What Power Supply Do I Need? How to Choose a Power Supply

Power supply units are the backbone of any PC. These devices pull power from your wall outlet and distribute it throughout your PC. Without the right amount of power, your machine will not function how you need it to — and it may not even turn on. Besides supplying power, a PSU can also provide fail-safes for misplaced currents or protect components during a PC crash.

The type or form factor of a power supply will tell you key features about the unit, including its size and the features it supports. Knowing these common form factors can help you choose the power supply you need. These tips can help ensure your power supply will fit in your case and operate as efficiently as possible:

In the past, ATX power supplies featured a single 20-pin connector as the main power to the motherboard, but they now feature a 24 -pin power connector (which is still compatible with older 20-pin motherboards). ATX power supplies also house an extra rail that is 3.3V. An example of a feature supported by ATX is soft-off, which allows specific software to turn off the power supply.

Shop ATX PSUs

ATX12V specification has surpassed standard ATX technically, but both power supplies share the same physical shape and size. Currently the most common form factor, ATX12V power supplies come in several versions that have been upgraded over time. ATX12V 2.1 and ATX12V 2.2 are the latest versions. These versions feature a 24-pin main power connector and have been tuned to maximize efficiency.

Shop ATX12V PSUs

Over the years, there have been a number of small power supplies designed for compact cases. These power supplies are smaller than the standard ATX but still get the job done and can be perfect for more economic builds concerned with desk space. Examples of SFF power supplies are the CFX12V (Compact Form Factor), LFX12V (Low Profile Form Factor), and TFX12V (Thin Form Factor).

Shop SFF PSUs

Designed to create a more stable environment for servers, EPS12V power supplies feature an 8-pin CPU power connection in addition to the 4-pin that is standard in ATX12V units. Be careful when choosing your power supply, however, as most common home PC components are NOT compatible with EPS12V form factor power supplies.

Shop EPS12V PSUs

In addition to form factor, it’s important to understand the differences between the various connector types featured on power supplies. These are the ends of cables that come from your power supply and plug into all your different components. Each of your PC’s components uses specific cables. Knowing which connections your PSU supports and which cables to use to connect your components to your PSU can speed up the build process and help to prevent any accidental damage. When you choose your power supply, check for compatibility with your other components. Here’s what you need to know:

These cables provide power to SATA storage devices like 3.5" hard drives. Multiple SATA connections are often found on one cable to eliminate the need to run excessive wiring. Recently, SATA power has been adopted by other devices such as RGB hubs or fan controllers as well.

While the CPU is directly installed on the motherboard, the 24-pin connection does not supply enough power for modern processors and other motherboard features. For this reason, 4/8 pin connections are now used to power CPUs.

6/8 pin GPU cables connect to your graphics card or GPU. Different strength GPUs require specific configurations. For that reason, 6/8 pin connections often come with multiple configurations on a single cable, such as 6, 8, 6+8, 8+6, and 8+8.

Each motherboard utilizes power differently. 24-pin motherboard cables typically power motherboard features like PCIe and other components directly connected only to the motherboard.

Rarely seen in modern systems, 4-pin Molex connectors are now considered legacy hardware. Typically replaced by SATA connections, Molex 4-pin cables can still be found on some liquid cooling pumps.

Rails are what the PSU uses to supply power through cables to the PC's components. Power supplies can have one or more rails, and individual rails can have different amperage outputs. The rails on modular and semi-modular PSUs will look like rows of ports or cables that plug into the various PC components.

While discussing rails and amperage can get incredibly technical and detailed, there is one key fact to remember: a single +12V rail can provide power for all your PC's components without issue. The major difference is that when a single a rail system fails, every component is at risk. When a multi-rail system fails, however, only the devices connected to the failing rail are at risk, potentially saving other costly components.

To put it plainly, multi-rail power systems can provider better protection for components from stray currents in the event of a failure.

Knowing the efficiency specifications of your power supply can save you money on your electric bill. Efficiency is so important when choosing a power supply that the entire industry follows the same standard, called 80 plus. An 80 plus rating tells you valuable information about the device and its capabilities. The lowest tier of 80 plus certification verifies that the power supply provides 80% of its total wattage to your system while losing 20% to heat. As the rating goes up, so does price and overall efficiency.

80 plus bronze is sufficient for most builds. Only when a high-end GPU and CPU are being used is a higher efficiency rating truly needed.

Making sure your power supply fits inside your case and has the proper cables to power your components is essential. However, secondary factors that are not critical can greatly impact your satisfaction with your PSU as well. 

Fan noise is an issue for many users. When a power supply starts outputting significant wattage, it heats up. Excessive heat will trigger a significant increase of the fan speed of the PSU, and it can get incredibly loud. Paying attention to factors like efficiency and wattage limits, you can find a PSU that will power your system without sounding like a leaf blower.

Connectors on a PSU are critical to its function, and the cables connecting your PSU to your computer’s other components have characteristics that can help simplify cable management and personalize your PC. All power supply cabling comes in three styles: modular, semi-modular, and pre-installed.

• Modular power supplies save space by allowing users to connect the minimum amount of cables for each device. The modular design helps with cable management and adds an option for personalization with custom cables.
• Pre-installed or non-modular PSUs have all the necessary cables pre-installed on the power supply, and they are NOT removable. These units can be more convenient to build with but can often lead to dangling cables or unnecessary connectors causing clutter in the case.
• Semi-modular power supplies are a combination of the two styles mentioned above, with some of their cables pre-installed but space left for users to add more as desired.

Now that you know all the key terms and factors to pay attention to when purchasing a PSU, it's also important to consider what your machine's daily workload will look like and how much power that will require.

Once you know the core components you will be including in your build, you can use a third-party tool to add and double-check their power consumption. Now that you know how much power the machine is going to use, you can begin looking at PSUs and narrowing your choices by wattage and efficiency.

Different equipment pulls considerably different amounts of power, so be careful. Say you need to build a workstation PC that utilizes a powerful CPU and GPU. Those components will draw significantly more power than a machine with integrated graphics and will thus require more power from the PSU. Conversely, if you need to build a small form factor PC for basic office use, a less powerful PSU can save you money while still getting the job done.

Power supplies can make or break a build. While not as flashy as other components, selecting the right PSU is crucial. When choosing your PSU, details and specifications are of the utmost importance, and you should always double-check your power supply and other components to ensure they are compatible. Here’s a quick summary on how to choose your power supply:

• Determine the amount of power your CPU, GPU, and other devices will need to run properly.
• Ensure your PSU connections match your other components.
• Double-check to make sure your PSU will fit in your case.
• Consider long-term factors like acoustics, cable management, and upgradability.

Purchase the perfect power supply for your needs at CDW today!

Shop PSUs