus the maximum theoretical wattage (amps * volts). As you can see there is also another line below that tells you the maximum load you can get.
For example the label tells you you have 2*12V rails, other one 12V*17A=204W and 12V*15A=180W but in the label it totals to 336W under maximum load, while the total should be 384W. This is because the power supplies are designed to share some components among rails and the maximum values only apply when you would have only one rail giving out maximum, so the manufacturer of the PSU has given the corrected maximum output value of 336W for the 12V rails.

iloveToms said:
Thanks for everyone's recommendations!

But what about all this 'rail' stuff? What numbers should be looking at on the back of the PSU to figure out if it'll run my system? Where does the power to the PCI express hook up come from (The 12V, etc)?

Thanks, I'm trying my best to understand this better so I don't have to keep bothering you guys :) 


The most power hungry components (CPUs,GPUs, harddrives...) use the 12V rails so it is the most important value. The ATX12V specifications state that any given wire/rail can give only 240W output. So when more powerful computers with more power hungry graphic cards, processors, etc.. would need more power than any single line can give, the manufacturers added more rails to power supplies in order to balance and increase the maximum amount of power output.

As for the efficiency rates on power supplies the optimal load/sweet spot for maximising the efficiency varies depending on the power supply, but usually it is around 50-60% of the maximum load.

Here is a good article if you want to educate yourself more about power supplies. Hope this helps :) 
http://www.hardwaresecrets.com/article/181/1
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January 15, 2010 9:52:29 PM

Hi Nowa,

Wow, great article, thanks for posting! They do a great job of breaking stuff down. I think I am pretty PSU savvy now thanks to the tips posted here and that article

Nowa said:
As for the efficiency rates on power supplies the optimal load/sweet spot for maximising the efficiency varies depending on the power supply, but usually it is around 50-60% of the maximum load.


Thanks for that, that makes a lot of sense.

I just bought a really cool 550 Watt PSU that is guaranteed to be at 88% efficient at all loads!! 80+ Silver certified, yeah baby. I'm really excited as my current power supply purchased 4 years ago is (or was) 60% efficient, and that's without capacitor aging....yikes!

I hope to save a lot on my electric bill, what with this PSU and the components listed in the OP.

Thanks again everyone, this forum has been great to me :D 
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January 15, 2010 10:40:00 PM

iloveToms said:
Quote:
You may not need 5 , but 1 x 80mm isn't going to provide much airflow to help cool your components.


I'm using an old 8600 GT I had lying around and my CPU/MB temps never get above 35C. So far, I see no need to buy an extra 4 fans!

I bought my components with energy efficiency in mind. The 95 W CPU does really well and doesn't give off a lot of heat

Let me know what you guys think about my current PSU! Thanks


You seem to be going for the absolute minimum with your thinking IE: one fan is plenty, minimum PS. This is a very dangerous way of thinking. Spend a few extra dollars on a better PS and a few more fans whether you think you need to or not. When you spend lots of money on other high end components and then go the cheap route with your PS and fans, you are risking major damge to your computer. Soend a few extra dollars, it will be very much worth the money.
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