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April 13, 2009

The 6 Most Important Factors when Choosing a Power Supply

“Which power supply do I need?” “What should I look for?”

Have you ever asked yourself these questions? Many of you did ask me for some help regarding power supplies, so here we go!

When you have to pick a new power supply, you have six main factors to consider:

* Maximum power output, in Watts (W)
* Maximum amperage output, mainly on the 12V line(s), in Amperage (A)
* Power efficiency, in %
* Brand/Series
* Number and type of connections
* Price

How much power do I need?

It’s impossible to give a single answer, as everyone have different computer systems. To answer this question, I recommend that you use the two following power supply calculators.

NewEgg Power Supply Calculator (Basic, for simple systems)

eXtreme Power Supply Calculator (Advanced, for advanced systems, overclocking, etc.)

I recommend that you add up 100W to your result, to be on the safe side. For example, I get 402W out of the eXtreme calculator, for my own overclocked system. In this case, I’d recommend getting at least a 500W power supply.

Keep in mind that overclocking raise power requirements.

Now that you figured out how much power you needed, let’s move on to amperage.

As much as the power requirement matters, the Amperage requirement is as important if not even more important. This is especially true if you have a or several dedicated video card(s). As a general rule of thumb, you’re aiming for the highest number on the 12V line.

There’s also the debate about using a power supply with a single or multiple 12V lines. How do you figure out which one a particular model is? See the following pictures, which I’ll use for a comparison at the same.

Both power supplies are rated for a maximum of 600W power output in total overall.



Can you spot the difference? On the OCZ model on the right, with multiple 12V rails, you have +12V1, 2, 3 and 4, each one rated for 18A.

On the Ultra model on the left, with the single 12V rail, you simply have a +12V, rated for 37A.

You’re probably telling yourself that the OCZ model, on the left, has more much potential, with 72A in total!

Not quite. See, the thing that the marketing folks don’t tell you is that each of those multiple rails can deliver up to 18A, but they cannot do it all at the same time.

To find the true total of Amperes that those 12V lines can deliver together, here are some maths for you:

The maximum combined rated wattage for the 3.3V, 5V and all the 12V lines is 580W. In the markerting world (read: In a dream world) where the 3.3V and 5V don’t use any power you’d have 580W available in total, to be separated between all the 12V lines.

580W / 12V = 48.33A. That’s the maximum amount of 12V amperage that the power supply would be able to supply in an unrealistic situation.

Now, back to real life, let’s assume that the 3.3V(@10A=33W) and 5V(@10A=50W) used 88W. This is a guess, not too far from real usages though, which varies depending on your setup of course. In this assumption, that means that the 12V lines would have 492W now.

492W / 12V = 41A.

In both cases(48.33A or 41A), that’s far from the 72A you would have first thought of right? Over 40% off what you would have thought in the second case! Imagine buying a car only to discover that it offers more than 40% less power than you thought, because of confusing marketing tricks. You would be pissed right?

So this goes to show you that you have to be careful with multiple 12V rails power supply for that. Marketing makes it look like this particular power supply from OCZ does deliver up to 72A on the 12V lines, like anyone could easy conclude by adding up the 4 lines amperage together.

Power supplies manufacturers, I ask you to clearly indicate the maximum Amperage available at a given time. Kudos to SilverStone for indicating this information on some of their power supplies. I’m sick and tired of this confusing marketing bullshit! Give me real specs!
Power efficiency

Why is power efficiency is important?

* Wasted power is converted to heat, which need to be dissipated with a fan. The lower the efficiency, the more heat. The more heat, the faster the fan will have to run to dissipate it. The faster the fan, the noisier it will be. So, high efficiency = less noise basically.
* Electricity bill. You have to pay for that wasted power , even if you’re not really using it ;)

You’re basically aiming for the highest % that you can get. 80% is decent, while 85% and higher is better.
Number and type of connections

You have to make sure that you have enough of the right type of power connections for all your components to be plugged and working.

The most common types are:

20+4 pin Main Connector: For the motherboard
4 pin 12V: Plugged into the motherboard, used by the CPU. May be 8 pin in some cases.
6 pin PCI-Express: Plugged into and used by video cards.
6+2 pin(or 8 pin) PCI Express: Same as the regular 6 pin, except that it can deliver even more power to the video card.
SATA: A “L” shaped power connection, for recent hard drives and optical drives.
Molex or Peripheral: 4 pin connectors for older hard drives, optical drives, case fans, etc.
Floppy: Use this connection to power a floppy disk drive.

Your power supply is powering your whole PC, which is worth hundreds if not thousands of dollars. The differences between a good and cheap brand/series are the following:

* Reliability
* Power protection
* Clean power

Reliability: Cheaper brands power supplies are much more prone to breakdowns. The problem with that is not only downtime, it’s that it can also break another part of the, if not the whole, computer!

Power protection: It protects your computer from power surges and short lack of power ( in the milliseconds range). This is especially important if you live in a old house/neighborhood or if you have frequent thunderstorms in your area. Cheaper models/brands will not offer any type of protection or only minimum, leaving your PC unprotected!

Clean power: By clean power, I mean that power does not fluctuate. A cheap brand power supply power will fluctuate and that in return will cause your PC to be unstable or worse, make it crash. For overclocking, clean and stable power is crucial!

For a list of many power supplies, see this extensive brand/series power supply list, divided in tiers, according to performance, quality and reliability.


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