Corsair Blog

In our effort to properly assist our customers in selecting a Corsair power supply unit, or PSU, the most frequently asked questions by far deal with how and why to make certain selections. Let’s face it, computer enthusiasts are definitely an educated and inquisitive crowd so the old "because I said so" type answer we got from Dad as a kid just doesn’t cut it. Of course, we’d love to sell everyone on the planet an AX1200i Digital ATX Power Supply, but that’s also not the correct approach.

In order to help our customers intelligently select the ideal power supply for their system, we developed our PSU Finder. It is a very simple tool that allows a user to enter important criteria relevant to their systems to determine the best PSU for them. Click on the link to access the Corsair PSU Finder.

Corsair PSU Finder

The PSU Finder is very streamlined and easy to use. However, many enthusiasts want to know how to evaluate their system for themselves. Many of the PSU selection questions we field can be addressed by understanding some basic fundamentals. This how-to guide will discuss these fundamentals, and help you to choose the PSU that is ideal for the system you are building.

PSU Selection in Four Simple Steps

The four steps for selecting a PSU are very simple. The first step is to estimate the power needs of your CPU and motherboard. Next, determine how much power your graphics solution will require. Then, factor in additional components, and overclocking impact. Finally, determine what wattage PSU you will need by totaling these power requirements and factoring in some efficiency considerations.

Just a little bit of online research will tell you all you need to know to determine your power requirements. We’ll show you how to use this information to select a PSU that will meet your needs perfectly.

Step 1: Determine CPU/Motherboard Power Requirements

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.

The current six core CPUs shown here 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 typically 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.

So, assuming that we are using the AMD, 6-core Phenom II CPU shown above, the power allowance of the CPU/Motherboard combination will be 125W for the CPU plus 75W for the motherboard, or 200W.

Step 2: Calculate GPU Power Requirements

Modern GPUs can pull tremendous amounts of power. Some dual GPU cards are rated at almost 400w for their TDP. The AMD Radeon™ HD 5970 (left) and the NVIDIA GTX480 (right) both consume large amounts of power under full load. They are rated at 294w TDP and 250w TDP respectively.

Again, TDP information is readily available on the web. Type "HD 5970 TDP" or "GTX480 TDP" into Google or Bing and you’ll see what we mean! Or, you can go to the Geeks3D GPU TDP Database for a complete listing of TDP for most GPUs.

Do you plan to use two or more GPUs using SLI or CrossFireX™? If so, multiply your GPU TDP accordingly! Also make sure you count the number of PCIe power connectors you need. 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 score some adapters. The Corsair PSU Finder also takes this into consideration.

For our hypothetical system, let’s select a single GTX480 GPU. Now our power requirements are 250W for the GPU, plus 200W for the CPU and motherboard, for a total of 450W.

And what about "Haswell Compatibility?"

During the Spring of 2013, we heard a lot about Intel's new "Haswell" Core CPUs and whether or not power supplies will be compatible with Intel's 4th generation Core processors' C6 and C7 sleep states.

At Corsair's labs, we tested a number of Haswell based PCs with every Corsair power supply.  All of Corsair's power supplies work with the Haswell platform, but certain combinations of components can cause the power supply to shut down unexpectedly when the PC goes to sleep or may prevent the PC from coming out of sleep mode.  In the DIY world, it's almost impossible to guarantee 100% compatibility with 100% of the possible build combinations out there.  But fortunately, if a problem is encountered, disabling the C6 and C7 sleep states in the BIOS solves virtually any problem.  Of course, if you want to ensure 100% compatibility with Haswell's low power sleep states, any Corsair power supply from the CX750 on up will function properly.  For more information about compatibility with these low power sleep states, please see our blog post here.

Step 3: Factor in Other Components and Overclocking

Most of the other items that go into a computer consume relatively little additional power. We’ll list a few parts here as an example to finish out our power rating. The following components make up a fairly well equipped gaming rig:

• 5 fans , which typically consume 3-5w each
• 1 Corsair SSD, which consumes a whopping 2w maximum
• A "green" type 500GB  HD which typically consumes a maximum of 10w
• A CD/DVD drive which typically consumes a maximum of 30w

This totals 67W. To keep our numbers simple, lets allow 75W.

Of course, overclocking a CPU or GPU can easily add 50w-100w or more of power consumption (each!). Why build a system that doesn’t have the ability to support a little overclocking?!? We’re going to add 50w each for a CPU and GPU OC to account for a moderate overclock of each.

