Time to choose a power supply for your next PC build? Here’s an overview of the key factors you’ll need to consider
When you’re preparing to build a new PC, the first topics on your mind are likely powerful multi-core processors, fancy high-end graphics cards, ultra-fast NVMe storage, or maybe even unique case lighting and effects. But one component that often fails to get the attention it deserves is the power supply. After all, virtually every other component in your PC can be affected by the type, quality, reliability, capabilities of your PSU.
That makes the process of choosing the right power supply just as important as any other component. Therefore, here’s a quick high-level overview of the factors you should consider when looking at power supplies for your next PC build.
Power Supply Wattage
The first step in choosing a power supply is determining your required wattage. Choosing the correct wattage is important: going too low will prevent your components from performing at their expected level, cause system instability, and can even damage components in some cases. Going too high doesn’t carry those same risks, since the system will only draw the power it needs. But, since higher wattage power supplies at a given quality level are more expensive, it wastes money that could be better spent on adding or upgrading another component.
Despite the heavy marketing in recent years of 1,000+ watt units, most systems, even those with some high-end components, won’t need more than around 650 to 750W. To get an estimate of what your planned build needs, you can use a PSU Calculator, which allows you to choose the specific components you plan to include in your PC build and then produces a recommended power supply wattage based on the advertised power requirement of each.
Some PSU Calculators hosted by power supply manufacturers themselves, such as be quiet!, also check for connector compatibility instead of just wattage. For example, if you use the calculator to plan a high-end rendering workstation with four GPUs, it won’t display power supply models that lack the required number of PCIe power connectors even if that model’s wattage is otherwise sufficient.
Power Supply Form Factor
Beyond the pure wattage, you also need to consider the physical size and layout, or form factor, of your power supply. There are a variety of historic/obsolete, server, and specialty form factors, but in general when looking at the modern consumer and enthusiast PC market, power supplies fall into the ATX/EPS and SFX/SFF form factors.
The ATX form factor is likely the standard design you think of when picturing a power supply, although there are several variations. The original ATX standard from the mid-1990s required a 4-pin +12V CPU connector. The introduction of more powerful systems in the late-1990s necessitated the introduction of the EPS form factor, which allowed for an 8-pin +12V CPU connector. Today, almost all power supplies marketed as “ATX” meet the EPS standard and include two 4-pin +12V connectors that can be used depending on the motherboard and processor requirements.
The standard dimensions for an ATX power supply are 150mm wide by 86mm tall. The depth of the power supply can vary depending on the device’s features and design. The default depth for a low-to-mid wattage ATX power supply is 140mm. The official “ATX Large” form factor increases that depth to 180mm, but some of the most powerful ATX power supplies on the market today have depths of 200mm or greater. The key takeaway here is that any power supply you choose that is truly ATX will have the same width and height, but depth may vary. So be sure to check the PSU manufacturer’s website for the depth dimension to see if your case can accommodate it.
The SFX or “small form factor” standard is designed for smaller cases. Upon its introduction in 1997 this meant less powerful PCs for general consumers and offices. But in recent years, the increase in power efficiency has made higher-end small form factor PCs popular. And although current SFX power supplies can’t match the wattage or connector flexibility of their full-size ATX counterparts, they can accommodate some of the highest-end components on the market.
The standard dimensions for an SFX power supply are 125mm wide by 63.5mm tall by 100mm deep. However, like the ATX form factor, there are some variations, the most common of which is SFX-L.
The SFX-L form factor maintains the same width and height, but extends the depth to 130mm. This allows for the use of a larger fan and, in turn, higher wattages.
SFX power supplies will generally be more expensive than ATX power supplies at the same quality level and wattage, so you’ll usually want to go with one only if your chosen case requires it. If you end up using an SFX power supply in a case designed for an ATX PSU – either because you need the extra space in the case for cable management, extra storage devices, or case mods, or if you’re reusing an SFX PSU from a previous build – you just need to pick up an ATX to SFX PSU adapter if your case or power supply didn’t already include one.
Power Supply Efficiency
Most power supplies are rated by the 80 PLUS program, which is a voluntary certification for measuring a PSU’s efficiency. There are six levels of increasing requirements, ranging from the basic “80 PLUS” to the most demanding “80 PLUS Titanium,” that certify a power supply’s ability to use electricity with as little waste as possible.
For example, if a power supply draws 647 watts from the wall but provides 550 watts of power to your PC components, that’s an efficiency of about 85 percent, which would equate to 80 PLUS Silver.
Power supplies with higher ratings are more expensive, but also more efficient, which means that they lose less electricity to heat/waste, so they’ll be cheaper to operate in the long run. However, the difference in operating cost between efficiency levels for a typical PC can be very small, and it may take many years to recoup the initial up-front cost of the higher rated PSU.
The 80 PLUS program also isn’t perfect, since the ratings are based on a small sample size and, aside from Titanium, don’t factor in efficiencies at very low loads. Therefore, going with a higher rated PSU isn’t going to hurt, but be sure to weigh the cost difference when comparing models that otherwise meet your needs. In general, the 80 PLUS Gold rating is sufficient.
