PSU minimum wattage requirements: how important are they?

Megalodon

A few months ago I purchased a barebones shuttle, model SK22G2 V2. I didn't have the money at the time to purchase a video card and I've been running off the integrated graphics (which have worked great, surprisingly). Here are my questions:

After browsing for video cards, I've noticed quite a few of them have minimum wattage requirements, ranging from 400W to 550W. My PSU is only rated at 250W. I figure these ratings are based on a significantly more power hungry system than I have, but I don't have enough information available to me to feel confident making a decision.

Here are the primary components currently drawing power from my system:

• WD Raptor SATA hard disc drive
• DVD drive
• AMD Athlon 64 X2 Dual Core 2.6GHz processor

Currently I have the two dongles necessary to power the cards I'm looking at:

XFX PVT71PUDP3 GeForce 7900GS - the customer reviews on this one give varying opinions. One guy says he needed to buy a 700W PSU to run it, although I suspect this was for SLI, another reviewer says a 400W will do it. A third says the box states a minimum of 350W are needed. The specs pages on both newegg and the manufacturer's website are silent.

EVGA 256-P2-N624-AR GeForce 7900GS - says it requires a 400W supply.

Finally, the GeForce 7900GS is not on the official supported video cards list for this model. However, a few of the other cards that are on the list are even more power hungry, so clearly wattage isn't a criteria.

gneGne

Watch this: http://www.guru3d.com/article/content/377/3/

According to this article, their computer uses 187watt total. However they recommend having atleast a 350w PSU.

Megalodon

I wish I knew what other components they were running, because I don't understand the jump they make in their conclusion to needing a 420W PSU. The article does give me one more bit of info though...the maximum peak wattage for the 7900GS is stated to be 82W.

Amperage is an issue I never really considered--a friend of mine suggested it was pretty irrelevant. In the conclusion of that article it says that 22 amps minimum are needed "for the total of +12 volts rails." Does this mean if I've got 11 amps on the mainboard line and the additional power connector I'm O.K.?

I found a great website, but even they don't analyze amperes.

Rook

Shuttle build exceedingly excellent power supplies. You can safely ignore the minimum wattages that graphics card manufacturers recommend as they're usually trying to cover the fact that a lot of PSUs are horribly manufactured.

I would bet on you being able to run an 82W card easy on a shuttle.

Pheezer

The thing to bear in mind is that not all PSUs are created equally, and even the high end ones are only 70% efficient. So build with a buffer in mind, but remember that you're not likely going to ever have your CPU and videocard running at 100% utilization while your hard drives are all in seek mode and you're burning a DVD while your 7.1 soundcard is being used to push a 7.1 system at full volume and your network card is transferring at a full gb/s and you get the point, right?

Look at the max power usage specs for your CPU, video card and hard drive, and design based on a worst case scenario where all are drawing as much power as possible. That'll set a good high water mark for what you can realistically expect to need from a PSU.

tsmvengy

The guys on the Silent PC Review forums are pretty good about recommending how much power your components might draw (silent computing = efficient power supplies.

Just by looking at those components, I think your 250W psu MIGHT be able to run a 7900GS. From a quick google search, it looks like the 7900GS uses 45W. What video card do you have in there now?

Megalodon

I'm using the mainboard integrated graphics, I don't have a card. Thanks for mentioning the forum--and thanks to everyone else as well--I think I've got enough information to make a decision with, when I get the money to purchase the card. I'll update with the results when they come.

Barrakketh

pheezer FD wrote: »

The thing to bear in mind is that not all PSUs are created equally, and even the high end ones are only 70% efficient.

Assuming I'm interpreting what you said correctly, are you saying that a PSU with 70% efficiency is only able to deliver 70% of it's stated wattage?

My understanding is that the 70% figure is to take into account the inefficiencies of a switching PSU, and that the wattage that it's rated at is the maximum it can output. So a PSU delivering 200 watts will draw approximately 280 watts from the outlet.

tsmvengy

Barrakketh wrote: »

pheezer FD wrote: »

The thing to bear in mind is that not all PSUs are created equally, and even the high end ones are only 70% efficient.

Assuming I'm interpreting what you said correctly, are you saying that a PSU with 70% efficiency is only able to deliver 70% of it's stated wattage?

My understanding is that the 70% figure is to take into account the inefficiencies of a switching PSU, and that the wattage that it's rated at is the maximum it can output. So a PSU delivering 200 watts will draw approximately 280 watts from the outlet.

This is correct. The wattage rating on a PSU is what it delivers in DC current - it will draw more than that from the wall.

Fristle

Power supplies convert AC power from the wall into DC power inside the PC. But they convert it to three levels of DC power: 3.3V, 5V and 12V. They call these "rails" as in the 5V rail and the 12V rail. The important thing when estimating how much power your PSU can supply is to know how much power it is dedicating to each rail. Some devices take 5V power, some take 12V power. If I'm not mistaken, video cards use the juice from the 5V rail.

