There are countless instructables and how to articles on the web and in print on building your own PC. However, there aren't as many guides on building a PC that's energy efficient.

Throughout this instructable, I'll give you some tips on how to select the right components for your energy efficient PC. Whether you want to build an ultra efficient power sipping Linux network device or a PC with enough power to play today's demanding games but that's light on both your wallet and the environment, you'll find advice here.

If you're not convinced all this is worth the trouble, read the next step for a counter argument to that.

note: I use the term PC throughout this article. While most of the advice only applies to PCs in particular (for example: most people aren't building a Mac from scratch, but you may be able to replace the hard drive or other components in Apple's machines), some advice applies to Macs just as well. The advice in steps 1 and 2 apply to just about any modern computer in existence.

Step 1: Why Bother? Or, Turn It Off!

Why should you bother? Well, there's a lot of reasons. I could probably go on about the environment, carbon emissions, and dirty power generation all day. That's not going to change your mind if you're not convinced already. So let's talk about the fact that it will save you money on your monthly utility bill and the absolute easiest way to do it.

Turn it off! The most energy efficient PC is one that's off. Seriously! Many people leave their desktops on 24/7/365. If you're not using it, you're just throwing money down the drain. How much money? That depends on how much power costs in your area and what kind of PC you leave running. You can buy a device called a kill-a-watt that will help you measure the energy use of a device. They don't cost that much, and you'd be surprised how much energy things around your house suck up, sometimes even if they're "off". Next, open your utility bill or call your utility company to ask them how their pricing works. Once you know how much your PC uses and how much your electricity costs you can calculate how much money it costs you to run your PC constantly.

A recently released report concluded that US companies alone waste $2.8 billion a year powering unused desktop PCs. The average cost of running a single unused desktop PC? $36 annually. And these are business desktop PCs. If you're leaving your souped up gaming rig with its power sucking overclocked CPU running 24/7, you may stand to save a lot more.

Hey, you shouldn't do that! All that powering on and off will put wear and tear on your computer's components. Your hard drive will crash. Your mother board will fry. Your power supply will burst into flames. Your house will burn down. You'll be homeless and responsible for all the co2 emissions from your burning possessions. Actually, no. None of that will happen. Well, probably not anyway. Modern PC components are built to survive thousands of power cycles. That's not to say they won't fail eventually, but the odds are that powering off your computer when you're not using it won't make it explode.

In fact, there's evidence that indicates powering off your computer may actually be beneficial to its longevity. Components like desktop grade hard drives aren't designed to be used constantly. Using them 24/7 may shorten their life. When your computer is on, it generates heat. The more heat, the more likely component failure is. Also, any moving mechanical part of your PC will wear. If they are moving constantly, they will wear faster and fail sooner. Prime example, fans. If a case fan fails, the heat buildup inside the machine can cause component failure. If your power supply's fan dies, I would say that's even more dangerous. Not only could heat buildup inside the PSU kill it, it could dirty the power to other components and fry them too. If your video card fan dies, you'll start to see graphical artifacts from the over heating and it will eventually fry itself (this has happened to me twice).

I can't claim powering off your computer is guaranteed to lengthen its life. I can't even claim that keeping it powered on all the time won't do the same either. There's evidence on both sides of the debate, so I'll leave that judgment to you dear reader. What it will do though, is save money on your power bill.

Of course, you'll probably want to actually use your computer at some point. So let's talk about making it energy efficient when it's switched on.

Step 2: Power Management Options

The next easiest way to reduce your PC's energy use is to use the power management options of your operating system. I know, I know, I said we'd start talking about how to reduce it's energy use while you used it. We'll get to that in the next step if you want to skip ahead. However, for those of you who can't or don't want to turn off your PC while it's not in use, at least set your power management options to save electricity when you can.

Your PC probably has settings inside it's operating system to save power when it's sitting idle. You can usually do a number of things.

You can turn on a "screen saver". This alone usually doesn't do much, other than prevent CRT burn in. Some graphically intensive screen savers can even use more energy than a idle desktop. A blank screen would be much better. Better still would be turning the monitor off. Sometimes you can specify that hard drives should spin down when idle as well.

