How to choose the right power supply for your computer – 2020 edition

When deciding to build a new computer, it’s easy to overlook the humble power supply unit (PSU).

I mean, that’s not exactly the most exciting part of your new computer, with most of the coverage going to the parts it powers. Yet it is one of the most important parts of your build, if not the most important. Nothing else you buy will work without it, after all.

A few extra minutes taken into your buying decision will pay off in the long run. A quality PSU can not only power this build, but can also power your next build, and probably the next one. You see, the ATX spec for PSUs isn’t going away anytime soon, and it’s the other components in your build that get upgraded most often.

Determine the power you need

Image: Be quiet!

The biggest question when specifying a power supply is “How many watts do I need?” This seemingly simple question can be complicated to answer, as it depends on the rest of the pieces in your build.

Generally, the more components in your system, the more power you need to run them. A simple system won’t need the same power consumption as a complex system with multiple graphics cards, a liquid cooling loop, and high-end components.

This means that you need to determine the approximate power your build will consume and plan accordingly. There are several ways to do this, and we’ve put together a short list that should cover your needs. Most of them are produced by specific PSU manufacturers, but the result they give you can be used with any brand PSU, not just theirs.

  • PCPartPicker: This handy site should probably be your first stop when you start planning a build. It has most of the hardware on the market in its huge database, and once you put the parts you want in a list, it will give you a Estimated power based on these choices.
  • Be quiet! Power Calculator: This calculator lets you add major specs and includes things like if you have USB 3.1 Gen. 2 which uses more power, a price range and whether you want ATX or SFX/ITX form factors.
  • EVGA Power Meter: EVGA’s calculator asks you a series of questions about your components, then offers you a list of suitable power supplies from its own line. You can further narrow down the choices by efficiency level and series, and it does a good job of estimating your wattage needs.
  • CoolerMaster Power Calculator: Cooler Master’s calculator allows you to integrate the main parts of your build, from CPU to motherboard to GPU. It won’t allow you to factor in things like case fans, so be sure to add some extra margin to the number it gives you. Conveniently, it also offers you Cooler Master-branded PSUs that would suit your components while letting you decide if you want things like modular cables or preferred efficiency ratings.

Form Factor and Cable Choices

computer power supply form factors

Image: Joe Rice-Jones/KnowTechie

Okay, so there are two main form factors for computer power supplies. You’ll mostly choose the one that suits the chassis of the computer you want to build in, but sometimes you want a smaller form factor even if your case has plenty of space, like if you’re trying to cram two systems into one build personalized. .

  • ATX: This is the most common form factor, and it’s the one you’re most likely to find on the component shelves of your local big box store. The ATX standard specifies that the height and width of the PSU should be 86mm x 150mm, but the depth can be almost any size, with larger power units being deeper. Keep this in mind if you are building in a cramped enclosure.
  • Special effects : It stands for Small Form Factor and it’s for small cases where you don’t have a lot of room, especially those designed for M-ITX motherboards. They generally have lower wattage than ATX and are more expensive, as miniaturization, while maintaining power efficiency, becomes expensive.

The second thing to consider is how you want your cables to be connected on the power side:

  • Non-modular: All power cables are physically connected to the power supply and not removable. This can be a problem if space is limited or if you are not using all the power connectors.
  • Semi-modular: The main cables are permanently physically attached, and the rest of the peripheral wiring is done with modular cables with sockets so that you can only use the ones you really need. The 24 pin motherboard case will still be part of the permanently attached cables, and some units also permanently wire the 4/8 pin EPS connectors and/or some of the PCIe connectors.
  • Modular: If you could see where this was going, give yourself a cookie. Modular PSUs have receptacles for each PSU cable and a large bag of cables that you can pick and choose to power only the components you have. They are slightly more expensive than the other two options, but they give you more flexibility when building and if you want to use custom cables, this is your best bet.

