Choosing a power supply

I remember when this bit was simple.  Your case came with a 200W power supply and that was the end of it - you even plugged your monitor into it!  Now it's all become complicated.  First you have to figure out what size power supply you need, then you need to decide if you want a single rail or multi rail unit and then it really starts to get difficult.

After a fair bit of research and reading of reviews, I've brought together the factors that I think are important to this decision.

First, a Word About Efficiency and Heat Output

All power supplies draw more power from the wall socket than they provide to your computer equipment.  The extra power is wasted - mostly as heat - during the transformation from AC to DC and to the voltages that the various outputs require.

A decent power supply these days will be 80 Plus rated - meaning it will be 80% efficient at peak power draw.  The best ones might be over 85% efficient.  They will all be more efficient at 50-80% of peak.  I want my PSU to be at peak efficiency while my system is at peak load, because peak efficiency means the least amount of power being converted to heat - thereby helping case cooling.  If it can also be efficient at average load, even better.

Now - How Big?
High-end CPU's and GPU's require more power.  Peak power draw on an overclocked CPU or GPU can easily hit 300W and more.  Hard disks are in the range of 5-12W, when loaded.  (SSD's seem to stay below 2W, so they shouldn't cause too many headaches.)  A water cooling pump will be up to 25W.  Case fans range from less than 1W, up to about 4W.  Other items are typically not so big as to be a major consideration - unless you have a lot of them.

If I'm going to have enough juice for a CPU and 3 GPU's, there's around 1200W (peak draw) right there without even thinking about the rest of the system.  Figuring in 2 SSD's, (4W,) 3 HDD's, (25W,) 5 case fans (say, 10W) and a water pump, (20W,) leaves me at 1259W, with no headroom for anything extra at all.  There are a few power supplies that will pump out that much juice and more - all the way up to 1600W - but they are awfully expensive and tend to run hot, noisy and inefficiently.  It would be best if I could get away with one of the smaller units.

Fortunately, it is virtually impossible to have every component drawing peak power at the same time.  Even in a perfectly balanced system, at least one of my 3 GPU's will be waiting on the CPU to send it some work to do, which in turn might be waiting on the hard disk to return from an I/O request.  I am unlikely to be sucking data off of all of my hard disks simultaneously.  (To be fair, the pump is likely to be maxed when the rest of the system is maxed, because that's when it will be generating the most heat.)  The point is, I'm pretty unlikely to be drawing 1259W at a time.

I'm guessing that my GPU's won't be maxed out while I'm reading my hard disks - games generally avoid disk I/O during gameplay, as you don't want your expensive, lightning fast graphics card(s) waiting on a slow old disk operation.  I'm guessing that a more realistic peak power draw for the system will be around 75-80% of the CPU and and GPU's combined peak, plus the pump and fans, plus the rest of the system at idle.  That gives me a guesstimate of 1200 (CPU + 3xGPU's) * 0.8 + 17 (SSD's and HDD's at idle) + 30 (pump and fans) = 1007 watts, peak power draw.

Trying to fit my system into the peak efficiency band of most PSU's further increases this number.  1007W is 80% of 1259W - so I'm still gong to need a big power supply!  They don't tend to make them that exact size, but they get pretty close.  They seem to be available in 1000W, 1050W, 1200W, 1250W, 1500W and 1600W varieties.

The high-output PSU's tend to be less efficient, too.  I couldn't find any 80 Plus Gold rated PSU's over 1000W - and precious few rated 80 Plus Silver.

Do Overclockers All Need Single-Rail PSU's?
This post from jonnyGURU seems to be the authoritative word on single rail vs. multi rail PSU's; I recommend it to anyone that is considering buying a high-end power supply.  Basically, if you want any inbuilt protections against short circuit, (e.g. you do not want to get electrocuted or your house to burn down) get a multi-rail power supply.  This will incur the overhead of having to be more careful about how you plug your system together, so that no single rail's capacity is exceeded - or your computer will shut down.