So, now our power requirements look like this: 200W for CPU/motherboard, plus 250W for GPU, plus 75W for other components, plus 100W for overclocking. This totals up to 625W.

Step 4: Determine Target PSU Wattage

We recommend that you select a PSU that will meet your system’s target power requirements when running at 50 to 80% of its rated wattage. Why? The reason is that PSUs are most efficient in this range, and therefore will have quieter fan operation. The additional capacity also gives you some ability to upgrade with confidence later on, whether it is adding an additional hard drive or two, or overclocking more aggressively.

What Efficiency Do I Need?

Corsair PSUs typically operate at 80% efficiency or higher, and are certified by 80 PLUS®, an industry organization. Some PSUs are more efficient than others — our Platinum Certified AX and AXi power supplies are greater than 90% efficient! The more efficient a PSU is, the less energy is wasted in the form of heat. And, the less excess heat, the quieter a PSU is likely to be. Highly efficient PSUs are more expensive to manufacture, however, so they generally come with a higher price tag.

Modular or Non-Modular?

Some PSUs have their power cables permanently attached. This is the least expensive way to manufacture a PSU, and offer excellent price/performance.

Non-Modular PSU

We also offer modular PSUs, where many (or, in some of our model series, all) of the power cables are detachable. These modular PSUs allow you to connect only the cables you need, which makes system assembly easier, and offers great cable management advantages.

A semi-modular power supply is going to have at least the 24-pin and the 8-pin CPU power connector attached. Sometimes one or two more cables as well. These are cables that any computer is going to use, so it's not ever necessary to run the power supply without them.

Semi-Modular PSU

A fully modular power supply allows you to disconnect all of the power cables. While a particular cable, like the 24-pin, may never need to be removed to operate the computer, having all of the cables removable allow for easy installation and removal of the power supply, and also allows ease of cable routing. It is much easier to snake a power cable under the motherboard or behind the motherboard tray, when it is not connected to anything on either end.

Fully-Modular PSU

Our System Example

So far we have determined the total wattage we need, and thought about some of the key features we want. The determinant choice is usually made based on what type of system you are building. For the 781 watt requirement that we have identified, some excellent choices are listed below. Since the numbers we used are the upper echelon of every component running at 100%, it's safe to round down to 750W. Of course, we could round up to 800W as well.

The Price/Performance Build

For the user that is looking for features combined with, a non-modular 750W PSU would be a solid choice. A CX750 would fit into a wide range of budgets while staying in a good operating range for heat, efficiency, and noise.

The Overclocked Rig

A user that is into serious gaming or frequent overclocking could make great use of an efficient, modular PSU. The Professional Series™ HX750 will provide overhead for heavy usage, demanding gaming, and moderate overclocking, while operating efficiently and quietly.

The "Extreme" System Build

For the ultimate in expandability, you want a PSU with tons of headroom. The AX1200i Digital ATX Power Supply is ultra-powerful and ultra-efficient, with fully-modular cables that are ideal for multiple GPUs. This PSU has what it takes for the ultimate extreme performance system.

Conclusion

The  easy solution when selecting a PSU is simply to buy the absolute largest PSU that fits your budget and into the case you are using. And since a power supply only produces as much power as demanded of it, there really is no such thing as "too much power". However, for most people this type of shotgun approach is not always the best one. It is technically possible to have a PSU that has too much power if you have such a large unit in your system that your greatest load is well below 20% of the power supply's capability. In cases where budget is a concern, proper selection of the PSU is essential so that you do not spend any more than necessary. So, you would not necessarily want to put an AX1200i into your HTPC and there are far better choices from our lineup for such an application.

So, an informed and intelligent assessment of your power consumption needs is the place to start when selecting the ideal PSU for your PC. Once you know the size, it’s then a matter of balancing features, aesthetics, and your budgetary concerns.

And, going back to the PSU Finder, how accurate are those results? Well, our very own Jonny Gerow runs an overclocked Core i5 with three overclocked GTX670 cards and two hard drives. When that information is put into our PSU Finder, the AX860i is one of the suggestions. If you have a look at his blog post where he monitors the power consumption of his PC during different functions, you can see that he hits 771.6W by running Prime95 and Furmark simultaneously! I'd say the PSU Finder did a pretty good job of determining what kind of PSU Jonny should use!

We hope this guide will be useful to our customers contemplating a PSU purchase. Insert this information into your PC tool box and use it to select the ideal PSU for your system.

Additional Resources:

Corsair Support and Discussion Forums

Geeks3D GPU TDP Database