How to Choose a PC Power Supply

One of the least exciting, but most important, PC components is the power supply. PCs run on electricity, of course, and that’s not provided directly from the wall to every component inside a PC’s case. Instead, electricity routes from the alternating current (AC) provided by the power company into the direct current (DC) used by PC components in the required voltage.

It’s tempting to buy just any power supply to run your PC, but that’s not a wise choice. A power supply that doesn’t provide reliable or clean power can cause any number of problems, including instability that can be hard to pin down. In fact, a failing power supply can often cause other problems such as random resets and freezes that can otherwise remain mysterious.

Therefore, you’ll want to give your power supply choice as much time and attention as your CPU, GPU, RAM, and storage options. Picking the right power supply will give you the best possible performance and help contribute to a lifetime of reliability.

Power output: How much do you need?

While there are several important factors to consider when choosing a power supply – just like with every PC component – identifying one of the most important factors is refreshingly simple. You don’t need to pour through benchmarks or read reviews to know how much power output you need. Rather, you can use a tool like Newegg’s power supply calculator to determine exactly how much power your new supply needs to output.

To use the tool, you need to select your components from the drop-down lists for each category. The tool above is up-to-date with the latest options for central processing unit (CPU), motherboard, graphics processing unit (GPU), random access memory (RAM), and more. While the tool doesn’t drill down into the details of every component, it does so where necessary and takes the guesswork out of deciding how much power you need.

For example, if you’re building (or buying) a PC with an Intel Core i7-11700K CPU, an Nvidia GeForce RTX 3070 GPU, 16 gigabytes (GB) of RAM made up of two 8GB sticks, a 1TB solid-state drive (SSD), and a 1TB 7200RPM hard disk drive (HDD), then 512 watts of power would be recommended. You could choose a 600-watt power supply to be safe – and buying a suitable option is just a click of a button away.

Anticipate upgrades when buying a power supply

Of course, you might want to run some scenarios to make sure you can handle your long-term needs. For example, upgrading to an Nvidia GeForce RTX 3080 GPU bumps the recommendation to 602 watts, while doubling the RAM makes no difference. If you think you might upgrade your GPU, then, then you’ll want at least a 700 watt PSU.

You get the picture. Don’t just plan to meet your needs today, instead look down the road a bit and think about what changes you might want to make later. And if you’re buying a pre-built PC, then you’ll want to make sure you know what power supply it uses to make sure it can handle whatever you might want to add – or that it’s easy enough to replace at some point.

An important note regarding power: continuous power and peak power are different things. Generally, the “Maximum Power” figure of a power supply refers to the continuous (stable) power the PSU will deliver consistently, while the peak power refers to the elevated maximum (surge) power the PSU can deliver, albeit for a very short amount of time (e.g., 15 seconds). When buying a power supply, make sure its continuous wattage meets your needs or you’ll likely run into problems when your PC is running a full load.

Finally, don’t be worried that buying a higher-rated power supply means you’ll necessarily be using more power. A power supply will only pull the electricity demanded by your PC’s components – and so while it might be a waste of money up front to purchase a larger power supply than you need, it won’t cost you any more to operate your PC because of it.

PSU Protection Features

Some power supply manufacturers will build in protections to help keep your components safe from power-related issues. These protections often add some cost to a power supply, but they can offer some additional peace of mind as well.

The first is overvoltage protection, which refers to a circuit or mechanism that shuts down the power supply unit if the output voltage exceeds the specified voltage limit, which is often higher than the rated output voltage. This protection is important since high output voltages may cause damage to computer components that connect to the power supply.

The second is overload and overcurrent protection. These are circuits that protect the power supply unit and the computer by shutting down the power supply unit when there is excessive current or power load detected, including short circuit currents.

Efficiency matters with a PSU

Wattage is just one measure of a power supply’s performance. Another is its efficiency rating, which is a measure of how much DC power it sends to the PC and how much is lost primarily to heat. Efficiency is important because it affects how much you’ll spend on keeping your PC juiced up.