When a device manufacturer recommends a certain wattage PSU, they're estimating for what they need out of the 5V rail. Depending on what else is in your system, and how the PSU allocates its power to the different rails, you may be able to get by with less than the one the video card manufacturer recommends.

Here is a good calculator to get an estimate of your PSU needs: http://www.extreme.outervision.com/psucalculatorlite.jsp

Nova_C

A lot of companies use 'best case' power ratings. In other words, they rate the output of their power supplies under ideal conditions (Namely, seeing what it'll do in an environment much colder than any pc is actually going to be operating in - the colder it is, the better a PSU performs). Once a power supply approaches it's maximum output, voltage irregularities can start to appear. These irregularities can show up in dropped frames, corrupted data, lock ups, driver crashes, basically the whole gamut of computer problems. The cheaper the manufacture of the power supply, the sooner those irregularities will show. My philosophy is, if there's any doubt, get a better power supply. It is the single most overlooked and yet most important component of any computer system.

tsmvengy

Fristle wrote: »

Power supplies convert AC power from the wall into DC power inside the PC. But they convert it to three levels of DC power: 3.3V, 5V and 12V. They call these "rails" as in the 5V rail and the 12V rail. The important thing when estimating how much power your PSU can supply is to know how much power it is dedicating to each rail. Some devices take 5V power, some take 12V power. If I'm not mistaken, video cards use the juice from the 5V rail.

When a device manufacturer recommends a certain wattage PSU, they're estimating for what they need out of the 5V rail. Depending on what else is in your system, and how the PSU allocates its power to the different rails, you may be able to get by with less than the one the video card manufacturer recommends.

Here is a good calculator to get an estimate of your PSU needs: http://www.extreme.outervision.com/psucalculatorlite.jsp

A couple things:

#1: that PSU calculator dramatically overestimates the power supply you will need.

#2: as far as I know, video card manufacturers use "x-treem" systems as a spec to gauge how big a PSU you need: as in, if you have a Quad-core intel, 8 gigs of RAM, 6 WD raptors, 3 DVD drives, and 15,000 fans, you will need ___W PSU to run this card. Basically this covers their asses.

Megalodon

If that calculator dramatically overestimates the power supply I need, then I'm definitely in the clear. It's saying I'll need a 235W power supply.

Marty81

I have a question about 12V rails. Some PSUs advertise "dual 12V rails." How exactly does that work? If the card advertises 2 12V rails at 20 amps each, and my video card requires 30 amps, can the PSU power it?

Megalodon

That's related to my question above about amperage on the 12v rails. First of all, wow, 30 amps? Damn.

My experience working with electronics suggests that the values for amperage should just add up if you've got both rails leading into the card, if they're both powering the same thing, but I'm not sure what the card manufacturers for both my card and your card mean exactly. Also, presumably you meant if the power supply advertises two 12v rails at 20 amps each, right?

JWFokker

Consider that PSUs are not 100% efficient. Most are 70-75% efficient, so if the PSU is rated for 250W, only about 175W is available to your PC before the PSU overheats and shutsdown. The inefficiency is the power that the PSU draws but gets converted from electricity to heat along the way. You really need to buy a 300W PSU at the absolute minimum to be safe.

sirSolarius

P=IV and watts is just power over time. So you're looking at IV/time, and you know the voltage is 12V.

So the wattage divided by 12 is really telling us amperage/time, so any analysis of watts will be an analysis of current.

Barrakketh

Megalodon wrote: »

That's related to my question above about amperage on the 12v rails. First of all, wow, 30 amps?

They don't really draw that much. I think 15A is the most I've ever read about one drawing.

My experience working with electronics suggests that the values for amperage should just add up if you've got both rails leading into the card, if they're both powering the same thing, but I'm not sure what the card manufacturers for both my card and your card mean exactly. Also, presumably you meant if the power supply advertises two 12v rails at 20 amps each, right?

I don't think they both power the video card. Basically, in your standard ATX12V motherboard one of the connectors (that 4-pin one) is for the video card, and the the the other rail is used for everything else.

My PSU 20+4 pin connector for the motherboard (which isn't being used), the 4-pin connector, and a 6-pin connector for...hell if I remember, because it's not being used. I believe it's for PCIe and may be a replacement for that 4-pin connector in high-end motherboards.

JWFokker wrote:

Consider that PSUs are not 100% efficient. Most are 70-75% efficient, so if the PSU is rated for 250W, only about 175W is available to your PC before the PSU overheats and shutsdown. The inefficiency is the power that the PSU draws but gets converted from electricity to heat along the way. You really need to buy a 300W PSU at the absolute minimum to be safe.

Incorrect. We already addressed this earlier in the thread. Read my post above and the one below it that confirms what I said.