What you need to know about power supplies

It started with a clicky-popping noise and a black screen. There was an acrid smell of burning electronics, and a thin wisp of smoke rising from the computer tower. After recovering from my split-second of panic, I quickly realized that I wasn’t going to be able to play World of Warcraft or Quake 3 Urban Terror for a few days. My power supply had failed, and luckily I was just out of a PSU, and not an entire system.

(I should note that I used a PSU that came preinstalled in the case. I highly recommend getting a case without a power supply and choosing your own PSU.)

Ever since that unfortunate day, I’ve always told friends and readers to invest in a good power supply. Sure, losing a power supply to failure is inconvenient, but the risk of losing a motherboard or CPU is a very expensive possible side effect. Anyone who builds a PC owes it to themselves to pick a decent power supply. Here are the guidelines that I generally follow when choosing a PSU.

Page 1: Wattage and Efficiency and the 80 Plus system
Page 2: Warranties, Modular design, and PSU sizes

An information label on the side of a 650W SeaSonic PSU.

Wattage

Your PSU’s wattage rating is usually going to be the most obvious metric for choosing a power supply. If you choose a PSU with too little wattage, your system will shut off when it draws more power than the PSU can feed it. Conversely, buying a ton of wattage could be a waste of money. So, what is a builder to do?

The key is to come up with an estimated power draw for your PC. This estimate could be for the system you’re building today, but it is far wiser to build for the system you’ll be running in a year. Every new component will change how many watts your system needs to keep running. Generally, this tends to increase demand, though some component replacements can decrease demand (CPUs and GPUs have recently trended towards more power efficiency).

To estimate wattage demand, I personally like to use Extreme Outer Vision’s eXtreme Power Supply calculator. The calculator allows you to select your CPU and GPU make and model, storage, and other components. If you’re planning to overclock your system, you can also set CPU clocks and voltage, GPU clocks, and video memory clocks.

The eXtreme Power Supply Calculator helps you predict load for various component combinations.

When you’ve put in all the details you care to include, the calculator spits out three numbers: load wattage, recommended UPS rating, and recommended PSU wattage. The first number is the estimated load that you’ll typically see for the system. In the example in the screenshot, the system is estimated to pull about 320W. Under most circumstances, this load is pretty accurate, but the calculator provides the recommended PSU wattage, which is 370W. The higher number is meant to represent what you should buy for that system. The problem is, the calculator is just a number and doesn’t necessarily represent PSU wattages you can actually buy.

To settle on a wattage that I can find easily, I do a couple things. First, I round up the wattage to the nearest 50W mark (370W would round up to 400W). I’ll often add 50 to 100W to that, depending on the system. With this method, I can usually find a PSU that will deliver ample power, even if I upgrade to something more powerful down the road.

The eXtreme Power Supply Calculator will spit out numbers that are more of a starting point than an absolute.

For some systems, that extra 50W or more may not be warranted. Locked CPUs (Intel CPUs without the ‘K’ or ‘X’ designation) are far less likely to be used in situations where they draw more power than their spec calls for. Additionally, these CPUs tend to drop down from their maximum clock speed when they warm up, which helps save power too.

When it comes to unlocked CPUs and GPU overclocking, I like to have plenty of power overhead. That overhead can come in handy when pushing an overclock, or when adding components to an overclocked system. Overclocking often demands better cooling, and every fan and water pump will draw a few watts as well. The last thing you want to do while tweaking your build is find out that you don’t have enough power to do what you want. It’s a lot like a budget in real life: You may be able to get by with just the basics, but you should plan on having a little fun and some unexpected new expenses as well.

I think it’s also worth noting that your system won’t always be at maximum power. Most PCs only draw a hundred watts or less at idle, and rarely much more than 200W while doing “everyday” tasks like working on documents or browsing the web. But you want your PSU to handle peak power requirements, not typical loads.

The 80 Plus gold logo is an easy way to determine the minimum efficiency of the PSU.

