How to Choose the Right Power Supply

Mike Howell

By far the most overlooked component in new builds is the power supply unit, or PSU. Ironically, it’s one of the most important. Your power supply will be pumping the lifeblood of precious volts to each and every electronic part in your fancy new computer.

A catastrophic failure of a power supply can fry every other part in your PC, or in rare cases even start a life-threatening fire at 2:00 in the morning. Why risk a disaster like this to save the equivalent of four grande mochaccinos from that fancy coffee shop?

Form factor and physical size

The first things you will want to look at to narrow down your choice of power supplies are the form factor and dimensions. As the saying goes, form follows function, but be sure that you know the size of power supply that can fit into your chosen case.

ATX is the most popular and supported form factor, but some smaller cases need special “small form factor” power supplies such as SFX that can squeeze into tighter confines. Even amongst the same form factor sizing, the dimensions of power supplies can vary to the point where a quick check with your case’s allowable sizes is worth a few minutes of invested time.

Modularity and cables

Modularity can be a peripheral concern or an absolute deal breaker depending on case room and personal aesthetics. When a power supply is described as modular, it simply means that some of the cables that run between the PSU and PC components can be removed when not in use.

The modularity of a power supply generally falls into three categories: Non-modular, semi-modular, and full-modular.

  • Non-modular power supplies have all the various motherboard, hard drive, and PCIe power cables hard wired to the PSU.
  • Full-modular units allow you to remove and replace every single cable on the unit.
  • Semi-modular falls somewhere in between. Usually the main motherboard power connector is hardwired in, but the hard drive and PCIe power cables can be removed.

Aside from keeping a tidy case, modular power supply cables can also be replaced with more stylishly colored or braided versions for bit of extra visual appeal. I cannot stress the following enough: Never, ever mix modular PSU cables between brands or even models. There is no standard for how the wiring is pinned out on the power supply end, so you risk severe component damage by swapping them around.

As an aside to the modularity of a PSU, you also want to take a hard look at the number of PCIe power connectors offered. Two 8-pin PCIe power connectors is enough for current single graphics card configurations, but up to four connectors (6 and/or 8 pin) may be required for SLI or Crossfire. Consult the cable requirements of your graphics card to be sure you have what you need, and consider planning ahead for a future upgrade to a second card if that’s a possibility.

Efficiency ratings

Have you seen the “80 Plus,” “Bronze,” or “Platinum” ratings listed with the PSU while shopping? These are voluntary industry certifications related to the efficiency of a power supply to convert the high voltage AC power from your wall plugs to the lower voltage DC power that sustains your PC.

When a power supply wastes 20% or less of the incoming power as heat, then it is eligible for 80 Plus certification.

The certification comes in six levels:

  • 80 Plus
  • 80 Plus Bronze
  • 80 Plus Silver
  • 80 Plus Gold
  • 80 Plus Platinum
  • 80 Plus Titanium.

Beware: while higher labels do tend to correlate with better built units, it’s not necessarily an indication of quality. 80 Plus Gold tends to be the sweet spot that manufacturers aim for with consumer level units, but solid units can also be found in the 80 Plus and 80 Plus Bronze levels.

80 Plus Titanium is meant for datacenter-grade power supplies where a few points of efficiency loss can mean a giant bump in air conditioner and power expenses, so don’t bother searching those out unless you are dead-set on top quality with no budget considerations. If you’re interested in further reading, more detailed information on the various 80 Plus certification levels and what the labels mean can be found straight from the horse’s mouth at 80 Plus.

The added benefits of quality

Higher quality power supplies don’t just offer an efficiency rating named after a more precious metal. The engineering that goes into a unit with a 10-year warranty far exceeds the effort put into a $25 unit you would pick up off eBay.

From the major manufacturers, warranty is a very good indicator of the confidence the vendor has on their product’s quality and longevity. The peace of mind that you’ll have when you know that new power supply will reliably power your next two gaming rigs should not be dismissed.

In addition to greater lifespans, higher quality power supplies tend to be engineered for reduced noise. Noise in a power supply mainly comes from the fans used to cool the unit. The efficiency of the power supply impacts this as well, as the need for cooling in a power supply is directly proportional to the heat generated. As you remember from above, waste heat is the leftover percentage after the efficiency rating.  A power supply providing 500W at only 80% efficiency will have 125W of heat to dissipate, which is more than most top-end CPUs!

To combat this, most power supplies have one or more fans to run air through the unit for cooling. Fans are noisy, however, and so is air being forced through a tunnel of capacitors and transformers. Designs that account for acoustics, optimal fan speeds for a given temperature, and high quality materials all play a role in differentiating quiet units from noisy ones.

Coil whine or coil noise, the high-pitched resonance of power converting coils within electronics, is a secondary noise consideration. This can be caused by a lack of sound dampening materials used for the construction of the power supply. Alternative coil designs are also used to avoid audible whine from the units. Many low quality units aren’t designed with noise dampening or engineered to avoid coil whine in the first place.

Wattage considerations

Wattage can be a controversial topic among hobbyist PC builders. Some will argue that you can save a few dollars by getting a lower powered unit and it won’t make a difference at all in your build. I believe this is a bad idea, and I’ll explain why.

First, the wattage requirements themselves. When you build a PC, each component will draw some amount of power and that energy must come from the power supply. The thirstiest parts will likely be your graphics card and CPU, with the motherboard close behind.

