How to Pick the Right Power Supply

Maximum PC Staff

The Role of the Power Supply

The power supply is an oft-underappreciated component. Everyone knows it’s necessary, and most people who build their own PCs have some understanding of the basics of what a power supply does. But almost everyone tends to buy more power than they really need, or they’ll buy the cheapest PSU that delivers the wattage they think they need.

We’re not going to get too deep into the inner workings of the PC power supply, but it’s worth talking a bit about the basics of how they work before we dive into what to buy. Note that all references to household current are U.S. numbers, so adjust accordingly if you’re reading this in a different part of the world.

PC PSU Basics

The power supply’s job is to take 110-120V running at 60Hz and convert it to voltage and alternating current frequency suitable for your PC. Oddly, the current coming in from the wall is converted to DC (direct current), and then converted back to very high frequency alternating current. Trying to convert directly to DC current from 60Hz would require massive components, and PSUs would be unnecessarily large.  This high frequency AC is then converted to DC.

You’ll often see references to power supply rails. A rail represents a single voltage source. Most PCs use +3.3, +5 and +12V for driving motherboard and component circuitry. For most performance enthusiasts,  +12V current delivery is probably the most important. The +3.3V and 5V rail is for storage plus some motherboard components and expansion cards.

The 12V rail was once a fairly minor component in power usage inside the PC, but two things changed all that. First, Intel shipped the Pentium 4 CPU, which needed a 4-pin (now often 8-pin) 12V power connector to keep the power-sucking P4 running. The second thing that happened is that PCI Express graphics cards began using external power connectors for high performance products. So now the most power hungry parts inside the PC require 12V power delivery.

You’ll see heated debate among performance enthusiasts about the benefits of multiple +12V rails versus a single 12V rail. The thinking behind a single rail is that any particular 12V connector can draw all the power it needs, up to the total 12V limit for the PSU. Multiple rails are safer, with better circuitry for shutting down the PSU and protecting the system if current draw on one rail is too high. For most users, the number of rails is irrelevant to overall performance. If you’re interested, check out this discussion on multiple versus single rails .

How Much Power?

This is the biggest bugaboo power supply buyers face: how much power do I need? Various power supply calculators exist, like the ones at Outer Vision and vbutils . These are useful tools to give you a baseline, but most of them lack listings for every possible component. For example, the Extreme Outervision calculator didn’t list my Corsair sealed liquid cooler, so I had to take a guess. But it did show all the other components, and recommended a 718W PSU for my Core i7 980X CrossFireX system. That’s close to the 750W unit I actually use.

In point of fact, however, most systems don’t draw that much power at full throttle. I’ve been testing graphics cards on the same test system for several years. That system has an Corsair TX850 PSU. The single largest power draw I’ve seen with this system was when I reviewed the Asus Mars II graphics card, which has two overclocked Nvidia GTX 580 GPUs and no current limiter. Under a heavy load, that system drew 718W. If you seriously overclock your CPU and have three GTX 580 cards, you might want a kilowatt or more. Most normal systems can get by with less than 800W.

Another problem with these calculators is that not all PSUs are created equal. That 750W PSU I’ve got in my system is a Corsair AX750, which can deliver robust current load even as it approaches its rated maximum output. Other higher end power supplies from other companies like Antec, Coolermaster, Seasonic Enermax and others are equally well built.

Let’s take a quick look at two different 750W PSUs as a comparison. Both sport single +12V rails. One is the Diablotek PHD750 and the other is the PC Power & Cooling Silencer 750. Both have two 6-pin and two 8-pin PCI Express power connectors. But the Diablotek is rated to deliver 54A on its single 12V rail. If we do the math, 54A x 12V is 648W.

The PC Power & Cooling unit specs tell us it's rated to deliver 62.4A on its single 12V rail, so 62.4A x 12 is 748.8W. So the PC Power & Cooling unit can deliver almost its entire power load to the 12V rail. It’s also got a better warranty.

That doesn’t mean the Diablotek is a “bad” PSU – but I’d never use it in a system where I know the PSU might be under stress, like a dual GPU system with a significantly overclocked CPU. The PC Power & Cooling unit is rated at 88% efficiency, which translates to lower power usage when idling. Of course, it costs $110 versus the $60 for the Diablotek. But you get what you pay for.

Let’s reiterate that last though: with power supplies, you get what you pay for. Component quality doesn’t come cheap, and the power supply is critical in ensuring a stable system. I’ve often encountered stability issues with systems that were solved by installing a higher quality power supply.

Current Delivery

One other issue to consider is current delivery, particularly on the PCI Express power connections. Not long ago, some Nvidia-based graphics cards caused problems with systems that seemed to have perfectly adequate power supplies. The problem was that the PSU didn’t ramp up current delivery at system startup, and the system would sometimes refuse to boot. The GPU really wasn’t the problem—the PSUs—which were usually multiple 12V rail designs, rather than a single rail—just couldn’t deliver adequate current to the GPU. Even if the system booted successfully, stability problems could persist. So be sure to check the rated current on the PCIe power connections if you’re contemplating a multi-rail design.

Modular or Not?

