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Power Savings: 80 PLUS, Energy Star, Advanced Power Management

Power Supply Reference: Consumption, Savings, And More

Over the past few years, energy conservation has become a major focus for appliances, electronics, and especially for computers. Saving power can be accomplished through several means. One is to use more efficient components that simply use (or waste) less energy to do the job. The other is to properly manage the computer hardware so that components that are not being used are powered down or put into standby modes. By using more efficient components and turning off specific components of the PC when they are not in use, you can reduce the electric bill and avoid having to power the computer up and down manually. This not only saves energy, but it makes the system more convenient to use. The following sections discuss both of these approaches to saving power.


When it comes to computers, one of the major factors in overall energy consumption is the efficiency of the power supply unit. In 2004, the Northwest Energy Efficiency Alliance (NEEA) funded the 80 PLUS Program to encourage computer manufacturers to improve the energy efficiency of their machines by installing highly efficient power supplies. Ecos Consulting, who manages the program, tests and certifies power supplies as being 80% (or higher) in efficiency. To help offset the cost of producing more efficient designs, the program also pays incentives to manufacturers producing PSUs and systems that are certified.

Systems with more efficient power supplies consume on average from 15% to 30% less power than conventional designs. This can result in a significant energy and cost savings over the life of a system. In addition, the resulting lower heat output both improves system reliability and saves additional energy in cooling the system as well as the surrounding environment.

The 80 PLUS program currently has five levels of certification, from 80 PLUS to 80 PLUS Platinum. Each level of certification signifies different minimum levels of efficiency, which are measured at three different loads (20%, 50%, and 100%). The following table shows the details of each of the certification levels.

80 PLUS Certification Levels
80 PLUS RatingEfficiency at 20% Rated LoadEfficiency at 50% Rated LoadEfficiency at 100% Rated Load
80 PLUS80%80%80%
80 PLUS Bronze
80 PLUS Silver85%
80 PLUS Gold87%
80 PLUS Platinum90%

How is this efficiency determined, and what is the overall effect? The PSU in a PC converts the high voltage (120 V in the USA) AC wall current to 12 V and lower DC voltages for use in the PC. Unfortunately, no PSU is 100% efficient, meaning that some of the power is lost or used up during the conversion and ends up being dissipated as heat. Conventional PSUs are or were normally about 70% efficient, which means that 30% of the energy drawn from the wall socket is wasted and ends up as heat. As an example, let’s take a system that draws 250 watts total. The table below shows the resulting AC power draw and the amount of wasted energy if the PSU were 70%, 80%, or 90% efficient. 

The Effect of PSU Efficiency on AC Power Draw and Wasted Energy
PSU Classification 
80 PLUS80 PLUS Gold
DC Power Draw (W)250
AC Power Draw (W)
Wasted Energy (W)107

As you can see, when supplying the same 250 watts of power to the system, the actual amount of power used, and consequently the amount of energy wasted, varies considerably. A more efficient PSU can save a tremendous amount of energy and money over the life of a system. Because of this, I highly recommend 80 PLUS certified power supplies, especially those earning the higher efficiency ratings.

Energy Star

Energy Star is an international standard for energy-efficient consumer products, including computers and power supplies. The U.S. Environmental Protection Agency (EPA) introduced Energy Star as a voluntary labeling program designed to identify and promote energy-efficient products. The first products labeled in the program were computers and monitors. In the years since, Energy Star has become an international standard, and the label can be found on new homes, commercial and industrial buildings, appliances, office equipment, lighting, electronics, and more. Devices carrying the Energy Star logo generally use 20%–30% less energy than required by federal standards. In addition to Energy Star, many European-targeted products are labeled with TCO Certification, a combined energy usage and ergonomics rating from the Swedish Confederation of Professional Employees (TCO).

Starting in 2007, the Energy Star computer 4.0 specification required the use of a power supply that meets the 80 PLUS standard. In 2009 the 5.0 specification was released and now requires a power supply meeting the 80 Plus Bronze standard, for a minimum of 85% efficiency (at a 50% load).

