How to buy a computer power supply

Without a power supply, nothing in your computer works. So it's pretty important. Recent generations of computer hardware have become increasingly power-hungry, and power supply manufacturers have seized the opportunity to create ever-more-potent versions of their products. But more isn't always better. There are a few key things to keep in mind when you are shopping for a power supply.

Don't rely on power supply "calculators"

There are a number of websites providing power supply "calculators" of widely varying quality. The concept behind these is perfectly fine: total up the power needs of the different components in your prospective system, and then you just need to shop for something that can manage that. Done and done.

Unfortunately, these tools often fall victim to one of two problems. Often they will have inaccurate or incomplete information on different components, leading to mistakes or uncertainty. But just as bad, they rarely provide the numbers that you actually need. These tools commonly provide a single total wattage rating recommendation, suggesting that you should buy, say, a 450 W power supply. But this is a gross oversimplification; even among 450 W supplies of similar quality, two different models may be very different, especially if one is older than the other. The recommendation tools have to make a lot of assumptions to come up with their final number, and those assumptions are not always reasonable.

The process we will follow is pretty similar to what those tools attempt to do, but with a different set of assumptions. There are still some edge cases where those assumptions break, but in general they are pretty reliable.

Start with the processor

Your processor will nearly always be one of the two most power-hungry parts of your system, so it pays to make sure it's covered.

Modern processors virtually always include a Thermal Design Power (TDP) rating somewhere in their specifications. It should be the only thing rated in Watts (W). This information can be difficult to get at retail (there are no guarantees that it will be printed on the box), but is usually readily available on the Internet. Newegg, for example, lists it right in the product name. Intel provides TDP ratings for all of their products, back to the Pentium, on Processor Spec Finder. AMD provides information on its current and almost-current products at AMD Compare. And in a pinch, a Google search for your processor's name and model number and "TDP" will usually do a decent job.

Really you should try to be very specific in your search, as TDP can vary substantially between processors that appear otherwise very similar. But we will build enough of a fudge factor in later that you should be fine regardless.

TDP is provided for people designing cooling systems: it describes how much power the chip can be expected to output as heat. But all of that heat comes from electrical power that the chip takes in, and the conversion is 100% efficient, so TDP will also work nicely for our you, the power supply shopper. There are some quirks here (Intel and AMD do not measure TDP in quite the same way), but for our purposes it is adequate to take the TDP rating to be the amount of power that the processor needs.

If you have more than one processor (that is, multiple physical chips in multiple physical sockets, not just a single chip with multiple cores), just multiply TDP by the number of chips. If you simply can't find your processor's TDP, you can simply use 100 W as a rule-of-thumb estimate. This will sometimes be much too high and sometimes fairly low, but it is a useful ballpark guess.

Don't forget about the video card

Power hog number 2 is usually the video card. Video cards are a bigger challenge from a power perspective, because the companies do not generally provide power numbers to the public. If they do, then you're done: use the number the manufacturer provides and call it a day. If not, you will need to look for reviews that try to estimate graphics processing unit (GPU) power draw. Reviews done by Anandtech often include some sort of power numbers.

You don't need to find an exact match for your card; if you can find numbers on a video card with the same GPU and amount of memory (major variants of a card sometimes have different memory sizes), you will generally be OK. So if you have a Radeon HD 4850 with 512 MB of RAM sold by Powercolor, and the only reviews with power measurements that you can find are for a Radeon HD 4850 with 512 MB of RAM sold by Gigabyte, you can use those numbers without too much worry.

If you can't find any numbers for your particular card, you can again use 100 W as an all-purpose guess, though again this can be well off the mark. If you would rather go by the worst-case scenario, the most power that a single PCI-Express 2.0 video card can draw is 300 W. Again, if your system has more than one video card, just multiply. And if you have integrated video rather than a separate video card, use 10 W to cover your bases.

Is your system weird?

We've already noted some of the special cases to watch out for, but there are more.

If you have a fairly old system (Pentium III and earlier, many Athlon XPs and all earlier AMD products) that you are buying a new power supply for, it will supply power to the processor from the 5 V rail, not the 12 V rail. You can spot such motherboards because the don't have the squareish, 4-pin "ATX12V" connector (the one with 2 yellow and 2 black wires) or the newer 8-pin "EPS12V" connector (which looks a lot like 2 of the ATX12V connectors joined together).

If you have a large number of hard drives, you will need to be extra cautious. Hard drives draw a large "inrush" current when they first spin up when the computer is powered on. When one hard drive does this, it's no big deal. If you have 10 hard drives all doing it, your power supply may not be able to cope. Invest in a power supply with beefier 12 V and 5 V rails than you think you really need. If possible, go with SATA drives and a motherboard that supports the staggered spinup feature, which will power the drives up one at a time, with a slight delay, to reduce the load on the power supply.

If you have an (older) AGP graphics card or floppy drive, you will want a power supply with at least 1, preferably 2, "floppy drive" power connectors.

Take stock of connectors

Your motherboard will most likely have 2 connectors to worry about. One will be the ATX "main" connector, which carries several voltages and provides power to most of the hardware on the motherboard (the chipset, RAM, expansion slots). This connector may have 20 pins (the old style) or 24 pins (the new style, meant to better support power-hungry PCI-Express cards). Most power supplies have a "20+4" connector that can accomodate both. The other connector will have 4 or 8 pins, carries only +12 V and ground, and provides power only to the processor.

