PC Build — Power-Supplies

2014 May.24 — Added info about efficiency and active-PFC that I forgot to include, the first time around

I’ve jumped into the middle of my series of notes on building a PC. I’ll write the intro to this series, later. I start with this, now, because this is what I spent too many hours researching, this weekend.

Corsair AX860i PSU
Image source: corsair.com
The power-supply is the gas-tank for your PC. It is also the refinery for the fuel. An insufficient power-supply and your machine will not run very well; bad power from the power-supply and your components can burn out, becoming useless pieces of metal and silicon.

How Big?

The first consideration is the amount of power supplied. This can be hard to gauge. First, you need to know all the components you need for the build. Then you need to determine the power needed for each component, then total it all up, add some margin and get a power-supply that will cover, at least, that power requirement. The power-supply needs to supply power to everything running within the box. The most important pieces to include are the power-hungry ones:

  • Video card (especially high-end, gaming cards)
  • CPU
  • Memory
  • Hard drives
  • Any other mechanical component

One handy tool is the PC Part Picker website. Choose all the components you ever think you will use and it will give a wattage requirement estimate. Feel free to throw in several additional hard-drives, optical-drives, and high-end video cards—even if you never plan to actually get all those components. Then choose a power-supply substantially bigger than that.

This is where the “efficiency rating” comes into play; the higher the better, quoted as a percentage. A power-supply with an efficiency rating of 500 watts at 70% efficiency will only deliver 350 watts of useful power (500 × 0.70 = 350); 90% efficiency will yield 450 watts. More watts of power costs more and efficiency costs more, but is less wasteful.

In order to give myself headroom, in addition to the components I knew I’d have, I threw in 3 additional, high-end hard-disk drives, a high-end video card, and optical DVD drives, not all of which would I probably ever get. The total was just over 400 watts. So, I’m looking for a power-supply of at least (450/0.70) 650 watts. Even with all the overhead I planned for, I will probably shoot for 700+ watts; I can’t imagine I’d ever need more than that.


In addition to output power, there are other characteristics to consider:

  • Noise
  • ATX standards
  • Number of Connectors
  • Modularity
  • Active PFC, Voltage support

We don’t need any more noise in our environments. A constantly running power-supply fan will add to the noise generated by the hard-disks, CPU fan, and case-fans. Supplying power generates heat, so they include heat sinks to help draw that heat away and fans to cool the heat sinks. There are some fanless power-supplies, but they are more expensive and limit the versatility of the power-supply due to design limitations that exclude fans to help to manage the heat buildup.

Today’s modern power-supplies are switching power-supplies, so their power usage (and, therefore, heat buildup) is roughly proportional to the power required. You may have noticed that computers’ fans adjust to the power usage and heat buildup within the computer. Some power-supplies will even stop their fans altogether, when not necessary—some of the Corsair power-supplies do that.

What Kind of Power?

Despite attempts to maintain power-supply standards, evolving technology changes demands for voltage and power as well as the types of connectors needed by motherboards and peripherals. This is why I spent so much time researching a new power-supply; while I have a bunch of old PC chassis and power-supplies, it is unlikely that they will have the features necessary for contemporary CPUs, motherboards, video cards, and storage devices. The ATX standard, in addition to defining the motherboard form-factor, defines an evolving power-supply specification.

Make sure not to skimp on the “active PFC” (Power Factor Correction) feature. It digitally manages the power usage to improve efficiency and isolate the power-supply from input voltage variations. Any decent power-supply will implement active-PFC.

Even new CPUs’ specs can impact power-supply compatibility. According to “The big Haswell PSU compatibility list,” my choice of the i7-4770K Haswell CPU means being more careful in selecting power-supplies that fully support that family of chips. Haswell CPUs can run at lower wattage and older power-supplies may be unable supply such low power.

Modular Cables

Cable modularity also can add to the cost. In general, this isn’t necessary, but it provides two advantages:

  • Allows more versatility when the final build is not fully defined; how many components need to be connected and their distances between components.
  • Cleaner builds—only the necessary wires need to be included in the build.

Power supplies may have no modularity, be semi-modular, or fully modular. Fixed cables are like those found in any OEM build—where the number and distances to components are well known. Semi-modular have fixed cables for the “required” cables, with removable cables for others. I like the idea of fully modular, but that depends on whether it will be cost-effective. If I ever build another machine, I could always replace a modular power-supply with one that matches the final build and move the modular one to my next build.

Regardless of whether the cables are modular, the most important thing is that it has enough cables to support the peripherals that will be installed. Beyond the obvious connections to the motherboard, you’ll want to make sure that it has enough SATA and video power connectors to handle all devices you need. If you have particular legacy devices you want to support, then you may need connectors for those devices too.


I haven’t decide what I will get yet. A lot may depend on price. I will update this article as I narrow in on my top picks.  Here are some of the brands that look good so far:

  • Corsair RM or AX Series
  • EVGA SuperNOVA
  • Silverstone
  • Thermal Take

The specs on the Corsair power-supplies look to fit what I am looking for, but reviews on Newegg and Amazon indicate that their quality control can spotty.  I happen to have bought a relatively inexpensive power-supply tester, a long time ago. Before plugging it into my components, I plan to plug it into the tester for a while before risking burning out my motherboard.


Leave a Reply

Your email address will not be published. Required fields are marked *