Advanced Power Management

Advanced Power Management (APM) is a specification jointly developed by Intel and Microsoft that defines a series of interfaces between power management–capable hardware and a computer’s OS. When it is fully activated, APM can automatically switch a computer between five states, depending on the system’s current activity. Each state represents a further reduction in power use, accomplished by placing unused components into a low-power mode. The five system states are as follows:

  • Full On—The system is completely operational, with no power management occurring.
  • APM Enabled—The system is operational, with some devices being power managed. Unused devices can be powered down and the CPU clock slowed or stopped.
  • APM Standby—The system is not operational, with most devices in a low-power state. The CPU clock can be slowed or stopped, but operational parameters are retained in memory. When triggered by a specific user or system activity, the system can return to the APM Enabled state almost instantaneously.
  • APM Suspend—The system is not operational, with most devices unpowered. The CPU clock is stopped, and operational parameters are saved to disk for later restoration. When triggered by a wakeup event, the system returns to the APM Enabled state relatively slowly.
  • Off—The system is not operational. The power supply is off.

APM requires support from both hardware and software to function. In this chapter, you’ve already seen how ATX-style power supplies can be controlled by software commands using the Power_On signal and the six-pin optional power connector. Manufacturers are also integrating the same type of control features into other system components, such as motherboards, monitors, and disk drives.

OSs that support APM trigger power management events by monitoring the activities performed by the computer user and the applications running on the system. However, the OS does not directly address the power management capabilities of the hardware. All versions of Windows from 3.1 up include APM support.

A system can have many hardware devices and many software functions participating in APM functions, which makes communication difficult. To address this problem, both the OS and the hardware have an abstraction layer that facilitates communication between the various elements of the APM architecture.

The OS runs an APM driver that communicates with the various applications and software functions that trigger power management activities, while the system’s APM-capable hardware devices communicate with the system basic input/output system (BIOS). The APM driver and the BIOS communicate directly, completing the link between the OS and the hardware.

Thus, for APM to function, support for the standard must be built into the system’s individual hardware devices, the system BIOS, and the OS (which includes the APM driver). Without all these components, APM activities can’t occur.

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  • 1 Hide
    de5_Roy , January 11, 2012 4:07 AM
    very informative!
  • 0 Hide
    palladin9479 , January 11, 2012 4:47 AM
    Holy cow. Thanks for that Asus PSU link. I now know what's causing my system instability.

    AMD Phenom II x4 980BE OC'd
    4 x 4GB DDR3-1600 memory
    2x NVidia GTX-580 SLI'd
    4x SATA HDD's
    7x FANs (Water cooled system)

    Comes to 1150W recommended. I have a Corsair HX-1000 1000W PSU.
  • 0 Hide
    sincreator , January 11, 2012 4:57 AM
    Still running a Thermaltake 750w toughpower here. Been 5/6 years now. Man this PSU has seen some upgrades. lol. I'll probally buy another toughpower/Corsair sometime in the near future.(If this one ever dies. lol)
  • -4 Hide
    Dacatak , January 11, 2012 5:41 AM
    Still using the same Enermax Liberty 500W from 2006 for my new Sandy Bridge upgrade with GTX 560Ti.
    The only reason you'd need more than 500W is if you need to power more than one GPU.

    Of course, as stated in the article, not all 500W PSUs are equal. The Enermax Liberty was among the best 500W PSUs in its day, and its quality is still exceptional even by today's standards.
    It has dual 12V rails with 22A on each with a combined output of 32A total. Most of the dual-rail 500W PSUs sold nowadays max out at 18A per rail.

