How important is the power supply (PSU) when building a PC?


The power supply (PSU) is an essential part of any PC. It powers every component in your PC, and a bad or faulty one can bring it all down. Here’s what to look for in a power supply when assembling a PC.

A power supply is an essential part of the PC infrastructure

When we turn on a light, turn on a tap, or walk down a well-paved street, we don’t often think of the wonderful infrastructure that makes it all possible. But if someone hadn’t taken the time to think about it, things wouldn’t be so wonderful.

It’s the same when building a PC. We are obsessed with the number of cores in a CPU or the number of compute units in a GPU. But we rarely think twice about the power supply unit (PSU) that powers everything else in your PC.

You don’t need to think too much about your power supply, of course. But, if you don’t consider the PSU at all, chances are you’ll think about it a lot once it starts causing problems.

If your computer is not getting enough power or if the power supply is malfunctioning, a number of issues can arise. Your system may fail to boot, the entire system may become unstable, or simply shut down when power demand exceeds capacity. It is also possible for more expensive components to be damaged by instability.

The good news is that you don’t have to go into too much detail when choosing a good diet. There are many tools online that will help you determine the right type of power supply for your build.


A black box with 1000W written in gold letters.

As cores are for a processor, power is for a power supply. This is the main feature people look at because it tells you how much power a PSU can deliver. A good rule of thumb is to shoot about 25% or more of the headroom against the expected output for your PC. So if your expected maximum power is 400 watts, a 500W or 550W PSU would run it smoothly and provide some future proofing if you ever upgrade your PC with a component that needs more power.

So how do you determine your expected output? You can use a site like PC Part Picker, which will show you the expected wattage requirements based on your components. There are also many online power calculators, such as those from Newegg and Extreme Outer Vision, the latter being a popular choice. Don’t be surprised if each calculator offers a different recommendation as these are only estimates. Newegg’s tends to be a bit upscale, for example.

One last thing before leaving this topic is that you may come across people who talk about the importance of rails in a power supply; however, it is not as important as it once was. If you want a quick breakdown on the subject, check out this Techquickie video.


The number 80 above the Plus world enclosed in a rectangle with rounded edges.
The 80 Plus logo

When you look at power supplies, you’ll see they have 80 Plus ratings named after different metals, including bronze, silver, gold, platinum, and titanium. There is also a simple 80 Plus rating with no metal name attached to it.

These are efficiency and reliability ratings. 80 Plus means the power supply is 80% or more efficient at loads (power demands on the power supply) of 20%, 50%, and 100% at 115 volts and 230 volts. Efficiency requirements change with capacity and voltage, and the more valuable the metal name in the classification, the more efficient the power supply must be.

For example, at 50% load and 115V, an 80 Plus Bronze PSU is expected to achieve at least 85% efficiency, while a Titanium PSU at this load and voltage should be 94% effective.

Part of the reason these efficiency ratings were developed is that, like most things electrical, power supplies aren’t 100% efficient. In fact, they lose energy in the form of heat. That doesn’t mean, however, that a 400W PSU can’t deliver that much power, as the wattage on the case tells you a PSU’s maximum throughput.

You can find a list of 80 Plus certifications online, but the important thing to note is that the higher you go on the “value” scale for each metal, the more efficient the power supply is supposed to be.

In general, you want a power supply that is as efficient as you can reasonably afford at the power you need.

Other important features of a power supply

The different plug options on a semi-modular power supply.
A semi-modular power supply from EVGA. EVGA

Whether you’re building your own PC or just replacing the PSU on an older machine, there are a few more important considerations. The first is the question of a modular, semi-modular or non-modular power supply.

A modular power supply comes without any cords and is usually referred to as “fully modular” in online stores. With a fully modular power supply, it’s up to the PC builder to connect the cords they need. A semi-modular power supply, on the other hand, has a number of cables that are not detachable. These are usually the essentials, such as the 24-pin power cable, CPU power connector, etc. Everything you need can be added like with a modular power supply. When shopping, semi-modular power supplies may be called “modular” or simply “semi-modular.” Finally, there are also non-modular power supplies with all cables permanently connected to the power supply.

The biggest advantage of a fully modular power supply is that you control the number of cables hiding in the back of your PC. Even with a semi-modular unit, you can end up with excess cables that aren’t connected to anything and take up space on the back of the case.

Non-modular PSUs tend to be a bit cheaper than the other two, but you’ll have to stash a ton of cables in the back of your case, which can be a real pain. If you can spend the extra money, it’s worth going with at least a semi-modular power supply for a cleaner build.

We also suggest sticking with a power supply from a well-known company. There are tons of power supplies from many companies, most of which you’ve never heard of. This could set you up for a blown PSU sooner than expected, not to mention potential damage to other parts of your system.

Stick with companies like Corsair, EVGA, Cooler Master, Thermaltake, and other well-known ones to be reasonably certain you’re getting good quality power. It’s by no means a guarantee, but it’s a safer bet than a random power supply from an unknown company.

Finally, we suggest that you pay particular attention to the warranty. Although we often don’t think about warranties for other items, it’s a crucial part of a power pack. There are many power supplies that come with 5 or 10 year warranties, which means you can be more certain that these units will last through multiple PC builds. And if they don’t, you can always file a warranty claim.

You don’t need to obsess over powering a PC, but consider a few important issues and you can rest easy with a solid choice.

Alan A. Seibert