You will also need to be careful to buy a high quality PSU that has enough rails, with enough capacity, to power your equipment.  If your highest capacity 12V rail is only rated at 20A, then you won't be able to drive a high-end CPU or GPU, as it will shut down if you try to draw over 240W.  (12V x 20A.)  If you're overclocking, you will likely need even more current on your rails.  You may also find yourself in the situation where you are unable to draw the full rated power from you PSU because (e.g.) you have a component needing 30W and your 6 rails only have 10W of capacity left, each.  This last is more likely to be a problem when you are near the limit of your PSU's capacity, anyway - another reason to allow yourself a little headroom.

Single rail PSU's do away with this configuration headache and allow you to push your PSU closer to its limit by providing its full capacity with one gigantic rail.

How big did you say?
The more mundane side of buying a high-end power supply is that a higher output rating will typically mean a physically larger unit.  Some PSU's today are so big that they won't fit in some cases.  Others will fit, but might prevent a 5.25" bay, or perhaps a hard drive enclosure, from being used.  Whether this is a problem depends on your case and what you are trying to fit into it.

I'm planning on using the Lian-Li PC-P80, one of the few 10-slot cases on the market, giving me the option of 4 dual-slot graphics cards.  This case will fit a maximum PSU length of 220mm - just short of the largest PSU's about.

Noise
PSU manufacturers - particularly high end ones - are starting to pay more attention to noise reduction now.  Units with bigger fans, moving more air at lower revolutions, tend to stay cool for less noise output.  Some of the less powerful units even get away without cooling fans.  I want mine to be as quiet as possible, but higher end units are going to have some fan noise.

So, which one?
I looked at the reviews for a number of units, before settling on the Enermax Revolution 85+ 1250W.  It is 190mm - leaving 30mm more space than the Silverstone Strider ST1500.  It gets a "Best in Class" award from Hardware Heaven. JonnyGURU has good things to say about its cousins.  It is over 89% efficient between 500W and 1125W power draw, which should be my normal power draw.  If/when I eventually hit 4-way SLI and max this unit out, I can always add one, or even two, of these in 5.25" bays to take care of the excess.

Hard Disk Drives

With the better SSD's offering at least 10 times the real-world performance of the best HDD's, the lure of super-fast Windows startup and application load times is hard to resist.  Tom's Hardware has some interesting benchmarks on SSD performance and on HDD performance.  Putting the OS on a 160GB Intel X25-M seems like the way forward here.

The Fusion-io ioXtreme is another interesting player here.  Having one just for the pagefile and windows temp directory seems like the best use.  While the review on Tom's Hardware shows some blistering performance figures, it also shows some so-so figures for the consumer version.  It seems unlikely to have a big enough effect on frame rates to justify the cost.  Perhaps the price of the professional version will come down enough for this to be revisited at a later phase?

Upping the Ante

In this post, I explained the reasons for my initial decision to go down the ATi route for graphics.  Since then, new evidence has come to light, which has swung my thinking in the other direction.

• This review shows the relative performance of 2 Radeon HD 5970's in Quadfire vs 3 GeForce GTX 480's in Tri-SLI.  While not decisive, nVidia has an advantage at max settings, with AA enabled, plus more room for optimisation with their brand new drivers and the possibility of scaling to a 4th GPU for either PhysX or Quad-SLI.  (Assuming that they can get their Quad-SLI scaling sorted out!)
• This news item, shows a dual CPU motherboard from EVGA with SATA 6Gb, USB 3.0, and 2 NF200 chipsets, for 72 lanes of PCIe goodness!  That will support 4 graphics cards at a full 16x speed!  Being based on the EVGA Classified 4-Way SLI, the EVGA board will be more overclocker-friendly the the ASUS P6T7 WS Supercomputer board, which seems aimed more at professionals than enthusiasts.