The main setting you'll usually see is for "suspending" the system, putting it to "sleep", or into "standby" mode. In this mode the system holds its state inside RAM, which doesn't need much power in comparison, and then shuts off power to things like the hard drives, processor, etc.

Finally, if you have a laptop especially, you may be able to "hibernate" your machine. Hibernation does almost exactly what you might think. Your machine saves its current state to your hard drive, so all your work is safe, and then completely shuts off. When you're ready to work again, power is switched on, the machine's state is retrieved from the hard drive, and you can pickup right where you left off.

Here's how to find these settings in various operating systems, see images for specifics:

Mac OS X: Apple menu (that's that...apple...at the top left of the screen) -> system preferences -> energy saver

Windows: Start -> settings -> control panel -> power options

Ubuntu: System menu -> preferences -> power management

Step 3: Embedded Computers

Embedded computers are computing devices that are designed to do a specialized task. They aren't used as general PCs like most computers. They are designed and built to perform a small subset of tasks in an efficient manner. Think ATMs, digital photo frames, your wireless router, and so on. These devices are all technically computing devices, but they aren't general purpose computers. You wouldn't load Windows on them, that's for sure!

Some examples:

Soekris Engineering net series boards
http://www.soekris.com/

These boards are compact, low-power, communication computers available with up to 500mhz processors. They are often configured to be a firewall, router, vpn, wireless access point, or other network device. Because they have no moving parts, they are extremely reliable. And because they sip power (typically 10-20 watts), they are extremely affordable to run in 24/7 applications.

PC Engines WRAP or ALIX boards
http://www.pcengines.ch/
WRAP stands for wireless router application platform. ALIX is its slightly faster more modern replacement. It should be no surprise then, that these boards from PC Engines have features very similar to the Soekris boards that make them ideal for network devices or other low end computing tasks.

SheevaPlug
http://www.marvell.com/featured/plugcomputing.jsp

This $99 embedded computer from the folks at Marvell sports the shape and size of a standard wallwart! It sports a 1.2 GHz Sheeva CPU, 512 MB RAM, 512 MB flash storage, gigabit Ethernet, and a usb 2.0 port. Marvell says it sips 1/10th the power of a typical desktop (couldn't find any real numbers) and that they'll eventually cost only $49. Ideas for uses include network attached storage, print server, home automation, VOIP, and other home network devices.

Gumstix
http://www.gumstix.com/

The SheevaPlug still not small enough for you? Check out gumstix, the linux computers that are as small as a stick of gum! These specialized embedded computers are perfect for applications where space is a concern. You'll have to do some more work though, whether it's soldering on wires for external control and sensors or buying and attaching add on modules for things like networking. Still, you can't beat the size of these lilliputian linux devices.

Step 4: Low Power Computers

Unlike embedded computers, these "low power" computers can and are often used for general purpose computing. Anything that doesn't need a lot of horse power but needs the flexibility of running something like Windows applications is a perfect target for these machines. Whether it's a web browsing kiosk or simply a machine for basic office tasks like light word processing and email, you'll find something here to like. These machines often have the flexibility to be used in embedded applications too!

Examples include:

VIA series of processors (C3, C7, Nano, etc). These processors are designed from the ground up to be energy efficient and provide good performance per watt. Many of them can run without active cooling, meaning they need only a heat sink to dissipate heat rather than a heat sink with a fan. You don't typically buy a VIA processor separately, instead you'll buy it bundled with a motherboard and possibly RAM. Below you'll see a Jetway J7F series board featuring a VIA C7 processor.

Intel's atom series of processors. Intel designed these processors to target mobile and low power computing platforms. Netbooks, among them Asus' Eee PC, often use these processors. Intel has stated that the performance of these chips is roughly half of a Celeron 430 running at 1.8GHz. Again, as with the VIA chips you'll buy them with a motherboard. Below is an example of an Intel manufactured motherboard featuring an Atom 230 processor.

Step 5: Desktops, the Behemoths

The following steps will talk about individual components you can use to build a standard desktop. Whether it be your general purpose computer or a tricked out gaming rig depends on which components you choose, how much you're willing to spend, and how much you're willing to sacrifice power savings for speed.

