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Why do we use such large PSUs?

J

Jorenm

n00b
Joined
Jan 15, 2009
Messages
54
I'm curious. Every time I see a power consumption graph on a review website for a graphics card, they showed whole-system draw @ less than 400 watts under load with any card I've ever looked at.

Why wouldn't a 500 watt be sufficient for anything short of a SLI setup or perhaps lots of hard drives and addon cards?
 
That is the thing, 500w is sufficient for most anything.

People just buy big ones for a couple reasons: e-peen, thoughts of future expansion, or they actually need it. In that order. IMO.
 
simple, it enlarges your e-penis.

since mine is already large enough, i only pick a quality psu that's within my power consumption needs. just remember that the higher rated the psu is, the less efficient the psu is if you don't use at least half of its rated power output.
 
Are you all forgetting silence?

Since the fan will generally spin faster as loads go up, by buying a PSU with 2x the needed capacity, you can be much lower on the fan curve. If you have a PSU thats maxed out on the other hand, it will be at its max fan speed.
 
Are you all forgetting silence?

Since the fan will generally spin faster as loads go up, by buying a PSU with 2x the needed capacity, you can be much lower on the fan curve. If you have a PSU thats maxed out on the other hand, it will be at its max fan speed.
Click to expand...

Is this just how you think it works, or how it actually works?

It seems to me like a 1000w PSU only pulling 400w is going to run just as hot (and require just as high a fan speed) as a 500w rated PSU only pulling 400w.

I'm no expert, so if I'm wrong feel free to correct my thinking.
 
Jorenm said:
Is this just how you think it works, or how it actually works?

It seems to me like a 1000w PSU only pulling 400w is going to run just as hot (and require just as high a fan speed) as a 500w rated PSU only pulling 400w.

I'm no expert, so if I'm wrong feel free to correct my thinking.
Click to expand...

No, hes right, because the 1000watt psu pulling only 400watts has to work alot less, all the parts are only being stressed at about 40% of their rated capacity and will run cool, compaired to something thats being used near its top capacity, near the redline so to speak.
 
I do find it odd that it works that way, but yes, it does.

[ A large, say, 1000W at 200W load, 70% effeciency would have 60W of heat to dissipate. A 300W at 80% effeciency has just 40 watts to dissipate. ]

While the 1000W would be at its minimum fan speed, cool, the 300W would be nearly max fan speed, and very hot.

Anyone care to explain this to me? I could understand that maybe if the heatsinks were smaller, sure, the fan needs to ramp up more. But why is the air so hot? If the air is hot, there must be more heat being generated.

Am I just crazy, or is this.. some phenomena?

EDIT: Or under stress, do components make additional heat? My understanding was that power supplies only make heat for wasted energy, but maybe that's where I went wrong..
 
a larger PSU doesn't always give you silence or better efficiency, because their efficiency ratings aren't static across the board (and in some cases its less than ideal, if you're only using ~5-10% of its rated capacity)

ideally you want to load a PSU between 40-60%, which means on a 100W unit, for peak efficiency, you want like ~50% load, and you'll get whatever 83-85% effciency on that (meaning it'll draw whatever 600W at the wall), this means lower heat dissipation (in theory, and only slightly) than say a 600W unit putting out 500W, which is 83% rated load, so its going to dip into the upper 70's or scrape on 80% efficiency, draw slightly more power, and heat up slightly more

generally the larger PSU has nothing to do with eCock (TM), its just about headroom, because most people who overbuy a bit on their PSU have likely seen a crap unit tank out at some point, given that extra capacity generally doesn't cost a lot (you don't pay double for double output), why bother fighting it, when you can have headroom

in other words, if your system will draw between 400-500W, why not have a ~700W unit, ensuring you basically won't have issues, and you can plug in another HD without it exploding, instead of a 400-500W unit thats at nearly 100% draw
 
I would also agree with headroom. The psu is a part that you only have to replace maybe once per 3 maybe 4 rigs, thats a rare commodity in this [H]obby. I had a 500w pcpc that lasted nearly 6 years because I knew I would upgrade the rest of the rig at some point. The 500 was loud when I only used 200w back in the day btw :cool:
 
Keep in mind where you are. Most people do not have 800W+ power supplies, just a large portion of people that hang out here and some of us are a little bit crazy.
 
