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Choosing a Power Supply for Your Homebuilt Computer

There are certain parts of a computer that you definitely do not want to skimp on, and the power supply is at the top of the list.

A bad power supply can literally destroy your whole computer. Power supplies that produce too much voltage, too little voltage, inconsistent voltage, or that bleed AC into the DC can all damage your computer's delicate components.

That's why I only use power supplies from industry-leading companies like Seasonic, Corsair, Antec, Cooler Master (shown in this picture), or Thermaltake.

I'm sure there are many other companies who make excellent power supplies; but Seasonic, Cooler Master, and Thermaltake have never disappointed me, so I've been loyal to them for many years. Seasonic is my personal favorite at the time of this revision (in February of 2021), but all of the companies mentioned are reputable manufacturers.

Even the best manufacturers occasionally put out duds, however; so make sure to research the reputation of the exact model of PSU that you're planning to buy. Reviews on Amazon and other sites are helpful for this purpose.

In addition to choosing a high-quality power supply, it's also critical to choose one that is efficient, that fits in your case (form factor), that produces adequate power for your computer's current and future needs, and that has enough of the correct connectors for the type of system you are building. Let's look at all these factors individually.

Power Supply Efficiency

In simple terms, the efficiency of a power supply refers to what percentage of the input wattage is converted to output wattage rather than heat. To determine the efficiency, simply divide the output wattage by the input wattage. For example, if a power supply draws 600 Watts on the input side to produce 500 Watts on the output side, that power supply is 83.3 percent efficient (500 ÷ 600).

Wattage is voltage multiplied by amperage. Because the input voltage is constant, the component of wattage that changes with load is the amperage, or "current." Because various metallurgical and electrical factors that affect efficiency are current-dependent, power supplies usually have different efficiencies at different loads. So how do we determine how efficient a power supply is overall?

The 80 PLUS standard simplifies power supply efficiency ratings by looking at their efficiency at 20 percent, 50 percent, and 100 percent of their rated loads. Power supplies certified to be at least 80 percent efficient at all three levels are allowed to label and market them as 80 PLUS-compliant.

Companies who certify that their power supplies meet additional efficiency standards at load levels of 20 percent, 50 percent, and 100 percent of their rated loads are additionally allowed to label and market them with "metal" ratings ranging from Bronze to Platinum.

There are different standards for different types of power supplies. For computer builders based in the United States and other countries that use 110 - 120 volt AC as the household power standard, a power supply's compliance with the 115V Internal Non-Redundant standard is what we're interested in. Here are the basic efficiency requirements for that standard at 20 percent, 50 percent, and 100 percent of rated load as of the date of this revision:

80 PLUS: (80%, 80%, 80%)
Bronze: (82%, 85%, 82%)
Silver: (85%, 88%, 85%)
Gold: (87%, 90%, 87%)
Platinum: (90%, 92%, 89%)
Titanium: (92%, 94%, 90%)

The "Titanium" rating has an additional requirement that the power supply be at least 90% efficient at 10 percent load.

Choosing an efficient power supply will save you money on electricity, reduce your carbon footprint, and reduce the total heat produced by your computer. It's also a rough indicator of a power supply's quality: It's more difficult and requires better-quality materials to produce a high-efficiency power supply than a low-efficiency one.

Most quality power supplies for hobbyist computer builders nowadays have an "80 PLUS Gold" rating. That's also my personal minimum when choosing a PSU.

Form Factor

There are many different power supply form factors. The most common one for do-it-yourself computer builders is ATX. If you walk into a computer parts store and say you need a PSU without specifying a form factor, they'll assume you want an ATX power supply.

Other somewhat-common PSU form factors include:

ATX12V. This Intel design eliminates the 3.5V and 5V rails, leaving only the 12V. It's designed to improve power efficiency.
Micro-ATX. Electrically identical to ATX, but smaller.
EPS12V. Used mainly on servers and has an additional 8-pin connector.
WTX12V. A 12V only PSU for workstations.
FlexATX. One of the more popular smaller form factor power supplies. Longer and narrower than standard ATX.
Numerous small form factor power supplies including SFX12V (small form factor 12 volt), CFX12V (compact form factor 12 volt), TFX12V (thin form factor 12 volt), and many others.

Make Sure it Will Fit

In a perfect world, a power supply designed for a specific form-factor case would fit in any case using that form factor. In practice, double check to make sure. One common conflict is that the power supply is too long to fit behind the top bays of the case that are intended for optical drives. In other cases, the PSU may get in the way of the CPU fan.

