How to Choose a Power Supply: The Definitive Guide

Written by Azzief Khaliq
Last updated Feb 10, 2023

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a power supply unit (PSU)

The power supply unit (or PSU) is likely a gaming PC’s most critical yet underappreciated component, with many builders choosing to put more money towards other parts. But getting a cheap, low-quality PSU can lead to a ton of issues, ranging from general instability to damaged parts. Knowing how to choose a PC power supply can save you a lot of pain, and that’s precisely what our guide will teach you.

From wattage to electrical performance via form factor and efficiency, we’ll run through most of the crucial specs you should focus on when shopping for a power supply. We don’t have the space to cover everything in detail, but we’re confident that this guide should help you when shopping for your next power supply. Let’s get started.

Step 1: Determine the Form Factor

Before choosing a power supply, you’ll have to determine what size PSU you actually need. There are a surprisingly large number of power supply form factors out there, but most of you will choose between ATX, SFX, and SFX-L for your builds.

ATX, SFX-L, and SFX PSUs side-by-side

ATX is the standard PSU size you’re likely familiar with. Measuring 150 x 86 x 140 mm (5.9 x 3.38 x 5.51 inches), ATX is the power supply form factor of choice for mid-towers, full-towers, and most Micro-ATX cases. If you’re building in a tower case like the Lian Li Lancool 205 Mesh, you’re most likely shopping for an ATX PSU.

Corsair RM750x ATX PSU

Source: Corsair

That said, you still need to check case clearances, as many higher-end ATX power supplies are longer than the 140 mm (5.51 inch) that the ATX standard calls for. The Corsair RM750x, for example, is 160 mm (6.29 inches) long. While most cases will accommodate these long power supplies, some are more restrictive and require PSUs that adhere to the ATX standard.

If you’re building in a Mini-ITX or small form factor (SFF) case, you’ll be shopping for an SFX or SFX-L PSU instead. SFX PSUs measure 125 x 63.5 x 100 mm (4.92 x 2.5 x 3.93 inches), while SFX-L PSUs have a slightly longer 125 mm (4.92-inch) length.

Most small form factor cases will fit SFX and SFX-L PSUs, so you can choose whichever suits your needs. That said, some tiny Mini-ITX cases only have room for SFX power supplies, so double-check before buying.

Cooler Master V850 SFX Gold SFX PSU

A Cooler Master V850 SFX Gold PSU. Source: Cooler Master

Note that SFX and ATX power supplies are not interchangeable, as they have different mounting plates. You can buy SFX-to-ATX adapters like the SilverStone PP08 that let you install SFX PSUs in an ATX case. However, we recommend sticking to ATX PSUs in tower cases, as SFX power supplies come with short cables designed for small form factor builds. These will be too short to cover the distances required in an ATX case.

Of course, these aren’t the only PSU form factors on the market. But unless you’re building in relatively niche cases requiring even more compact power supplies, these are generally the three sizes you’ll encounter.

Step 2: Figure Out Wattage

Now that you know what type of power supply unit you should be looking out for, it’s time to determine how much power you need from your power supply. Different builds will require different amounts of power, with higher-end components almost always needing more power than their lower-end counterparts.

The easiest way to get an idea of your rig’s power consumption is with a power supply calculator. There are a bunch of them on the internet, but we often turn to OuterVision’s Power Supply Calculator. Select your PC components from the drop-down menus, and the tool will calculate a rough power draw and recommend the right power supply wattage for your rig.

OuterVision power supply calculator

Source: OuterVision

You’ll notice that the calculator will always recommend a PSU with more wattage than your rig needs. That’s a good idea, as it gives you some overhead to handle power spikes and provides more power for future upgrades. Unless you’re on a super-tight budget, try to over-spec your PSU slightly to accommodate for these factors.

However, you don’t want to go overboard and buy a 1000-watt PSU for a 400-watt rig. Not only is it a waste of money, but it’s also not a particularly efficient use of the power supply.

Step 3: Decide On Efficiency

What Is Efficiency?

Efficiency refers to how much power a power supply needs to draw from the wall to generate the watts it needs. The higher a PSU’s efficiency, the smaller the difference between its power draw and power output.

