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How to pick a Power Supply Unit that matches your hardware

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psu.jpgWe keep getting a lot of questions in the forums about what power supply to purchase, so we thought we'd put together a quick guide as to what to look for when you're buying a Power Supply Unit (PSU). We'll go into some basics with regards to how a PSU works, but we're not going to cover some of the more technical aspects here, as we want to keep this as a fairly general guide that most of you will be able to understand easily.

Let's start with the actual PSU itself, as long as you're getting an ATX (this is a PC form factor) compatible PSU, it should fit in any standard ATX or mATX chassis. Note that some high power models might use an extended form factor, but as long as the PSU is no deeper than about 160mm it should fit in most cases. For smaller mATX chassis you might want to consider looking for a PSU that is no deeper than 140mm.

The next thing to look for is that your power supply features Power Factor Correction (PFC), these days all good power supplies come with what is known as active PFC. This isn't going to make you save any money on your electricity bill unless you're a large corporate business, but it's generally a sign that the PSU is of better quality and that it's a more power efficient unit. It's a little bit too technical to explain what PFC does here, but for those interested in more details, we suggest hitting up Wikipedia.

Another thing to look for is 80 Plus certification, although these days things have gotten a bit more complicated as there are several different grades here. At the inception of the 80 Plus certification the goal was to create a set standard that would allow consumers to easily know that they were buying a power efficient PSU, but things have moved on quite a bit since then. As such, we know have 80 Plus which is the most basic certification and it guarantees that the PSU is at last 80 percent efficient. By efficiency in this case we're talking about input power minus power loss in the AC to DC conversion, so an 80 Plus certified power supply that draws 100W from the wall socket, would have to deliver at least 80W through its outputs, or in other words, 20 percent of the power is wasted in heat and general conversion loss.

Many of you may think that an efficiency of 80 percent isn't very good, but keep in mind that a decade or so ago, your average PSU had an efficiency of somewhere between 60-75 percent, so the 80 Plus certification was a big step forward. Today we also have Bronze, Silver, Gold, Platinum and Titanium 80 Plus certifications, with Platinum being up to 92 percent efficient at 50 percent load with the PSU connected to a 115V power source or 94 percent on a 230V power source. We're not going to go into any more details here, but you can find the various ratings in the table below for the different levels of the 80 Plus certification.

80 PLUS Test Type 115V Internal Non-Redundant 230V Internal Redundant
Fraction of Rated Load 10% 20% 50% 100% 10% 20% 50% 100%
80 PLUS 80% 80% 80%
80 PLUS Bronze 82% 85% 82% 81% 85% 81%
80 PLUS Silver 85% 88% 85% 85% 89% 85%
80 PLUS Gold 87% 90% 87% 88% 92% 88%
80 PLUS Platinum 90% 92% 89% 90% 94% 91%
80 PLUS Titanium 90% 94% 96% 91%

Source: Wikipedia

Another aspect that's important to look at is what is in general referred to as "power rails". A typical computer PSU delivers 3.3, 5 and 12V – as well as a few other Voltages that aren't relevant with regards to this topic. The combined output of the 3.3, 5 and 12V power rails times how many Amps each rail can deliver adds up to the total Wattage of a PSU. However, this is where you have to pay close attention, as some power supply manufacturers are cheating a bit here by shifting a lot of the power over to the 3.3 and 5V rails which are not used as widely as the 12V rails. So what you need to look at is the label of the PSU under what is generally referred to as "Maximum Combined Wattage".

You could end up losing a lot of usable power here, as some PSU's can have as much as 300W of the total load on the 3.3 and 5V rails, while having an underpowered 12V rail. This would not have been rare on a 600W power supply half a decade ago, but luckily more modern PSU's don't appear to suffer from this, as long as you're buying a unit from reputable brand. Keep in mind that a PSU can never deliver more power than the maximum combined Wattage under any length of time, although some PSU's have a "boost" mode where they can go above the maximum rated Wattage for a few seconds, i.e. during boot.

The second part to power rails include more modern power supply designs, since at one point in time Intel decided that it was time to re-design the computer PSU and use what is known as a multi-rail 12V design. The reason behind this was to be able to produce both more power efficient units and to reduce the Amperage per rail. Sadly a lot of early multi-rail PSU designs were poor, some couldn't handle load balancing between the rails properly and a couple of years later the first really high-end graphics cards appeared that required additional power and as such, some "enthusiast" power supplies appeared on the market that went back to using a single rail.

psu_comparison.png
A lot has changed since then and today there are a wide range of very high quality multi-rail PSU's in the market, as well as some very good single rail power supplies. The advantage of going for a good multi-rail PSU is that you can spread the load evenly, although with a single rail PSU you don't have to put much thought into things. On top of this, there are some high-end power supplies that allows you to switch between single and multi-rail mode. In all honesty, there isn't much in it here, but keep in mind that if a single rail, high amperage PSU goes wrong, it's a chance that it'll kill a lot more of your hardware than a multi-rail PSU where each rail has a much lower power rating per rail. We've included a screenshots above that hopefully helps explains the difference between single and multi-rail quite easily based on the power supply specs. Note that the two power supplies have slightly different specs and aren't as such perfectly comparable when it comes the total power output.
[PAGE][/PAGE]So what else is there to think about? Well, cables is one aspect that matters. For one, make sure that they'll reach, especially if you have a chassis with a bottom mounted PSU, as on some PSU's the cables have a hard time reaching things like the 4/8-pin 12V connector. Modular cables are also worth considering, although for most of us a fully modular PSU makes little sense unless you're planning on modding the cables. As such, what is referred to as semi-modular PSU's are in general the best thing to get, as you then only have to attach as many power cables for drives and additional hardware as you need and this helps reduce clutter and improve airflow inside your case.
atx_cable.jpgLast, but most definitely not least, we're going to take a look at how much power you need. We see many of you aiming for 700 or even 800W power supplies, some even higher than this. So how much power does your system actually need and which components draw the most power? Well, to give you something of an idea, we used a simple plug-in power meter and powered up a Gigabyte GA-Z77X-UP5 TH motherboard with an Intel Core i7-3770K CPU at stock clock speeds, 8GB of Corsair Vengeance memory and a 60GB Corsair Force GT SSD. This is the minimum set of components you'd need to have a workable CustoMac and you'd be relying on the Intel HD 4000 graphics. So how much power did this system draw?