With so many different style cases out there and so many deviations from the ATX standards, all I can say is carefully check the dimensions of all the parts you plan to use to make sure they'll all physically fit.

Active PFC

From the computer builder's perspective, Active PFC (Power Factor Correction) power supplies provide cleaner, more stable power to your system despite ordinary fluctuations in utility line voltage. They also somewhat reduce and stabilize overall power consumption on the utility side (but generally won't reduce your electric bill because you'll still be using the same amount of wattage at the wall outlet).

Nearly all high-end power supplies now have Active PFC circuitry; so there's really no need to consider it as a deciding factor in choosing a PSU. If you're looking at modern, high-quality PSUs, they'll almost certainly feature Active PFC.

The one downside to Active PFC power supplies is that you'll need a sine-wave UPS designed for Active PFC rather than a less-expensive battery backup, whose output only approximates a sine wave. If the UPS doesn't provide a clean sine wave to the PSU, then then PSU (and the computer, of course) will not stay on if the power goes out.

How Big a PSU do You Need?

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One of the most important parts of designing and building your own computer will be doing some math to calculate the capacity of power supply that you'll need.

To figure out how big a PSU your computer will need, you need to add up the nominal maximum power consumption of every component as stated in their specifications or data sheets, and add a generous margin for headroom. You can make this step easier by jotting down the power-consumption information while you're planning your computer and looking at parts.

"Headroom" in this context means an additional margin of available power over and above the sum of the power consumed by all the components. The three reasons why we want some headroom are:

Because it's not unusual for components (especially CPUs and GPUs) to consume more power than stated in the specs.
To provide some room for future expansion. You don't want to have to replace your PSU if you decide to add a HDD some day.
To prevent the power supply from having to put out max power all the time. Electrical components burn out more quickly when they're constantly working their hardest.

Finally, if you plan to overclock, add on another hundred Watts over and above the normal headroom.

When I'm building a system, I look up the maximum power requirements for literally every component I plan to install, add them all up, and then add on at least 25 percent more for headroom. If I think I will be upgrading the system in the future, I add as much as 50 percent.

For example, if the total power consumption adds up to 400 Watts, and I don't plan to upgrade anything, the math would be:

400 + (400 x 0.25) = 500 Watts

If I do plan to upgrade (or at least want to have that option without replacing the PSU), I would add on 50 percent for headroom:

400 + (400 x 0.5) = 600 Watts

Finally, I would add on another 100 Watts if I planned to overclock.

Power Cables, Connectors, and Cable Management

Computer components use a variety of connectors. You'll need SATA-type power connectors for the drives, the ATX connector to the motherboard, probably an 8-pin connector or two for the processor and video card, a PCIe cable, maybe a Molex or two, and possibly others depending on your computer's build. You also need to decide whether you want to use a non-modular, semi-modular, or full-modular power supply.

Non-Modular Power Supplies

Once upon a time, power supplies all came with all the cables you could possibly need permanently attached to the power supply. Nowadays, these kinds of power supplied are called non-modular power supplies. Non-modular power supplies are convenient because it's highly unlikely that you won't have a cable that you need. They also tend to be inexpensive.

On the down side, when using a non-modular power supply, you'll have many more unused cables to tie off so they don't block the airflow inside the case.

Fully-Modular Power Supplies

At the opposite end of the spectrum we find fully-modular power supplies. Fully-modular PSUs come with literally nothing attached to them. They have outputs into which you plug only the cables you need. The manufacturers usually include an assortment of the most common cables needed to build a "typical" PC, but check first to make sure. You may need to buy additional cables.

Note that modular and semi-modular cables from different manufacturers may not have the same wiring. As a rule, if you need more cables, buy the ones made for your brand and model of PSU.

Semi-Modular Power Supplies

In between the two extremes we find semi-modular power supplies. The ATX connectors to the motherboard connectors are permanently attached to the PSU on the assumption that you'll always need those connections; but you'll need to hook up the rest of the cables yourself. Like full-modular power supplies, they usually include a sensible assortment of cables for a typical build, but check first to make sure.

Semi-modular power supplies are a good choice for most builders because you'll always need the main ATX connection. But the difference between fully-modular and semi-modular is literally one cable to connect. That's why I really have no preference between the two. I choose the PSU based on its performance and reputation, not on whether I have one more or less connector to push into the box.

In closing, let me say once again that the power supply is not a place to look for a bargain. Do yourself and your computer a favor by buying a high-quality PSU to power your system and protect it from damage.

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- Choosing a Power Supply (This page)