PSU Voltage Rails
The power in a PSU is supplied to the components by 3.3V, 5V, and 12V rails. While the 3.3V and 5V rails are important, they are generally standardized among modern power supplies and the key differentiating factor is the design and functionality of a PSU’s 12V rail, since this is the source of power for the demanding PCIe devices like GPUs.
At a basic level, a power supply has a total amperage output (a minimum of 18A for the most basic PSUs to at least 34A for higher-end multi-GPU-capable units) that is delivered to components via the rails. A PSU can use a single +12V rail to deliver all that power, or it can split the load between multiple +12V rails. In an example 50A system, using a single rail means that all components receive up to that full 50A. If there were two +12V rails, each rail may deliver 25A individually.
While a single-rail configuration offers the most power to any given component, a multi-rail configuration offers additional protection against power surges or other issues, since your components are supplied by separate connections. The key with a multi-rail setup, however, is to pay attention to the PSU’s manual to ensure that you make the correct cable connections so that your components are indeed on separate rails.
You can find the information on the number and configuration of a power supply’s +12V rails in the manufacturer’s technical specifications for each model. If you’d rather not be stuck with a particular rail configuration forever, some power supplies, such as the be quiet! Dark Power Pro 11, allow the user to manually switch between single and multi-rail modes depending on the needs of their components and system.
Modular vs. Semi-Modular vs. Non-Modular
Modern power supplies in any form factor offer different levels of modularity for the connectors. This can affect both the price of the power supply as well as the unit’s flexibility when it comes to building the system and cable management.
Non-Modular: with non-modular power supplies, all cables are physically and permanently connected to the PSU. This is the usually the cheapest option, but it requires the builder to keep all unused cables tucked away in the case, which can be a challenge especially with small form factor builds.
Semi-Modular: for semi-modular power supplies, primary cables are permanently attached while optional and accessory cables are modular. Exactly which cables are non-modular depends on the PSU: at the least it will be the 24-pin ATX connector although some models also include the 4/8-pin ATX12V/EPS connector and/or a primary pair of PCIe connectors. Semi-modular strikes a balance between price and flexibility, since the “mandatory” cables are permanently connected and you only need to connect the “optional” PCIe/accessory cables you intended to use.
Modular: with a modular power supply, no cables are permanently attached. These are generally the most expensive at the same wattage and quality, but they allow a builder maximum flexibility. Not only can a builder use just the cables they need, but the included cables can even be replaced with custom options in specific designs (individually sleeved, different colors to match the case, etc.) or at specific lengths.
One factor you don’t want to overlook is noise. With ultra-efficient processors and quiet air coolers and fans, the wrong power supply may end up being one of the loudest components in your system.
If noise level is important for your build, go for a power supply that uses high quality components and fans. This is especially true for SFX power supplies since smaller fans will need to spin at higher RPMs, so the quality and noise level of that fan will make all the difference.
Also be sure to look for a power supply that supports a “quiet” or “silent” mode. Available in mid-to-high end power supplies, these modes automatically reduce or stop the fan speed when the PSU is under a low power load. Just like your CPU and GPU fans, therefore, the PSU fan will only spin up when necessary.
Power Supply Quality
Unlike the other factors discussed here, the quality of your power supply can be a bit trickier to evaluate. While there’s a risk of failure with any component, in general any power supply from a major brand purchased from a reputable retailer should be safe. But you’ll still want to consider your intended usages. An inexpensive, less efficient power supply with a short warranty may be fine for a basic office PC that will be used a few hours each day, but if you’re building a high-end workstation or gaming PC that is likely to experience heavy loads for long periods of time, investing in a higher quality model with a strong warranty is probably a good idea.
When attempting gauge a power supply’s quality, look for detailed reviews like those we do here at PC Perspective. Knowledgeable reviewers will open the power supplies up to examine its design and components and perform tests to measure quality.
Absent detailed reviews, look at the power supply’s warranty period. In general, the warranty length represents a manufacturer’s confidence in the quality and longevity of their product and, of course, provides additional protection to you if a failure does occur. Power supplies in lower price ranges will unsurprisingly include shorter warranties, but at the mid-to-high end of the market you’ll want at least five years.
Putting It All Together
Considering the different factors discussed above, those building a custom PC have the flexibility to take a number of approaches to choosing the right power supply. You can focus on a primary feature – super silent, extremely efficient, maximum modularity – or go all out and get everything.
And if you are after a power supply that has balance that would do the job for most PC builds you may be interested in checking out be quiet!’s Straight Power 11:
- Available in a range of wattages: 450/550/650/750/850/1000W
- 80 PLUS Gold Certified
- Fully modular with sleeved cables
- Offers four independent +12V rails
- Uses a 135mm Silent Wings 3 fan with variable speed
A PSU that matches the specs of 850W Straight Power 11 PSU would do the job even if you are looking at a very demanding build, including high-end components such as an overclocked Ryzen 9 3900X, RTX 2080 Ti, 32GB RAM, 4xSATA drives, and 6 case fans.
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