As an example, consider a PC that requires 300 watts of power. If you use a power supply with an 85% efficiency rating, your PC will pull about 353 watts of input power from your power company. A power supply that’s only 70% efficient, on the other hand, will pull 428 watts of power from the wall. Choosing the more efficient power supply will save some cash on your monthly power bill.

At the same time, a power supply with a higher efficiency rating will allow your PC to run cooler as well. Every PC component generates some heat, and that tends to work against top performance. A more efficient power supply will dissipate less heat, which will mean a quieter system thanks to fans that don’t have to run as fast or as long, better reliability, and a longer lifespan.

What is 80 PLUS certification?

As you’re searching for power supplies, you’ll see many that carry 80 PLUS certification labels. 80 Plus is a certification program that manufacturers can utilize to provide some assurances that their power supplies will meet certain efficiency requirements. 80 PLUS has various levels ranging from the basic certification to Titanium, and power supplies are rated by independent labs to provide the following efficiency levels for consumer 115-volt power systems:

When you’re shopping for a power supply at Newegg, you can choose to filter by 80 PLUS certification level. That makes it easier to dial in exactly the level of efficiency that you want to achieve in your new PC.

Understanding +12V PSU Rails

Wattage isn’t the only measure of a power supply’s ability to support all of your components, however. Power is provided to the components by rails, and while each voltage rail requires attention, the most attention needs to go to the +12V rail(s) that provide power to the most power-hungry components, as the processor and PCIe video cards receive their power from them.

A modern power supply must output at least 18A (amps) on the +12V rail(s) for a mainstream up-to-date computer, more than 24A for a system with a single enthusiast-class graphics card, and no less than 34A when it comes to a high-end SLI/CrossFire system. The output amperage figure we're talking about here is the combined figure for PSUs offering more than one +12V rail.

Of course, it is the combined total output number you should look for, and you can't always add up the+12V rails to calculate the combined output. For instance, a PSU labeled with rails labeled +12V1@18A and +12V2@16A may only have a 30A combined power output instead of 34A. Look for this information in the detailed item specifications or on the PSU information label.

If you are going to run an SLI/Crossfire configuration, you should make sure the +12V rail(s) provide no less than 34A combined. Different power supplies are labeled differently – some show the maximum amperage provided by each rail, and some will provide the maximum combined maximum wattage, e.g., 396W, which equals 396W/12V = 33A.

Another important consideration is the number of rails a power supply uses to provide power to its components. Simply put, a power supply can provide just one +12 volt rail to provide all of the power to your PC’s components, or it can have multiple rails. Using one rail means that all the power is available to all components connected to it – that makes configuration easy because you don’t need to worry about matching components to rails, but it also means that a power supply failure, such as a surge, will affect all components. Conversely, having multiple rails gives some security against catastrophic failure but requires more care in setting things up.

PSU Form factor – Will your power supply

fit?

The next consideration is a simple one – you’ll need to pick a form factor that you’re sure will physically fit into your case. Fortunately, there are standards in terms of power supplies just as there are in cases and motherboards.

This topic can get quite complicated, but the important thing to remember is you’ll want to match your power supply with your case and motherboard. The following is a basic overview of the most important power supply form factors today.

ATX

Although there are still AT form factor power supplies available for purchase, AT form factor power supplies are undoubtedly legacy products, on the way out. Even the later ATX form factor power supplies (ATX 2.03 and earlier versions) are falling out of favor. The major differences between the ATX and AT power supply form factors are:

1. ATX power supplies provide an extra +3.3V voltage rail.

2. ATX power supplies use a single 20-pin connector as the main power connector.

3. ATX power supplies support the soft-off feature, allowing software to turn off the power supply.

ATX12V

The ATX12V form factor is the mainstream choice now. There are several different versions of the ATX12V form factor, and they can be very different from one another. The ATX12V v1.0 specification added over the original ATX form factor a 4-pin +12V connector to deliver power exclusively to the processor, and a 6-pin auxiliary power connector providing the +3.3V and +5V voltages. The ensuing ATX12V v1.3 specification added on top the 15-pin SATA power connector on top of all that.