Well, you might be surprised when we tell you that even at heavy load running LuxMark 2.0 using the CPU to render the scene, were peaked out at just below 100W. Admittedly it's going to be possible to load the system more than this, but it's a good example of how power efficient a modern PC really is. That said, we're not recommending that you run out and by a 120W Pico PSU to power this system with, since as soon as you start adding a graphics card, a few mechanical drives and what not else, the power draw will increase.

Calculating the power requirement for a system isn't all that hard, as the power hungriest components in a PC is the CPU and the graphics card, unless you're building something out of the ordinary, but even then, this is likely to be the case. Nvidia helpfully provides the maximum graphics card power for each of their models on its website, although do check non-reference card designs as they may vary from Nvidia's ratings. As such, we can see that adding a GeForce GTX 660 Ti would add another 150W worst case scenario, where something like a GeForce GT 640 only adds 65W. Sadly, AMD isn't quite as forthcoming, but another way of looking at things is how many power connectors the graphics card has.
geforce.jpg
To put things in perspective, the PCI Express slot on the motherboard can deliver up to 150W of power to the graphics card if the motherboard has a PCI Express 2.1 or 3.0 slot, older motherboards are limited to 75W per x16 slot. Each 6-pin power connector on a graphics card then delivers up to an additional 75W of power. Some high-end cards also have an 8-pin power connector and this add up to 150W of additional power. As such, a card with no extra power connector draws 150W or less, a card with a single 6-pin power connector draws 225W or less, a card with two 6-pin connectors draws 300W or less and a card with an 8-pin and a 6-pin power connector can draw as much as 375W. Admittedly there are some cards that have even more power inputs than this, but most of these aren't suitable for CustoMac usage.

The easy way to calculate your power budget is to add up the TDP – which is the Thermal Design Power and not the power draw – of the CPU, the power draw of your graphics card, about 100W for your motherboard, RAM and a single drive and you got a rough indication of how much power your system will use. Note that regular PCI and PCI Express cards draw very little power, a x1 PCI Express slot for example can't deliver more than 25W.

To sum things up, your average PC will in general not need more than a 500-550W PSU unless you add multiple graphics cards. It makes little sense getting a higher Wattage unit unless you know for certain that you'll be taking advantage of it at some point in the future, as not only is it a waste of money when you buy it, but technically you'll also waste money when you use it, as you might not reach the ideal efficiency level of the PSU if you're not hitting at least 50 percent load. We'd also suggest sticking with well-known brands when it comes to power supplies and stay away from the bottom of the barrel units, as they're almost guaranteed to cause problems.
 
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Very interesting in deed. To get a first grasp of your estimated power consumption I suggest this very detailed PSU calculator. You can even check which effect overclocking your CPU to a certain speed will have on the required PSU power. http://www.enermax.outervision.com
 
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Very enlightening!
One factor I have come to attach a lot of importance to is noise.
After trying power supplies from OCZ and Corsair which had good reviews but turned out to have quite audible fans, I found one that I'm really happy with, the Fortron Aurum 400 W -- a "80 PLUS Gold" labeled one. In Europe this sells for ca 50 Euro (60 US$) -- somewhat pricey, but it has a FR4 pcb rather than bakelite or varnished cardboard as some of the cheaper Chinese brands, and it's still a lot cheaper than the truly fan-less ones, and unless one puts one's ear right up against the case, its fan turns so slowly that it's inaudible.
 
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Article: How to pick a Power Supply Unit that matches your hardware

Joncas> Thanks for the tip! ;-)
I was on my way to buy a Be Quiet! Pure Power L7 350W, I've read good reviews about it. (I plan a very basic pc ;) so no huge power needed)
Does anyone know and has opinions about this brand? (though it's only 80+)
 
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Article: How to pick a Power Supply Unit that matches your hardware

Joncas> Thanks for the tip! ;-)
I was on my way to buy a Be Quiet! Pure Power L7 350W, I've read good reviews about it. (I plan a very basic pc ;) so no huge power needed)
Does anyone know and has opinions about this brand? (though it's only 80+)

Bought one 5 years ago. Never had any issue about it. Quite silent. Seems to have a solid built. Very aesthetic too. German quality at it's Best in my humble opinion.
 
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Good writeup :) It's why I picked the Corsair TX650 for my Hackintosh and for my UnRAID setup, both are running the same power supply. Supports a large number of drives (I have 14 hanging off the 12V rail in the Unraid NAS/Plex box).
If I had my way again I would have picked the HX650 though for the modular ability (no unnecessary cables cluttering my case).
 
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