Jorenm said:
Why wouldn't a 500 watt be sufficient for anything short of a SLI setup or perhaps lots of hard drives and addon cards?
Click to expand...
A 500W PSU is sufficient for that purpose, and you won't see anything more being recommended around here by anyone who knows what they're talking about.
 
With power supplies, it's much better to have too much than too little. I don't want to bother fudging around with power supply calculators and computing how much I actually need, so I generally just get one that's a fair bit larger than I think I need. Within reason of course.

A good quality ~500W power supply will work fine for most people.
 
Thanks for all the informative replies.

I've seen so many people in so many different places (not necessarily here) say things like "Oh, you're getting X high-end graphics card, better get a 800w PSU". I was wondering if that was truly necessary.
 
This might be a stupid question but is a really good quality 500w PSU more efficient than let's say a cheapo 650w? Efficient in terms of energy used to power the same let's say 400w load, and maybe even a 550w load (ya I know how it sounds).
 
WildMonkey said:
This might be a stupid question but is a really good quality 500w PSU more efficient than let's say a cheapo 650w? Efficient in terms of energy used to power the same let's say 400w load, and maybe even a 550w load (ya I know how it sounds).
Click to expand...
Yes, most likely.
 
You usually have to pay more for more efficient PSUs as it takes premium quality components to make them efficent.

Oh and simple answer to your question is yes.

WildMonkey said:
This might be a stupid question but is a really good quality 500w PSU more efficient than let's say a cheapo 650w? Efficient in terms of energy used to power the same let's say 400w load, and maybe even a 550w load (ya I know how it sounds).
Click to expand...
 
And another thing to note, it seems that low-wattage units don't squeal or buzz, while expensive, large ones do, no matter the quality.
 
Value is also a factor. My 750TX is overkill for my rig, but it was only 5-10 dollars more than the 650 at the time so i thought why not? Although 650 was overkill as well :rolleyes:
 
obobski said:
a larger PSU doesn't always give you silence or better efficiency, because their efficiency ratings aren't static across the board (and in some cases its less than ideal, if you're only using ~5-10% of its rated capacity)

ideally you want to load a PSU between 40-60%, which means on a 100W unit, for peak efficiency, you want like ~50% load, and you'll get whatever 83-85% effciency on that (meaning it'll draw whatever 600W at the wall), this means lower heat dissipation (in theory, and only slightly) than say a 600W unit putting out 500W, which is 83% rated load, so its going to dip into the upper 70's or scrape on 80% efficiency, draw slightly more power, and heat up slightly more

generally the larger PSU has nothing to do with eCock (TM), its just about headroom, because most people who overbuy a bit on their PSU have likely seen a crap unit tank out at some point, given that extra capacity generally doesn't cost a lot (you don't pay double for double output), why bother fighting it, when you can have headroom

in other words, if your system will draw between 400-500W, why not have a ~700W unit, ensuring you basically won't have issues, and you can plug in another HD without it exploding, instead of a 400-500W unit thats at nearly 100% draw
Click to expand...


this and is also why i went with a 750w because @ the wall i pull 400w

 
I buy a little larger than I actually need, but don't go overkill on a 1000w that I may never need. I know I'm not going to have a Tri-SLI system, and if I could I would go ahead and just purchase a brand new 1000w at the same time. I could easily afford a new PSU if I could justify the cost of Tri-SLI.

The reason is upgrading. Pure and simple. I want more money to spend on upgrades. Buying quality PSU's with 200+ watts more than I need means the PSU will last atleast 2 to 3 rigs worth of upgrading (which leads to more money saved). I want to be prepared if I want to splurge on the newest video card that draws even more power than the previous one.
 
mrorange43 said:
Value is also a factor. My 750TX is overkill for my rig, but it was only 5-10 dollars more than the 650 at the time so i thought why not? Although 650 was overkill as well :rolleyes:
Click to expand...

QFT. While a 500W PSU would be enough for many systems out there, if you check out the current PSU market, there are barely any reasonably priced good quality ~500W PSUs out there. For example, the EA650 650W PSU is $80 as of this post at newegg.com. The Seasonic 550W and the Corsair 550W is $90 at newegg as of this post.. Considering that the EA650 is a good quality PSU, there's very little reason to go with the other 550W PSUs.

So value is an important factor in addition to efficiency, temperature, noise, future headroom when larger PSUs are recommended.
 
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