Efficiency and the 80 Plus system

As much as we’d like them to, electronics never operate at 100 percent efficiency in the real world. (If you have a device that does, I know a few engineers and physicists who would love to speak to you.) The label “80 Plus” on a PSU denotes that the PSU has been rated for a certain level of efficiency. Before we get into the 80 Plus system, let’s talk about efficiency.

When a PSU (or other device) is 80 percent efficient, 80 percent of the rated power is actually delivered to the system and the other 20 percent is lost in the form of heat. If a PSU is drawing 500W from the wall and is 80 percent efficient at 100 percent load, it will only be able to deliver 400W at maximum output. Such a PSU will be rated at 400W, since that’s the maximum power that will actually be delivered to the system.

Since the rated power of the PSU takes into account the PSU’s efficiency, there’s not a lot of math you have to do on your end. That is, unless you care about power bills. If you like to keep your PC on all the time (like I do), or you spend long hours gaming, a more efficient PSU can save you money. If that same 400W PSU is 90 percent efficient, it will only pull 444W (instead of 500W) from the wall to deliver the 400W to your PC. That difference equates to about the same energy as a 60W light bulb. And the more you play demanding games, the more those kilowatt hours start to stack up.

The 80 Plus specification requires a PSU to be at least 80 percent efficient at 115V (U.S.) at all loads of 20 percent and above.

It’s important to note that PSU efficiency isn’t linear and changes depending on load. The 80 Plus specification requires a PSU to be at least 80 percent efficient at 115V (U.S.) at all loads of 20 percent and above. For 230V (EU) AC connections, a PSU must be 82 percent efficient at 20 and 100 percent load, and 85 percent efficient at 50 percent load. 

Part quality is another big reason to get a more efficient PSU. The more efficient a PSU is, the less heat it generates. That tends to mean that the component parts last longer, and it doesn’t need to use its cooling fan as much. (Some power supplies are efficient enough to not have to engage the fan all the time.) Depending on your case, a less efficient PSU has the potential to raise the ambient temperature inside. PSU partitions and mounting your PSU fan-side-down in a case with floor vents can help mitigate these effects, but the best approach is to go with an efficient PSU in the first place.

This goes without saying, but a more efficient PSU is also “greener”. A high-power gaming PC under load is a major appliance, just like a washing machine or refrigerator. Unless you’re lucky enough to live off-grid with an array of solar panels and/or windmills on your property, chances are you draw your power from the grid. Drawing less power reduces the load demand of the entire grid, which can help keep peaker plants offline, especially during times of high-demand.

Talking about efficiency can be a bit mind-numbing. Labeling systems tend to make things easier. Energy Star adopted the 80 Plus specification in 2006 to help consumers buy more energy efficient computers. PSUs are tested by Ecova and EPRI for compliance, and Ecova maintains a list of every 80 Plus-rated PSU on its Plug Load Solutions website.

This table shows the thresholds for various 80 Plus ratings at different input voltages.

I personally recommend choosing PSUs rated at gold or higher, as 80 Plus Gold are at least 90 percent efficient at 50 percent load, and 87 percent efficient at 20 and 100 percent load. Like I mentioned earlier, a more efficient PSU is likely to be made with higher-grade parts. Higher-grade parts mean they can hold up under load and will often last longer.

If you’re looking for the very best rating, titanium, be prepared to search and pay a hefty price. Though we used EVGA’s SuperNOVA 1600 T2 in the 2015 Dream Machine, I haven’t seen a whole lot of titanium-rated PSUs out in the wild. After checking the list at Plug Load Solutions, there’s a reason why: Most consumer-facing PSU manufacturers (like Corsair and Enermax) only offer one titanium-rated PSU in their lineups. EVGA had the most at four. Titanium-rated PSUs also tend to come at the higher wattages, so you’ll be paying a pretty penny for that efficiency. My advice is to stick to platinum or gold for most builds.