It seems that motherboards are on the verge of a technology refresh.  Setting aside SATA 6Gb and USB 3.0; the dual CPU option offers a far better upgrade path.  I'm willing to bet that the price premium on the dual-GPU board will be worth it.

In a year or two, 2 cheap, mid-range CPU's will likely keep my frame rates comparable to whatever n-core top-of-the line monster chip Intel is offering at the time - just as 2 Core i7-930 chips offer more CPU power for less money than a single Core i7-980X does today.  Sure, some applications and games don't take advantage of multiple cores at the moment - but that's changing.  DirectX 11 is multi-threaded, so games that use it will be, too.  Both nVidia and ATi have gone multi-threaded with their drivers.  Multiple cores is the brave new world of high-end 3D graphics.

With this in mind, it strikes me that I might be best advised not to invest too heavily in that area just yet.

• I can save £130 in phase 1 by going with an ASUS Rampage II GENE. That should perform exactly the same as any other board on the market, up until the point that I want to add a 3rd graphics card - by which time it might be time to think about upgrading to a next-generation motherboard, with dual CPU and Quad-SLI support -  I even get Creative SupremeFX audio thrown in!
• I can save another £100 by choosing the GTX 480 over the Radeon HD 5970, which gets me a faster GPU and more headroom for expansion.

Unfortunately, this change of direction means my previous decision on case needs revisiting.  If the EVGA dual-processor motherboard is the same odd size as the Classified 4-Way SLI, then it won't fit most cases, due to the 10 expansion slots required for a quad-GPU config.  The few cases that will fit the EVGA board all look like they belong on the bridge of the Starship Enterprise, but the Lian-Li PC-P80 is the least offensive of them, with good air cooling, while supporting water cooling and a massive 390mm graphics card length.  Once I replace the fans with LED-less ones, it should be acceptable.

Revised build roadmap and phase 1 parts list to come soon.

Phase 1 in Detail

Phase 1 will be air-cooled, with just one graphics board and stock clocks.  This phase is about getting a working system up and running with minimal expense and hassle.

That doesn't mean ignoring my design goals, though.  I want to leave myself with the longest and smoothest upgrade path possible, avoiding buying anything that I'll just have to throw away in a later phase.  To that end, I'll be investing in the best motherboard I can find, along with a case that will go the distance and a graphics board that will scale well.

Parts List
Monitor: I happen to have an old Samsung 22", 1680x1050 LCD hanging around, so I'm going to use that for the early build phases.  With all the detail turned up, I doubt I'll have enough grunt to max most titles out on this screen until at least phase 2, depending on how well the overclocking goes.
Motherboard: ASUS Rampage III Extreme

This one is important to get right, as replacing it later is both expensive and a serious hassle.  The OC'ing potential of this board, along with its support for future CPU, IO device and RAM models means it should last the distance as well as any motherboard could.  Also supports both nVidia SLI and ATi CrossfireX, which means I can switch ponies later on, when/if nVidia takes the lead in the 3D graphics race.

Graphics Card: ASUS EAH5970/G/2DIS/2GD5/A
One would hope that using an ASUS graphics board with an ASUS mainboard might reduce the chances of compatibility woes by a smidgen.  This ASUS board is pretty reasonably priced anyway.  nVidia vs ATi was discussed in and earlier post, so I won't go into that again here.
CPU: Intel Core i7 930 (LGA1366)
While not the best CPU available, this one ought to see me through until the rest of the system out paces it.
RAM: 3x2GB DDR3 1600
No need to go all out here.  RAM isn't the most critical part of the overall system when it comes to frame rates.  As long as I have a reasonable amount of moderately spec'ed RAM it shouldn't be the bottleneck - at least in the early phases.  I can upgrade this in a future phase, without throwing away too much money.
Case: Zalman GS1000 Plus
Really pushing the boat out on this one.  This is the one part of the system that will outlast all the other bits.  Until the ATX form factor goes out of fashion, I'll be able to keep on building systems in this enclosure.  Ironically, it's actually the cheapest component in this list.  Further discussion here.