A substantial change occurred in the ATX12V v2.0 specification, which changed the main power connector from a 20-pin to a 24-pin format, removing the 6-pin auxiliary power connector. Also, the ATX12V v2.0 specification also isolated the current limit on the 4-pin processor power connector for the 12V2 rail (+12V current is split into the 12V1 and 12V2 rails). Later, the ATX12V v2.1 and v2.2 specifications also increased efficiency requirements and mandated various other improvements.

All ATX12V power supplies maintain the same physical shape and size as the ATX form factor.

The EPS12V power supply form factor utilizes an 8-pin processor power connector in addition to the 4-pin connector of the ATX12V form factor (this isn’t the only difference between these two form factors, but for most desktop computer users, knowing this should be sufficient). The EPS12V form factor was originally designed for entry-level servers, but more and more high-end desktop motherboards are featuring the 8-pin EPS12V processor power connector now, which enables users to opt for an EPS12V power supply.

The Small Form Factor (SFF) designation is used to describe a number of smaller power supplies, such as the SFX12V (SFX stands for Small Form Factor), CFX12V (CFX stands for Compact Form Factor), LFX12V (LFX stands for Low Profile Form Factor) and TFX12V (TFX stands for Thin Form Factor). They are all smaller than the standard ATX12V form factor power supply in terms of physical size, and small form factor power supplies need to be installed in corresponding small form factor computer cases.

PSU Connectors

A power supply is useless if it won’t connect to and power every component in your PC. That means it needs to have all the required connector types.

The first connector to consider is the main connector that powers the motherboard. This connector comes in two types, 20-pin and 24-pin. The latter is increasingly popular, and it’s likely that your power supply will provide both options. Just check to be sure.

Next up is the processor power connector, which comes in 4-pin and 8-pin versions. As with the main power connector, many modern motherboards have switched to the larger format. Again, make sure your power supply is compatible.

The most frequently used power connector is the 4-pin Molex connector. It’s used for a variety of components including older HDDs, optical drives, fans, and certain other devices. Newer SATA components have their own SATA power connector, and you can also use Molex to SATA adapters if you run out of either. And, you can even use splitter cables to increase the number of components you can connect – but keep in mind your power supply’s upper limits.

Fan noise and cable convenience

Now that we’ve considered the most important power-related factors, there are a couple of other things to consider when choosing a power supply. These aren’t as vital, but they can affect how pleasant a power supply is to live with throughout your PC’s lifespan.

Fan noise

As we’ve already discussed, power supplies generate heat. That means they require fans to stay cool and run efficiently. You’ll want to give some thought to how quiet you want your PC to run, which will be determined a lot by your environment. If your PC operates in a quiet space, then larger fans that spin more slowly to move the same amount of air will likely result in a quieter PC.

There aren’t any real standards around power supply cooling, and so you’ll need to compare marketing materials for your power supply options. This is one area where in-depth reviews will be particularly helpful, as they tend to measure how loud a power supply is during various levels of operation and so offer some guidance as to how loud you can expect your PC to run.

Cabling

Finally, there are three basic types of power supply cabling. Whether you choose a hard-wired, modular, or hybrid system will determine how clean the inside of your case will be and how much work you’ll need to put in to keep your PC uncluttered and organized.

Hard-wired cabling means that every connector is directly connected to the power supply and so will be present whether needed or not. The advantage – and it’s a small one with modern power supplies – to hard-wired systems is that it’s both simpler and doesn’t impose any additional resistance with extra connectors.

Modular cabling means that each connector can be added in as needed. That makes it easier to keep your case clean and uncluttered, but it also introduces some additional complexity – and price – and some additional resistance thanks to extra physical connections. That’s most likely irrelevant for most users, though.

Hybrid systems have some cables, such as the main power connect, physically connected and the others being optional. A hybrid system can present a good compromise, in that certain cables are required and even if the additional resistance of modular connections is minimal, it’s easy enough to avoid.

Time to power up

Obviously, there’s a lot to picking a power supply, and it’s an important decision in putting together a new PC. But spending a little time up-front to make sure your power supply provides your PC components with reliable, consistent, and safe power will save you huge amounts of time in the long term, and it will help make your PC a better and more efficient machine.