PSU = Power Saving Up

[Chimel]

Someone mentioned in another thread that big PSUs draw more juice than smaller ones for the same task. Is there any data on that?

For instance, if your system draws 300WH at full load, is there a big difference between a 450W and a 850W PSU from the same manufacturer and series, say the Corsair 85% efficient HX PSUs?

Both are 85% efficient, so they should draw an extra 45WH (300x15%), but I suppose there is an extra power consumption price to pay for the extra wattage between the 450W and 850W models, I just can't figure out how much.

Still, I would assume that difference does not approach the 17WH savings between a 85% and a 90% PSU, even between a 450W 85% and a larger 850W 90% PSU.

My example for argument's sake:
255 + 45 (300*15%) = 300 (85% efficiency)
255 + 28 (283*10%) = 283 (90% efficiency)
Difference = 17WH

 

[Zero82z]

PSU efficiency curves are inverse parabolas. They peak at around 50% load, and the closer you get to minimum or maximum load, the more the efficiency drops off. If you are trying to power a 300W load and you have a 600W and a 1000W PSU that are both 80Plus Silver rated (just as an example), the 600W PSU will use less power since it will be more efficient at a 300W load (50%) than the 1000W unit would be (30%).

This is also why 80Plus has three different requirements for each of their efficiency categories. They require PSUs to have certain efficiencies at 20%, 50%, and 100% load, and a PSU must meet all of those targets to be certified.

 

[Chimel]

Thanks, Zero82z.
So based on the load efficiency for these different certifications, the figures are something like:
HX450 (Bronze) 84%: 252 + 48 = 300W
HX850 (Silver) 86%: 252 + 41 = 293W, savings = 7W ($7/year)
AX850 (Gold) 88%: 252 + 35 = 286W, savings = 13W ($14/year)

Which means that since the higher spec PSUs seem to also have higher 80+ certifications, it does make sense to upgrade to higher wattage. At least in this case study.
The savings offered by a higher 80+ grade are much more important than any savings made by staying at the lowest viable wattage PSU of the same 80+ grade level.
If there was a 80+ Bronze 850W PSU, the figures would be:
850 (Bronze) 83%: 252 + 52 = 304W = -4W ($4/year)

I added the amount in dollars (at $0.12/KWH) to show that any savings is really immaterial for just one computer, but higher 80+ grades probably also represent better quality build, more stable current, and eventually better performance and longevity.

http://en.wikipedia.org/wiki/80_PLUS

 

[blastingcap]

If you're like most people you idle way more often than you put load on your PC. Therefore it's probably a good idea to find something that is efficient at your idle load but still has enough juice to handle a full load (active CPU, GPU, hard drives, optical drives, etc.). Overbuying wattage loses money not only up front, but also in lower efficiency while you are idling.

If you're a 24/7 folder though, or only boot up to game and then shut down right after, or something like that, you are better off with a PSU that is twice your load wattage, rather than twice your idle wattage.

Thanks, Zero82z.
So based on the load efficiency for these different certifications, the figures are something like:
HX450 (Bronze) 84%: 252 + 48 = 300W
HX850 (Silver) 86%: 252 + 41 = 293W, savings = 7W ($7/year)
AX850 (Gold) 88%: 252 + 35 = 286W, savings = 13W ($14/year)

Which means that since the higher spec PSUs seem to also have higher 80+ certifications, it does make sense to upgrade to higher wattage. At least in this case study.
The savings offered by a higher 80+ grade are much more important than any savings made by staying at the lowest viable wattage PSU of the same 80+ grade level.
If there was a 80+ Bronze 850W PSU, the figures would be:
850 (Bronze) 83%: 252 + 52 = 304W = -4W ($4/year)

I added the amount in dollars (at $0.12/KWH) to show that any savings is really immaterial for just one computer, but higher 80+ grades probably also represent better quality build, more stable current, and eventually better performance and longevity.

http://en.wikipedia.org/wiki/80_PLUS

 

[Chimel]

Agree, the second graph and comments show it all:
http://www.anandtech.com/show/2624/3

There's also the boot spike when all devices are powered simultaneously when you switch the computer on, but most modern PSUs seem to handle that now.

But I don't recommend overbuying wattage, only getting a better 80+ grade, to 80+ Gold, for instance. Only it so happens that Corsair doesn't have lower wattage PSUs in 80+ Gold grade, only Bronze.

 

[Sycraft]

If you're like most people you idle way more often than you put load on your PC. Therefore it's probably a good idea to find something that is efficient at your idle load but still has enough juice to handle a full load (active CPU, GPU, hard drives, optical drives, etc.). Overbuying wattage loses money not only up front, but also in lower efficiency while you are idling.

If you're a 24/7 folder though, or only boot up to game and then shut down right after, or something like that, you are better off with a PSU that is twice your load wattage, rather than twice your idle wattage.

Well three other things you need to consider:

1) You use much less power at idle so efficiency problems aren't as pronounced. Say you have a PSU that's 80% efficient at idle, and your computer uses 80 watts at idle. That means it draws 100 watts from the wall, 20 are wasted. However if you computer uses 500 watts under load it draws 625 from the wall, 125 are wasted. As such efficiency under load is a consideration, even if you idle a lot.

2) The efficiency curves are usually pretty narrow. For example the Corsair AX1200 is 87% efficient at its low point (full load), 89% at its high point (half load). Quibbling over trying to hit the most efficient point is a bit silly when talking differences that small. You are much better served simply getting a more efficient unit over all.

3) You never want to run things like power at capacity. You want overhead. There's a lot of reasons for this, but you don't want to run things right up to the limit, if you can help it. You want to have reserves. Any and all savings from trying to optimize the supply as small as possible will be lost if it burns out and takes your motherboard with it.


So, if you buy a PSU that is roughly 1.5-2x your calculated max load (if practical, can't do that for real high end systems) you are good to go. It will have plenty of power to spare.

 

[blastingcap]

Well three other things you need to consider:

1) You use much less power at idle so efficiency problems aren't as pronounced. Say you have a PSU that's 80% efficient at idle, and your computer uses 80 watts at idle. That means it draws 100 watts from the wall, 20 are wasted. However if you computer uses 500 watts under load it draws 625 from the wall, 125 are wasted. As such efficiency under load is a consideration, even if you idle a lot.

2) The efficiency curves are usually pretty narrow. For example the Corsair AX1200 is 87% efficient at its low point (full load), 89% at its high point (half load). Quibbling over trying to hit the most efficient point is a bit silly when talking differences that small. You are much better served simply getting a more efficient unit over all.

3) You never want to run things like power at capacity. You want overhead. There's a lot of reasons for this, but you don't want to run things right up to the limit, if you can help it. You want to have reserves. Any and all savings from trying to optimize the supply as small as possible will be lost if it burns out and takes your motherboard with it.


So, if you buy a PSU that is roughly 1.5-2x your calculated max load (if practical, can't do that for real high end systems) you are good to go. It will have plenty of power to spare.

Those are some great points.

The first point I dunno, I have one PC on pretty much 24/7 and it's idling for most of that 24/7, plus it's my HTPC and thus won't ever see truly high loads anyway. So for almost the entirety of the year I'm going to be at some miserly idle load. I think the other extreme is a machine that is on 24/7 doing folding and maxing the CPU on something else, or whatever. For people in-between the above two extremes I think your advice is very good.

The second point varies.. you chose one example but there are some lesser PSUs with larger curves. But I agree that efficiency curves might not be as steep in some PSUs.

I agree with the third for another reason too: aging capacitors.