Buying a PSU

[Computer-Ed]

Buying a Power Supply

When people build a PC they put a great deal of thought into the various components that will be in the computer. The processor, motherboard, ram video card and hard drive all are considered, brands and qualities weighted to find the best and most efficient way to build the PC and meet the needs in mind. Even the computer case will many times receive a great deal of attention and thought, but the power supply almost always ends up being put in almost as an afterthought.

Oh sure people will try to get a big PSU but they usually do not really understand why they make the choices they make, they see a number, some buddy said it was good and in it goes.

Over the last week I have a chance to speak with some folks at Enermax, Antec and Thermaltake. We discussed power supplies, how they really work and what to look for when making your purchase.

Let’s begin with the most easily identified information on the power supply the wattage. This number is the theoretic maximum power output of the power supply. For us to make an informed choice we first need to determine just how much power we really need.

Antec has a link to an online power calculator that does a good job of helping you determine your power needs.

eXtreme PSU Calculator

Using this calculator and knowing what our components will be it is easy for us to get an accurate feel for our PSU needs. Be sure when you put your information in the calculator that you enter the CPU Utilization TDP to 100% and the system load to 100%. This will ensure that the calculator will return the maximum need of your system under FULL load.

With this number in hand we now have a good indicator of what the maximum need of our system will be, but that is not where we want to stop. A PSU works best in a range from roughly 35% to 70% of its peak power. This is the best efficiency range for your PSU.

Now in theory our best solution here is to do some quick math and more calculations for an accurate number to be given. To simplify everything however the safe bet is to make sure the PSU is rated 50% higher than the rating the calculator gave. Let’s say for example the Calculator told you to get a 400 watt power supply, we would therefore get a 600 and make sure our peak usage was still in the efficient range and we had some headroom for upgrading as well.

Now that we have an idea of the wattage we want it is time to decipher all the other information we can find in order to find our best PSU. After wattage the second most important stat to look at, in my opinion, is the efficiency rating. The efficiency rating is the percentage of power your PSU takes from the wall and actually converts into DC power for the computer, the remainder is lost to heat. This number therefore will not only tell you how a PSU will make use of your power and possible cut its cost to operate, but will also give an idea of how much heat the PSU is going to put in the case.

Picking an efficient PSU is now a SIMPLE matter, look for 80Plus certification. This certification is given by a third party; the company cannot make the claim, they have to receive it from independent testing. For the consumer this means that the company cannot use tricks in calculation to make their system seem more efficient than it is.
80Plus means that the system has been verified to be over 80% efficient at power levels ranging from 20% load to 100% load. This will result in a savings on the electric bill as well as reduced heat in the system. Now let’s be real for a moment, the savings we are discussing is not all that huge, in the case of a typical PC in the home we are seeing an average savings of about $50 per year on the light bill compared to a generic PSU.

However 80Plus does not just stop at the efficiency, it requires the use of Active PFC. Now there are all sorts of long explanations of what PFC is about but for American users these explanations mean little. Let’s keep it simple and say that with Active PFC the PSU is even more efficient in power usage. So much more so that due to the Active PFC that is in 80Plus certification they are claiming that the number of PC that can work on a single circuit will increase by 50%. That means if right now at a LAN party you can get 4 PCs to run on a single circuit with Active PFC on ALL the systems you could in theory get that number to 6.

For purposes of this guide then we will recommend that you only use PSUs that have 80Plus certification.
The next consideration we will explore is cooling. There are a lot of options out there, passive cooling, single fans in back, 120 mm fans on top or bottom and an 80 in front and back. Is there one method that is better than the others? After speaking to various companies about PSU design I can tell you the general consensus is no. The 120mm fan is the current fashionable approach but in the newer PSUs coming down the line with 80Plus the fans are being moved to the single 80mm exhaust. The reason is simple the PSU generates a lot less heat since it has high efficiency.

Some will still swear by the 120 fan system but in fairness I think a lot of those people do so because they also believe it helps cool their computer. The PSU fan should not be needed to help cool the PC. A well designed case has at least a single 120mm exhaust fan which for most PCs is plenty of exhaust power. The fan in the PSU should have one job, keeping the PSU cool.

Now all that being said the point I am trying to make for this guide is that the fan system is NOT important for determing the best PSU for you with the exception of one that will provide good cooling for your PSU when compared to your case design.

Next we have the war that exists in the PSU community, a single 12v rail vs. multiple 12v rails. I was amazed at all the different answers on this I got. On the multi side you have the argument that this will keep any one component from overdrawing on the 12v side and possibly frying the system. I must admit I had never heard this argument before but it does make sense. When a component fails it will sometimes overdraw power and could fry other components. The multi 12 volt rail system limits the potential for this. The other reasoning I heard for multi rails is that the old standards system placed a limit on the rails and this was done to stay within the certification. The single rail side argues that you lose power by using a multi rail system and the single rail system will better be able to meet the needs of some of the new parts coming down the line.

With the current computer parts out there I have yet to see a PC that would not work correctly with the multi rail system. I will not say that in the future this might not change but I seriously doubt we will see anything that will require a single rail for the 12volt in the near future. As such my recommendation is not to be too concerned with this; either type of PSU should serve you well.

The final issue of consideration is modular cabling vs. fixed. Again we have two sides to this camp but they are not quite so diverse. Both sides agree that modular cabling can result in a power lose in the transfer to the modular plug. However all the companies I have spoke with have stated that the lose is so minor as to have no effect on the actual use of the PSU. Additionally a modular design offers some flexibility. You can use just the cables you need and thus keep cable management under control easier. Additionally you have more options with the cable choices. Let’s say you need a few more SATA power cables. With a fixed cable PSU you will have to put on adapters to fix this problem. With a modular system you could just put on the SATA plugs and remove any other plugs you did not need.

With 3 of the best PSU companies’ out there agreeing that modular does not do enough harm to be an issue I am inclined to agree. As such my recommendation is to buy a modular plugs PUS if it meets the other requirements we have discussed. The convenience of the modular system makes up for any potential shortcomings due to how minor they are.

By taking the time to weigh these factors we can find a PSU that will run our system most efficiently. This is a lot more important than people make it out to be. A good PSU can increase your system stability, reduce heat in the system, save on operating cost and possible allow you to avoid repairs on other components.

The next time you prepare to build a PC or want to buy a replacement PSU, take the time to look over these factors, make an informed purchase. It will usually pay for itself quickly, not just in energy savings but in system stability and longevity.

[Death_blooms]

I am one of those people who forget about the PSU until last minute. Next time I build I will be using your guide.

Some good info there

[Computer-Ed]

Thanks, I was amazed as I researched just how little many people look at the PSU and what we miss out on to make a good and effeicent choice.

[Dread]

People seem to have a stigma about spending money on a quality power supply. Guess it just isn't glamorous compared to the other system components. I've been saying it for a long time, don't skimp on the PSU.

Buying a crappy/cheap PSU for a good system is like putting a 4 cyl engine in a Ferrari 599...just doesn't make sense. But people do it all the time and then complain when they have problems.

I would rather save money elsewhere, than skimp on the PSU.