80 plus efficiency ratings

PSU efficiency is indicated on the 80 PLUS scale. The scale, from lowest (least efficient) to highest (most efficient), is as follows:

  • 80 PLUS
  • 80 PLUS Bronze
  • 80 PLUS Silver
  • 80 PLUS Gold
  • 80 PLUS Platinum
  • 80 PLUS Titanium

The basic 80 PLUS efficiency rating only requires a PSU to be 80% efficient across its power range. On the other hand, any PSU hoping to get hit the 80 PLUS Titanium PSU efficiency rating needs between 92% to 96% efficiency based on load and input voltage.

Why It Matters

If you’re concerned with your power bill, you may want to consider a high-efficiency PC power supply. For example, a cheap 80 PLUS PSU will pull 625 watts from the wall to generate 500 watts for your components. In contrast, an 80 PLUS Gold PSU with 90% efficiency only draws around 555 watts from the wall to generate that same 500 watts.

But efficiency isn’t just about power draw. More efficient power supply units tend to use better parts and output less heat, significantly extending a power supply’s working life. Manufacturers tend to back their Gold-rated and higher PSUs with extended warranties, too, with five-to-ten-year guarantees quite common at the high end.

However, a more efficient power supply will also cost more, so there’s a balancing act there when it comes to your budget. For example, you can get a 650-watt, 80 PLUS Bronze PSU like the Corsair CV650 for around $70, whereas the 80 PLUS Gold Corsair RM650 will set you back closer to $120.

You get a more efficient PSU and an extra two years of warranty coverage with the latter, but you’ll have to decide whether that’s worth the additional $50. If you’re building a mid-to-high-end rig, we think you may as well spend the extra and go for a good power supply with a Gold rating (or higher). Those on a budget, though, will likely have to compromise and go for a Bronze-rated PSU instead.

Step 4: Consider Cabling

When choosing a power supply, you’ll also have to decide whether you want one with modular, semi-modular, or fixed (non-modular) cables.

Non-Modular Power Supplies

Non-modular PSUs have all the power supply cabling permanently connected to the PSU. This makes them more affordable, as manufacturers aren’t spending extra on the connectors and PCBs required for modular cable connections.

The main downside is clutter, as you’ll likely end up with a bunch of unused cables in your rig. A non-modular power supply may have more SATA power cables than you need, a Molex connector or two, and maybe even spare PCIe power connectors. These take up space and require a bit more cable management to clean up.

Non-modular PSU

A SeaSonic S12III 650 non-modular PSU. Source: SeaSonic

Modular Power Supplies

Modular PSUs alleviate cable mess and management issues by making all cables detachable. A modular power supply unit has dedicated sockets you plug cables into, and you only need to connect the cables needed to get your system up and running.

This can lead to a much cleaner system with less cable management required. For example, you can ditch the SATA power cables if you’re building an NVMe-only rig, which minimizes cable clutter. However, the extra circuitry and connectors needed for the modular sockets make modular PSUs more expensive than their non-modular counterparts.

Modular PSU

Corsair’s fully-modular SF750 and its cables. Source: Corsair

Semi-Modular Power Supplies

Semi-modular PSUs combine both approaches, having some of the power supply cabling permanently attached while having sockets for other cables. You’ll usually get a fixed 20/24-pin ATX power connector, with some also having an attached 8-pin CPU power connector.

Semi-modular power supplies used to be more common, but they seem to have fallen out of favor with consumers and manufacturers over the past couple of years.

Semi-modular PSU

An EVGA 650 GQ. Source: EVGA

Most 80 PLUS Gold (and higher) PSUs are modular now, so there really isn’t much of a choice if you’re shopping at the higher end. But if you’re building a more value-oriented rig with a Bronze PSU, we recommend sticking with a non-modular PC power supply to maximize your savings. Sure, it won’t look as cool, but that’s a small sacrifice for a more affordable PSU.

Check out our guide to modular power supplies for a more thorough run-down of all three cable types.

Step 5: Research Rails and Protection

PSU rails and protections aren’t particularly sexy topics, but they’re important parts of the power supply-buying process. Protections are especially crucial, and you should only buy PSUs with adequate working protection circuitry for the safety of you and your rig.