Close attention should be paid to the specific wattage requirements of each component. You’ll find that many manufacturers will “helpfully” suggest a PSU wattage for their graphics card, but without the context of the rest of your build it’s a complete shot in the dark.

What you’re better off looking for on your graphics card or CPU is the TDP or Thermal Design Power rating. Technically, TDP describes the maximum amount of heat a component will generate during normal use. Thanks to the laws of thermodynamics and the tendency for TDP to be measured in watts, this is also an extremely close approximation for the maximum power draw of each component.

Alternatively, a manufacturer may simply describe the maximum power consumption in watts for a component. This should be taken as the equivalent to TDP. Thankfully, to ease consumer confusion, actual power draw numbers are becoming more common by the year.

It’s safe in most standard builds to only look up the power requirements for the CPU and graphics card for your build, and then estimate the rest. You’ll almost never find documentation on the power consumption of a motherboard, for example, so it’s safe to estimate 35W for a tiny Mini-ITX board. A Micro-ATX or ATX desktop motherboard clocks in around 65W. CPU or case fans can be estimated at 2W. All-in-one CPU or GPU cooler kits are a hair under 10W. Modern RAM uses about 3W per module, with little regard to size. Spinning hard disk drives (HDDs) can be estimated at 9W on the high side, while solid state drives (SSDs) use a mere 2-3W.

How do all those numbers add up in the real world?

As an exercise, let’s add up the TDP or maximum power consumption for a PC built from the Exceptional Tier on our homepage, and see what we would come up to:

Component Wattage
MSI Armor GTX 1080 Graphics Card 180W
Intel i7-7700K CPU 91W
Cryorig H5 Ultimate CPU Cooler 2W
MSI Z270 Gaming Pro Motherboard 65W
8GB DDR4 RAM 3W
2TB HDD 9W
525GB SSD 3W
HAF X Case (fans) 4W
Total wattage 375W

These same calculations can be done more easily with specific PSU sizing calculators like Outervision PSU Calculator.

Wow, that means I can go buy a 360W power supply and be fine, right? Well, maybe not. Aside from the dearth of decent desktop power supply units available below 550W, there are practical considerations that impact some basic sizing assumptions.

First of all, it is never a good idea to plan for a PSU to be at or near 100% load. Heat is the enemy of power supplies, and running at full load is a surefire way to get one nice and toasty.

PSUs are generally most efficient (and provide the most reliable, clean power) running at ~50-80% of their rated load. Secondly, if you size a PSU to the exact wattage requirements of your build then you’re leaving no room for expansion or overclocking. That second GTX 1080 you’ll see on sale for $100 off a year down the road isn’t going to do you much good if you need to buy a brand new $100 power supply to run it because you skimped on your build.

Overclocking can also add a significant boost to PSU needs, as you can see with the Outervision PSU calculator linked above. Just a 10% overclock and boost in voltage on a CPU can increase its power requirements by 30% or more! Similarly, graphics cards see around a 15-20% jump in power consumption for a moderate overclock. Let’s take a look at these components again and estimate what we really should be looking for in a power supply, assuming you want to both overclock for maximum gaming performance and have room for a second GPU later down the line:

Component Wattage
MSI Armor GTX 1080 Graphics Card 180W
Graphics card overclock room 24W
Room for a second overclocked graphics card 204W
Intel i7-7700K CPU 91W
CPU overclock room 28W
Cryorig H5 Ultimate CPU Cooler 2W
MSI Z270 Gaming Pro Motherboard 65W
8GB DDR4 RAM 3W
2TB HDD 9W
525GB SSD 3W
HAF X Case (fans) 4W
Total wattage with overclock room 613W
Overhead to keep PSU running <80% 153W
Final recommended PSU size 756W

Just by taking into account overclocking, a second GPU in the future, and a safe overhead for power efficiency we’ve doubled our expected PSU wattage. For reference, the price difference between a 550W EVGA G3 and a 750W model is around $20. With that extra $20 investment for a component in a $1700 computer, you’re allowing far greater potential for overclocking and expandability. Simultaneously, that added room ensures that even when you do expand in the future you’ll have peace of mind that your PSU will still be delivering clean, reliable power without excess heat affecting its longevity.

I know what I need, but what brand?

Now you’ve narrowed down what you need. Let’s say it’s an ATX full-modular power supply with at least an 80 Plus rating and 700 watts of power. There are still a hundred options — what do we do now?

Unfortunately, this is where things can get a bit tricky. Even amongst the same manufacturer, various power supplies can be top tier or budget bin quality. Corsair is a good example of this, with their AX series being a great buy while the CX series is meant for budget-constrained builds.

The top models that are recommended time and time again are:

Power supplies come and go. Old models age out while new shiny ones are introduced. As a consumer, you can’t judge the build quality of a PSU by specifications alone. I heartily recommend reliable review sites like JonnyGuru or Anandtech for thoughtful and thorough recommendations of the best products available. Their batteries of tests include short circuit protection for safety, power ripple that affects the quality and reliability of power delivered to components, actual wattage delivered, the thermal properties of the power supply, and even breakdowns showing the quality of the individual electronic components in the reviewed units.

We hope that you, armed with the information in this guide and the helpful hand of trusted review sites, will find the right power supply for your needs that provides a safe, reliable, and worry free experience for years to come.

Note: A previous version of this article contained an error regarding the amount of heat dissipated when a PSU is providing 500W of power at 80% efficiency.  Commenter Clayton K pointed out that the correct amount of heat would be 125W, which the article as been updated to reflect.