Connect everything together

power cables

Image: Joe Rice-Jones/KnowTechie

The next step is to determine exactly which power connectors you need for the hardware you are purchasing. All the cables you need will come with the power supply you decide to buy, you just need to figure out how many of each connector you need.

  • 24 pin motherboard: It’ll always be there, there’ll only be one, and it’s the main power connector on your motherboard and you can’t miss it – it’s huge.
  • 4/8 pin CPU/EPS: This connector powers your CPU, and only your CPU. Some PSUs have two, to power motherboards that have more than two 4-pin EPS sockets that are typically designed for overclocking. Usually located on the top left of your motherboard.
  • 6/8 pin PCIe/GPU: These are intended to power your graphics card or some add-on cards such as sound cards that have additional power requirements. They are usually split into 6+2 instead of a solid 8 pin because GPUs use both 6 pin and 8 pin sockets for power depending on how many watts they need. Make sure you have enough for the GPU you buy.
  • SATA: These small connectors power hard drives, solid-state drives, and things like fan hubs or RGB controllers. Most PSUs have multiple SATA connectors on a single cable, so you can plug in more than one thing without needing a lot of cables from your PSU.
  • 4 pin MOLEX: These have mostly been replaced by SATA, but you’ll still find at least one cable with MOLEX on it if you buy a modular power supply, because things like pumps used for water cooling and some older items equipped of RGB use it instead of SATA for power.

Spend a minute thinking about efficiency

computer power label

Image: Joe Rice-Jones/KnowTechie

While wattage is one of the most important considerations when choosing a power supply, it’s not the only thing to think about. An inefficient power supply creates heat, which leads to wasted energy, higher electricity bills, and could potentially shorten the life of other components.

For power supplies, the efficiency rating is based on the “80 Plus” scale. A PSU rated at 80 Plus should be at least 80% efficient, so only 20% of the energy is lost as heat. The rest of the scale is based on precious metals, with bronze being 82% effective, silver 85%, gold 87%, platinum 89%, and titanium 90%.

These efficiency ratings are all based on a 100% PSU load. Now PSUs are more efficient at 50% load, so you can get 94% efficiency on a Titanium PSU at that load. If it matters to you, you want the highest precious metal, and you should also buy a power supply that is twice the power needed by the rest of your system.

Oh, and while we’re on the subject of heat, consider the noise generated by the PSU fan. Many PSUs these days have “quiet” or “eco” modes, which don’t spin up the cooling fan unless the PSU temperature exceeds a certain threshold. You’ll want it if noise is a consideration for you.


custom fully jacketed psu cables in water cooled pc

Image: Joe Rice-Jones/KnowTechie

There aren’t many things you can buy to accessorize your PSU, but the most common cables are fully jacketed cables. These replace the annoying cables that come with most power supplies, but you need to make sure you buy ones made specifically for your power supply or you run the risk of blowing up your expensive computer.

The safest way is to get cable extensions, which connect to the end of your existing cables so that only the visible section inside your case is fully jacketed.

A final word on power supplies

Usually you won’t see your PSU while it’s inside your case, so it doesn’t really make sense to buy one for aesthetic reasons as well as all the ones we’ve covered. This may change depending on your choice of chassis or because you are building a demo version and want to show it off.

The last thing is to consider what percentage of your budget you are going to spend on it. Ten to fifteen percent of your total budget is a good rule of thumb, with some wiggle room for special features or if you want this PSU to last multiple versions.

We’re big fans of EVGA and their fully modular power supplies, so this is a good place to start if you don’t want to go through the whole process.

What do you think? Did you learn anything from this article on power supplies? Any tips you would like to add? Let us know below in the comments or forward the discussion to our Twitter or facebook.

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Maker, meme-r and shameless geek. The hardware guy here at KnowTechie, if it runs on electricity (or even if it doesn’t), I probably have one here somewhere. My hobbies include photography, animation, and hoarding Reddit gold.

Alan A. Seibert