How to Build a PC - The Power Supply

By Jonny Gerow, on 28 ottobre 2013

Selecting Components: PSU

Selecting a PSU is more than just knowing how much power you'll need to power your build, but also what features best suits your build and building preferences. There are also a number of features that you might want in a power supply like a Zero RPM mode that allows the PSU to be completely silent, Corsair Link compatibility that allows you to monitor your PSU, modularity that allows you to remove unused cables and different levels of efficiency that can save you money on your electric bill.

What's going to fit in my chassis?

Most chassis are standard ATX form factor. And while there are also chassis made for micro ATX and mini ITX motherboards, they will typically take a standard ATX power supply as well.

A standard ATX power supply always has a rear dimension of 150mm x 86mm:


What can vary is the depth of the unit:

So first thing's first: check to see how much room you have for a power supply. Take into consideration that cables will need to come out of the front of the power supply unit and that you'll need access to those cables.

The next thing that often gets asked is "what should the orientation of the PSU be?" In most pictures, you'll see the fan of the PSU facing up, but the fact of the matter is the PSU can be mounted with either the fan pointing up or the fan pointing down. So this shouldn't be a concern as long as the intake fan is not obstructed.

Non-modular, semi-modular or fully modular?

The modularity of a power supply refers to what cables, if any, can be removed from the PSU housing. If a power supply is non-modular, that means none of the DC output cables can be removed. For a lot of people this is perfectly fine. You may need all of the cables or you have somewhere to hide excess cables. But when a power supply is modular, meaning you can remove certain DC cables from the housing, you don't have to worry about hiding unused cables. Why hide unused cables? Well, for one, it looks good. The other benefit is airflow. Every extra piece you have running across a path of air inside the chassis is going to disrupt that airflow; even if it's something as small as a power supply cable. When you have a semi-modular power supply, typically the cables that cannot be removed are cables that every user is going to need no matter what kind of PC they're building. Take the modular CX series, for example:

The CX500M pictured above has two fixed cables: the 24-pin, which any standard ATX motherboard requires, and the 8-pin that's used to get additional power to the CPU. Other than these two, every other cable is modular. Meaning: every SATA, Molex or PCIe cable can be added or removed depending on the machine being built that this power supply is going to power.

A fully modular power supply like the RM Series (that's the PSU that's shown at the very beginning of this blog post) has NO fixed cables. The best part of this is during the initial building of the PC. You can bolt in all of your hardware, including your power supply, without any cables to move out of your way. Then, as a final step of your build, you can add whatever cables are needed and hide them any way you can. Even hiding the cables are easier because you can leave the cable detached from the PSU and whatever you need to power until you're done hiding it.

If budget is a concern, keep in mind that modularity isn't always free. That CX500M you see up there sells for about $70. If you need more power, but only have $70 to spend on a power supply, you can get the CX600 that's non-modular instead. So make sure you prioritize!

How much power do I need?

Now that you know what physical size can fit in your chassis, it's time to consider how much power you'll need. The first thing to remember is that a computer's power supply only puts out as much power as required of it. So even if you have a 1000W power supply, if your computer only needs 350W, the power supply is only going to put out 350W. That's not to say you should get the biggest PSU possible even if you know your computer won't use that much power, but it's better to get something bigger than what you need than something that's barely big enough.

First, let's try to figure out how much power we need. This actually isn't too difficult. Both CPUs and GPUs are given a factor called Thermal Design Power, or TDP. This number refers to the maximum amount of heat watts a cooling system must be able to dissipate and keep the CPU or GPU at or under its maximum operating temperature. You can easily determine the maximum power draw of your CPU and GPU using a very basic Google search. Even Wikipedia has tables that list most of the CPU cores out there, so I find it to be a good reference.

The six core CPUs shown below are powerful and consume a lot of power at 100% load. The AMD Thuban 6-core based Phenom II CPUs (left) and the Intel Gulftown 6-core based Core i7 CPUs (right) are rated at a TDP of 125w and 130w respectively according to their manufacturers.

Of course, the CPU is not the only component on the motherboard which dissipates power. We should go ahead and allow 75W for the components on the motherboard itself. While this number may vary, 75W is a nice conservative number, and is an adequate ballpark estimate to use when selecting a PSU.

Modern GPUs can pull tremendous amounts of power. Some dual GPU cards are rated at almost 400w for their TDP. An Nvidia Titan is rated at 250w TDP, while a Radeon HD 7990, which has two GPUs on one card, has a TDP of 375W!.

Again, TDP information is readily available on the web. Type "Nvidia Titan TDP " or "Radeon HD 7990 TDP " into Google or Bing and you’ll see what we mean.

Do you plan to use two or more GPUs using SLI or CrossFire? If so, multiply your GPU TDP accordingly. Also make sure you count the number of PCIe power connectors you'll need for your graphics card. If you have two cards that each require two PCIe power connectors, a PSU with only two PCIe power connectors isn't going to work unless you use adapters.