PSU Rails

One of the great, long-running debates regarding power supplies is about single- vs. multi-rail designs. Single-rail power supply units use a single, high-powered +12-volt rail to power the system, while multi-rail uses multiple smaller +12-volt rails.

Both rail types are perfectly adequate and will power a system without issue. However, single-rail designs are slightly higher risk due to their monolithic nature. A single-rail power supply will keep pushing current through the cables even when there’s a short circuit, eventually melting the cable(s) or even frying itself.

Corsair HX850

Some PSUs, like the Corsair HX850, have single- and multi-rail modes. Source: Corsair

On the other hand, a multi-rail power supply will have tighter limits on current and will shut the entire unit down when it detects issues on one (or more) rail(s). This makes them safer and more resistant to power or cabling issues in your system.

We recommend using a multi-rail power supply unit for safety, but both types work fine and will power even high-power rigs without issue. However, the extra safety net of a multi-rail design is worth any extra effort and price you might have to pay. For more info, check out this video with PSU legend Jonny Guru:

Protection Circuitry

All quality power supplies should have a suite of protection features designed to keep your PSU and rig safe. Now that you’ve narrowed down your selection based on wattage, size, and cabling, you’ll want to look out for these to ensure you get a safe PSU.

Over Power Protection (OPP) is a protection circuit that activates if components pull too much power from the PSU. A working OPP circuit will kick in and shut the power supply off at anywhere between 110% to 150% of the PSU’s listed power output.

Over Current Protection (OCP) operates similarly to OPP, except it works on the +12-volt rails on a multi-rail power supply. OCP uses shunt resistors to monitor the current going through the +12-volt rails and intervenes if any of them pulls more current than safe. OCP is only present on multi-rail PSUs, as OPP is sufficient for a single-rail PSU. Both are essential, although an overly-sensitive OCP setting can actually cause systems to shut down even when there’s no threat to the PSU.

Over and Under Voltage Protection (OVP and UVP) monitor the voltage going through your PSU rails (+12 volts, +5 volts, +3.3 volts, and +5VSB). They will shut your power supply down if the voltage on any of these rails goes too far over or under the spec. ATX spec doesn’t mandate UVP, but we recommend looking for one with UVP anyway to be safe.

SilverStone NJ700

Internals of a SilverStone NJ700 PSU. Source: SilverStone

Over Temperature Protection (OTP) uses a thermistor attached to a heat sink to monitor the PSU’s temperature. If the heat sink temperature exceeds a set limit (for instance, when a PSU fan fails), the circuitry kicks in and shuts the power supply down to protect it. The most recent ATX spec requires all PSUs have a working OTP circuit.

Short Circuit Protection (SCP) monitors the PSU’s output rails and activates if it detects a short circuit (indicated by a resistance of less than 0.1 Ohm). Short circuits in your rig can melt cables and even set them alight, so this protection is crucial. ATX spec requires PSUs to have SCP, so you’ll see this on almost all branded power supplies.

Power Good or PWR_OK Signal isn’t a protection circuit as such; instead, it’s a delay period that determines whether the power supply has enough stored power to run under full load for 17 milliseconds. The delay should ideally be under 250 milliseconds and more than 100 milliseconds.

Note that some PSUs may advertise these features but not have working circuitry, rendering certain protections useless. Don’t just go by the manufacturer’s claims; read reviews to see whether the power supply you’re interested in runs as it should. Which, handily, brings us on to the next step.

Step 6: Read Reviews

Unlike other components, you shouldn’t buy a power supply unit solely based on whether it fits all your requirements. Not all power supplies are created equal, and some power supplies that may seem decent on paper can be unreliable and unfit for use. Case in point, the “exploding Gigabyte power supply” debacle from 2022:

Situations like these are why you should always read PSU reviews before committing to a purchase. In fact, we’d recommend you actively avoid buying any power supplies that haven’t been reviewed by reputable outlets such as Cybernetics, Tom’s Hardware, or TechPowerUp.