Your video card will probably have a connector or two as well, unless you have integrated video. These will be 6-pin or 8-pin "PCI-Express Graphics" (PEG) connectors, and they also carry +12 V and ground. If your card has an 8-pin PEG connector, it may come with an adapter to connect to a 6-pin PEG connector from the power supply.

Lastly you will have drives, fans, and possibly other accessories to power. Most power supplies include enough Molex and SATA power connectors that you shouldn't have to worry about this, but take count anyway.

What do you need?

So it's time for the math. This part is important, so pay attention.

Add together the power draw for your processor and video card. Add another 50 W for miscellaneous components. Now divide this number by 10 *, and round up to the nearest integer. The 12 V rail of your power supply should be rated for at least that many amps (A) of current.

As discussed previously, this doesn't work for everyone. If you have an old motherboard, your processor may not draw power from the 12 V rail, and you will need a stronger-than-average 5 V rail. If you have a lot of hard drives, you should total up their power draw (and maybe add a little extra) on each rail (normally the 12 V and 5 V rails) and factor that in. If you have multiple, power-hungry video cards, you will hit some stumbling blocks later on.

But for the vast majority of buyers, this one number is the one critical thing to pay attention to.

What features do you want?

There are the things you need, and then there are the things you want. High-efficiency supplies, active power factor correction, modular cables... there are lots of features you can spend money on if you want to. We have discussed some of these features previously in the Big Power Supply Guide, so we will jump over them here.

We will discuss one feature here, though. Power supplies have fans to help keep them cool, and these fans come in two major configurations. The "classic" configuration places an 80 mm fan at the "back" of the supply (the side that screws into the case and is exposed to the world), sometimes with an additional intake fan at the "front" of the supply. Many supplies now have a larger 120 mm fan on the "bottom" of the supply instead, and a grating on the back. The larger fan is generally quieter (and as we discussed when we made your computer quieter, the noise it produces will usually be less annoying), although the airflow pattern inside the power supply is actually worse. Many people opt for the larger fan, or buy a power supply because it is quieter and, it just so happens, that supply has a larger fan.

Learn to read the label

This is where things get just a touch tricky. Power supplies have several different "rails", which supply different voltages. The details are mostly not worth worrying about. What you need to know is that there are three important rails: 3.3 V, 5 V, and 12 V. These are also sometiems written +3.3 V, +5 V, and +12 V, to distinguish them from the much-less-important negative-voltage rails.

In modern computers, the 12 V rail is by far the most important. It supplies power to the processor and video card, and as we have already noted, those are normally where most of the power goes. In fact, it's the only rail we're going to worry much about here.

Several years ago, Intel added a requirement to the ATX standard, effectively requiring that 12 V rails be limited to 20 A of current. This was meant to address safety concerns. Power supplies more or less complied by building supplies with multiple 12 V rails, which had separate current limiters. Over time even this eroded, and now many such supplies really just have one big rail.

The take-home is that you need to know to look for the combined 12 V rating on a power supply. Often separate ratings for each 12 V rail will be given (often in the form 12V1@18A, 12V2@18A, 12V3@18A). Don't worry about those. Look for a "total" or "combined" 12 V rating, and go by that.

This strategy works just fine (and saves time) unless you have multiple power-hungry graphics cards. In those cases, you may actually need to check the manufacturer's manual to see how the power from the 12 V rails is distributed to ensure you don't overload any one rail.

Pick some initial candidates

Go to your vendor of choice and get ready to build a big list of contenders. Consider this the qualifying round.

You know what your combined 12 V rating needs to be, you know the connectors you need, and you know what features you want. So use those as a basis to grab an initial list of hopefuls. And if a particular supply doesn't explicitly list a combined 12 V rating, don't feel too bad about skipping it.

You can do a little more filtering at this stage if you want. You probably have some idea of a budget, even if it's only "I don't want to spend $500 on a power supply". And maybe there are some brands that you just want to avoid (Aspire, Apevia, and maybe some others that made our bad power supply brands list). So avoid them. And if your list includes a $20 power supply that claims to have the same specs as a $100 supply, and the $20 supply isn't just $20 because it happens to be on sale, you may want to skip that one too.

Reduce your pool of candidates to something manageable

If you've done things right, that should still be a fairly big list. And it's hard to make a reasonable decision on more than 2 or 3 options. So your next goal should be to narrow it down to 2 or 3 options.

What you need for that are reviews. Good reviews. Don't put your fate in the hands of user reviews at a retailer like Newegg; they simply aren't reliable. Sites like JonnyGURU, Anandtech, and SPCR all do thorough, helpful power supply reviews.

Spend, spend, spend

If everything has gone according to plan, you should now be down to perhaps 3 models, and you should have enough information to make some sort of decision between them (even if that decision is just "any of them would work, and this is the cheapest"). Before you whip out your credit card, it is always a good idea to do some quick comparison shopping by plugging a model number into comparison engines like PriceGrabber, Google Product Search, or Epinions.

Footnote

* We cheated a little here to simplify the math. To get the current needed to deliver a specified amount of power at a specified voltage, we use Ohm's Law: P = I * V, where P is power, I is current, and V is a voltage of 12 V. So in effect, we are dividing the power by 12 to get a current value. However, we are simultaneously multiplying by a fudge factor of 1.2 to account for aging effects and manufacturer dishonesty. Multiplying by 1.2 and dividing by 12 is equivalent to dividing by 10. This sleight of hand makes the numbers easier to work with and easier to do without a calculator, while also providing some wiggle room.

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