Modular power supplies are popular because they allow you to easily and simply organize your power connections. With standard power supplies, there are always unused cables that you need to bundle up and tuck somewhere inside the case, and the result is not only unsightly, but can interfere with airflow in smaller cases.

On the other hand, modular PSUs aren’t perfect. Each connection can increase the impedance on the circuit, potentially making it less efficient. Modular PSU makers have improved connectors over time, the connection can never be as efficient as straight wire. For most users, the convenience of modular connectors outweighs minor losses in efficiency.

The real issue is the lack of standards for modular connections. Even pinouts for similar looking connectors can differ. The hodge-podge of different modular connectors is an incredible nuisance, particularly if you own different brands of power supplies. Even within the same brand, different PSUs may have different modular connectors.

Each PSU maker uses different styles of modular connectors, so hang onto extra cables!

If the PSU makers could agree on one set of connection standards, then the connectors themselves could be interchangeable. There might even be aftermarket connectors available. For example, I’d love to be able to add more SATA connectors and dump some of the old Molex-style 4-pin connectors in some setups. But that’s not possible in today’s environment of different connector styles for modular PSUs.

Count Your Connections!

Of course, power isn’t everything. In addition to how much power you need, you also should count your connectors. I’ve gotten myself into trouble in the past when I discovered that my PSU only has four SATA drive connectors, while I have five SATA devices in my system. Sure, I can use an Molex-to-SATA adapter, but I’d really prefer to have the correct connectors.

Can't have too many of these…

So be sure to do an inventory of all the connectors in your system. Don’t forget to take into account possible future growth, particularly with SATA and PCI Express connectors. Better to have four 8-pin PCIe connectors, and use them only in 6-pin mode, than to not have them and discover that new high performance graphics card you just bought requires two 8-pin connectors. Similarly, ensure the PSU has a minimum two extra SATA connectors beyond your existing installation.

Eight is better than six.

Note that some power supplies come with multiple ATX12V connectors – those connectors used to deliver additional dedicated power to the CPU. Some lower end motherboards only have a single 4-pin ATX12V connector, while higher end boards have an 8-pin connector. A few extended ATX boards for workstation and server apps may have an additional 4- or 8-pin connector. Most PSUs have a split 8-pin, in which either 4-pin half may be used in a motherboard with only a 4-pin connector, or may be combined to supply 8-pin ATX12V. A few PSUs ship with dedicated 4- and 8-pin connectors, so you use whichever one you need.

If your PSU comes with a fixed 8-pin ATX12V connector, there's probably a 4-pin one as well. Or you could have a dual 4-pin.

Note that most power supplies still ship with a number of those old Molex 4-pin connectors. These days, those are mostly used for driving fans that don’t connect to a motherboard, though a few older optical drives still exist that use them. On higher end systems, I find those Molex plugs going completely unused, because the case fans are either plugged into the motherboard and controlled by the BIOS or plugged into a dedicated fan controller.

Efficiency & Noise

Now that actual power supply efficiency certifications exist, it’s a little easier to choose a PSU based on a desired efficiency goal.

Before jumping into a discussion of PSU efficiency ratings, let’s discuss a key point. Let’s say you have a system that runs at 70W idle with a 500W PSU. If you drop in a 800W PSU that’s equally efficient, your system will still run at 70W while idling. The system consumes no more power at idle just because you have a PSU with a bigger maximum capacity, assuming both are equally efficient. The bigger PSU gives you more headroom – it will run more quietly and generate less heat when it’s drawing 500W. So having a little extra capacity headroom may reduce noise levels and even generate less heat.

PSU efficiency is rated at five levels for typical desktop PC PSUs; those levels correspond to how well the PSU maintains 80% efficiency over its load range. (There’s actually a sixth level for high voltage redundant PSUs.) Here’s a chart from the Wikipedia article on the 80 Plus standard :

The key takeaway is that higher 80 Plus levels are more efficient. Note that not all power supplies are equally adept at hitting the standards; there are always little variations among different samples. But in general, higher 80 Plus ratings means your PSU is more efficient – and also likely more expensive.

Physical Size

So you’ve just bought that lovely 950W, 80-plus gold certified power supply. It’s heavy and solid feeling, and has all the connectors you ever would want. So you go to install it in your PC chassis, only to find out that you can’t have your hard drives and the power supply installed at the same time.


This scenario has happened to me on occasion. PSUs actually come in different physical sizes. The size doesn’t matter much if you pop it into a gigantic, full tower case. But a number of mid-tower cases have constrained PSU spaces that are opposite the optical or hard drive bays. Buy a PSU that’s just a little too long, and you’ll find yourself unable to attach SATA drive connectors.

Two very good 750W PSUs. Guess which one fits into a small case.

Bottom line: be sure to check exactly how much space you have, and the physical size of the power supply, before dropping hard coin on that new PSU.

Final Thoughts

We’ve barely touched on the technology underlying today’s power supplies, which deliver more power and are more efficient than ever. However, this should arm you with enough information to make good choices.  Remember, if there’s one thing you don’t want to skimp on, it’s the quality of the power supply. If you’re on a budget, be willing to sacrifice a speed grade on your CPU for a better PSU. It’s much better to be stable and actually run your games than have a systems that can generate blue screens of death just a little faster.

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