The main reason is that only thorough PSU testing can ascertain whether its electrical performance is up to par. A good PSU review will evaluate multiple aspects of a PSU’s performance, including efficiency, load regulation, ripple, protection features, and transient handling. A good review can help you choose a power supply that’ll be safe and reliable for years to come.

We don’t have the space to discuss all the tests here, but let’s run through a handful of the more critical tests.

Voltage (or Load) Regulation

Power supplies feed your system through several different-voltage rails: 12 volts, 5 volts, 3 volts, and 5VSB (a standby voltage). In an ideal world, a power supply will deliver consistent voltages on all of these rails regardless of the load.

However, that’s never the case, so voltage regulation tests evaluate whether the power supply can keep the voltages within the allowed limits for each rail. The previous ATX spec allowed for ±5% variations on all rails, but the new ATX 3.0 spec increases this to ±7% (or ±8% for PCIe power).

Better power supplies will keep these rails running consistently within spec and close to their nominal voltages. The better the regulation, the smaller the deviation.

Ripple Suppression

Ripple suppression is closely related to voltage regulation. However, instead of measuring the overall deviations from the nominal voltages, power supply unit ripple tests evaluate how capably the PSU suppresses minor voltage fluctuations on these rails.

Ripple is, unfortunately, an inherent element of power supply units due to their switching-mode design. However, a properly-designed power supply can reduce voltage inconsistencies and deliver clean, accurate power to your components. Ripple suppression is crucial, as large amounts of ripple can shorten component lifespan and impede overclocking quality.

PSU ripple

Poor ripple suppression as shown on an oscilloscope. Source: Corsair Blog

The ATX specification allows for 120 mV of ripple on 12-volt rails and 50 mV on the 5-volt, 3.3-volt, and 5VSB rails. A good, high-quality power supply should be able to keep ripple beneath 50% of these limits on all rails, with some even managing to keep it below 20 or 30%. Lower is always better here.

Transient Handling

Computers don’t run at full blast all the time, and the loads we put on our rigs can vary in intensity and processing power from one moment to the next. PC components can also have big spikes in power draw, which can be dangerous if a power supply isn’t good at handling transients.

Transient testing evaluates how capably a power supply handles these rapid (and brief) power spikes. A good PSU should handle transients without tripping any protection circuits (within reason) or deviating too significantly from the nominal voltages. Like load regulation, a PSU should keep voltages within ±5% or ±7% depending on the ATX spec revision it aims for.

A PSU’s ability to handle power transients is closely related to its load regulation and ripple suppression. The results of all three tests should give you a decent idea of a PSU’s quality and whether purchasing it is a good idea.


A power supply’s 80 PLUS efficiency rating is useful, but it doesn’t tell the whole story. Manufacturers can rig and manipulate the testing, usually by using higher-quality components for evaluation units (or downgrading production units). So if you’re concerned about efficiency, you’ll want to read PC power supply reviews to see whether they perform as their ratings suggest.

Corsair SF600 efficiency curve

The efficiency curves for a Corsair SF600. Source: Tom’s Hardware

Generally, this is more an issue with smaller, no-name brands than it is with power supplies from big names such as Corsair, SeaSonic, or Cooler Master. However, never take things for granted; always read reviews to see whether power supplies, even those from trusted power supply manufacturers, are as efficient as their 80 PLUS ratings suggest.

If you’re curious about the entire PSU test suite, check out Cybernetics’ test protocol overview. It goes through all the important tests and discusses their importance.


Buying a PC power supply unit may seem simple initially, but there’s more to it than just getting one with enough wattage. A lot goes into making a good power supply, and knowing how to choose the right one can be the difference between a stable, reliable rig and one that constantly crashes and throws up issues.

We mentioned it earlier, but it’s worth repeating here: read power supply reviews. Once you’ve found a few PSUs that tick the boxes for wattage, form factor, efficiency, and price, spend some time to see if any trusted outlets have tested them out and read what they have to say. Taking a chance on untested and unproven power supplies isn’t something we recommend doing, so play it safe and choose a PC power supply with proven quality and performance.

Need help picking your other parts? Check out our high-level guide to choosing PC parts for some quick tips.

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