Step 6: Processor

The processor is often the first step in building a machine. You decide on a processor and build your machine around it. You want to pick something that will have enough horse power for your needs without over spending on speed you won't use. CPUs are complex devices with a myriad number of technologies in each that make one processor more suited for a particular task than others. A full discussion of choosing a processor is outside the scope of this instructable however, so we'll just consider power consumption and related characteristics.

The wattage of a processor (also called Thermal Design Power or TDP) is the total amount of heat which must be dissipated by cooling for the processor to function correctly. This isn't the maximum amount of power the processor can ever draw (this is a common misconception), but the maximum amount you're likely to be seen drawn running real world applications. This means a processor with a TDP of 100 W is probably going to be using a lot more power than one rated at 10 W. However, a processor rated for 100 W may or may not use more power than one rated for say 90 W. It's not a hard and fast rule.

That said, you'll want to look for processors with lower TDPs in general. The difference between 90 W and 100 W isn't massive, but the difference between 65 W and 125 W is probably going to be noticeable overall. The less power used, the less heat generated. The less heat generated, the less heat needs to be dissipated by the heat sink, fans, your home's air conditioning, etc. Money saved.

Examples:
Budget: AMD Athlon X2 4850e - 2 cores running @ 2.5 GHz w/ 45 W TDP
Midrange: Intel Core 2 Duo E8400 - 2 cores running @ 3.0 GHz w/ 65 W TDP
High end: Intel Core 2 Quad Q9650 - 4 cores running @ 3.0 GHz w/ 95 W TDP

Step 7: Power Supply

The power supply, or PSU, converts the high voltage AC electricity that comes into your home to well regulated lower voltage DC electricity for the components of your computer. This conversion is not perfect, there are inefficiencies in conversion that waste power. The more efficient your power supply, the less power it needs to draw to power your computer's components.

The maximum amount of power a PSU can output is measured in watts, and is a primary feature of a PSU. You can buy PSUs of a few hundred watts or ones that can output over a 1000 watts. Many PSUs are efficient at 100% of their load, meaning when they are outputting the maximum amount of power they were designed to. However, some PSUs become less and less efficient as the load decreases. This isn't good, because you often want to buy a PSU that can output more than you currently plan to use to allow for future upgrades that might consume more power.

80 Plus is an initiative to promote the use of more efficient PSUs. PSUs can become 80 Plus certified at various levels to show how energy efficient they are. Today, there really isn't much of a reason to buy a PSU that isn't 80 Plus certified as there are lots of them on the market. To be 80 Plus certified, a PSU must demonstrate that it is 80% or more energy efficient at 3 levels of load. That is to say, at various amounts of power draw from the PSU it has to waste 20% or less energy in the conversion process. Click here for more details on 80 Plus, and the different levels of certification or click here for the official 80 Plus web site.

How beefy a PSU you need will depend on the type and amount of components you need to power. There are various calculators on the web if you search for them. Components will often state how much power they draw at idle and under load. Using these two things together gives you a good idea of how much capacity you'll need. Make sure you plan for upgrades by allowing yourself some extra capacity in your PSU!

Examples (as of 4/09):
Budget: Enermax MODU82+ - 425 W - 80 Plus Bronze
Midrange: SeaSonic M12D - 750 W - 80 Plus Silver
High end: Cooler Master UCP RSB00 - 1100 W - 80 Plus Silver

Step 8: Video Card

This might be an easy section for some of you. My recommendation for a video card is the integrated graphics found on many motherboards. While not useful for most modern gaming, it will consume the least amount of power.

I know, I know, you had your heart set on playing the latest games. Okay, lets make a compromise then. A good midrange or higher end graphics card? What about SLI? Do two midrange cards in SLI beat one higher end card for performance/watt? That depends on a lot of things, not the least of which is exactly which cards you choose. The important thing to realize is that you should actually DO this comparison. Often, you'll find today's modern dual GPU single card solutions will satisfy your desire for SLI. You can start with the examples below.

Examples (as of 4/09):
Budget: any integrated solution
Midrange single card: ATI Radeon HD 4850
High end single card: ATI Radeon HD 4850 X2

Step 9: Putting It All Together

Once you've decided on your components, you can put it together! Instructions on this are outside the scope of this instructable. I recommend you read the How to Build a PC instructable for a good overview of what you'll need to do.

Congratulations, I hope you've learned a thing or two by reading this. Get out there and start saving some energy!