Most of the other items that go into a computer consume relatively little additional power. Most of these components have a power requirement on their label or you can pull it up on the manufacturer's website.

Now take your numbers and add them all up. Since this number is the maximum power requirement of all of your components added up, you're not going to need a PSU that large, but that doesn't mean you shouldn't consider one that large, or larger, if the budget allows. Why?

Typically, your power supply's maximum efficiency is between 40% and 60% of it's maximum capability. Have a look at this graph which represents the efficiency of an RM750 from 20% to 100% load:

Here we can see that peak efficiency is right at 50% load with just over 90% efficiency with 115VAC mains.

Another thing to consider is that you're probably only going to be near full load when you're playing games. Most of the time that you're using your computer, you're going to be around 90 to 120W load for the most part,so you don't want to go too crazy. So using the graph above, we can see that we're about 88.5% efficient at these low loads, which is still better than the 87% efficiency we see at full load.

Another thing to consider with Corsair power supplies is when the fan will actually turn on. In a typical 25°C (77°F) room, an RM750's fan will turn on at 40% load (300W). Once there, the fan typically spins at under 700 RPM as long as you're under 60% load (450W).

Here's a look at the fan noise curve for the RM750:

Now let's say that what we calculated earlier determined that our maximum load is going to be 450W. So do we want a 450W power supply? Let's have a look at the efficiency curve:

At 50% and 100% load, the 450W is a more efficient power supply than the 750W at 50% and 100% loads. But at 100% load, the 450W is still less efficient than the 750W. And at 120W, the 450W is only just as efficient as the 750W at 120W.

And what about fan noise?

The fan in the 450W stays quieter, longer than the 750W's fan, but if we've calculated our maximum load is going to be 450W, our fan is going to be louder than the one in the 750W at everything over 370W. It's not even fanless between 180W and 300W, while the RM750 would still be without fan noise at loads up to 300W. So if I were to select a PSU because I wanted the quietest and most efficient power supply, I'd actually select the 750W.

What about the HX, the AX or an AXi, and what about different efficiencies?

There's no doubt that Corsair has a number of power supply product lines to choose from. While the RM and HX Series are both 80 Plus Gold rated, on the surface one can consider the RM Series a better choice because it is fully modular while the HX Series are only semi-modular. But the HX is rated at 50°C, as opposed to 40°C, and uses all Japanese capacitors, so the HX is more robust than the RM. It has tighter voltage regulation and a 7 year warranty, as opposed to a 5 year warrranty. So while the RM is a fantastic power supply, the HX is that much better. And why would you want a better power supply? Fortunately, there's a blog post about that!

There's also the matter about efficiency. 80 Plus Bronze versus Gold. 80 Plus Gold versus Platinum. Each level of efficiency requires a different degree of efficiency under different load conditions. 80 Plus Gold, for example, means the PSU is at least 87% efficient at 20% load, 90% efficient at 50% load nad 87% efficient at full load. 80 Plus Platinum is 90%, 92% and 89% efficient at 20%, 50% and 100% loads, respectively.

While the return on investment is arguable (and I've provided some examples here), when a power supply is more efficient, it not only uses less power from the wall, but it also generates less heat. Less heat means the fan in the power supply doesn't have to run as much or as fast and therefore the power supply is generally quieter.

And then there's Link

How can you call yourself an uber-nerd if you can't tell me exactly how much power your PSU is putting out while playing Battlefield 4 versus playing Candy Crush Saga? Huh? Well, with Corsair Link you CAN! The Link software monitors and allows some control of various Corsair components. Temperatures and fan speeds can be monitored throughout your PC, as well as the ability to change the colors of little light strips you can stick around the inside of your case. With the RM series of power supplies you can see how fast your fan is spinning and how much juice your +12V rail is putting out (and since the +12V rail is 95% of what your PC uses, you have a pretty good idea of how much power you're using. The AXi power supplies give you a lot more monitoring and control. You can monitor the +12V, +3.3V and +5V, total power out, total power in, the internal temperature of the PSU as well as the speed of the fan. You can even change the speed of the fan. For example: if you allow your PSU fan to always spin, it may not spin as fast at higher loads because you're maintaining air circulation.

In conclusion...

Hopefully I've shown you today that you can narrow down your power supply choices starting with two criteria: how much total power is the power supply capable of and how loud does it get at different loads? And remember, a larger power supply does NOT mean you're going to use more power, so if the budget allows, you could walk away with something quieter than what a smaller sized PSU can offer. From there you can figure out how efficient you want your power supply to be by considering how much money it's going to save you on your electric bill and how much cooler the power supply is going to run, which can also determine the sound profile of the unit. And don't forget to take into consideration features like modularity and the monitoring ability of the Corsair Link software.