    The Enermax was definitely ahead of its time, and in general, PSUs sold directly by their manufacturer (OEMs such as Enermax, FSP, Kingwin, Seasonic) tend to be of superior quality than those sold by third-party rebranders (Antec, OCZ, Thermaltake, Corsair, etc.).
  • 7 Hide
    cumi2k4 , January 11, 2012 8:22 AM
    Was wondering about power cycling and thermal shock... The article said that thermal shock from powering on & off can cause deterioration in a system. You suggest S3 (Suspend to Ram), but does this also cause thermal shock to the system when resuming from sleep mode?
  • 2 Hide
    lordvj , January 11, 2012 11:59 AM
    ^ this. was wondering the same thing
  • 2 Hide
    jaquith , January 11, 2012 2:37 PM
    Great article and thanks, it'll 'hopefully' make my job easier in the Forum and stop the silly arguments I have recommending PSU's. I really wish folks would stop skimping on their PSU's on nice systems.

    Another important point that folks have a tendency to forget is 'electrolytic capacitor aging' which over time takes their once 650W and after a year or so reduces it to 520W~500W aka Capacitor Aging.

    Great PSU Sizer ->
    100% CPU Utilization (TDP)
    100% System Load
    30%~35% for Capacitor Aging
  • 0 Hide
    zak_mckraken , January 11, 2012 2:40 PM
    @cumi2k4 and lordvj : We can only assume it does cause a thermal shock, since only the RAM retains power in S3 mode. The other unpowered components thus cool down during stand by mode, like a regular shutdown.

    Very informative article by the way!
  • 1 Hide
    TeraMedia , January 11, 2012 2:40 PM

    Yeah, me too! I had significantly underbudgeted power for fans (9), ODD/HDDs (8) and USB devices (3), and was going nuts trying to figure out why the system was unstable at times. I thought I had a bad MoBo, or HDDs, or GPU, or ??!?!@#$? Now I know.
  • 1 Hide
    xenol , January 11, 2012 3:04 PM
    I'm kind of suspect about the ASUS power supply link. It tells me for my old system, I should get a 600W power supply but I ran a 500W on it for years without problem.
  • 0 Hide
    Onus , January 11, 2012 3:35 PM
    The statement about third party rebranders depends on who the OEM is. If Seasonic or Delta makes it (e.g. most Antec units), it is going to be a good PSU. Many Corsair and XFX are made by Seasonic too. Channel Well, Sirtec, and some others have some units that aren't so great.

    I found the article of some interest (and will revisit the sleep settings on my own system), but some of it was also years out of date. That's probably hard to avoid on a writing project of this magnitude.
  • 5 Hide
    chaz_music , January 11, 2012 3:38 PM
    Good collection of interesting PSU topics. I especially liked the ACPI information. I have several comments and suggestions to change in the article though. I work in the PSU industry and can shed some light on a few issues.

    On efficiency, most people leave out the fact that we tend to use air conditioning here in the USA a good part of the year. Here in the mid Atlantic, we tend to use A/C for about ~ 7 months annually. This adds a thermal penalty to any heat that you dump into the office/home air during those months. With most A/C systems, the cost to remove 1W of heat is an additional 0.5W of A/C power (50% overhead). Taking the above numbers and some rounding, I use an overhead rating of 30% total for any heat dumped into my home / office. So take your power loss numbers and multiply by 1.30 to get the total cost impact to your wallet. This also should be done for using CFL and LED lighting. They are not allowed to use A/C cost in their advertising, so the public does not get to see the true possible savings.

    There are several types of UPS systems that you should write about. The one you outlined is called a double conversion unit, which is always processing the power to give a clean regulated sine wave output. These are the least efficient and most expensive though. Double conversion is always taking the AC input, making DC, and using a PWM inverter to make regulated AC again for the output. Double conversion efficiencies are typically around 88-90% efficient, so this can impact you total system efficiency and operational costs. A cheaper UPS is the standby type, which allows the raw utility power to go straight to the load with some light duty surge clamping in between. When the input power voltage goes out of bounds, there is a switch over that is usually around 4-8msec which is faster than the PSU hold up time of 20msec. Since normal operation is straight pass through, the usual efficiency is close to 100% (minus the UPS internal power needs and charging). Note though that some UPS systems are crap and can use upwards to 100W just being plugged in.

    I did not follow your discussion on the alarm buzzer indicating overcharge, which should never happen in any UPS. Most modern UPS system implement a battery test to make sure that the battery capacity and internal resistance is able to hold up the load. If the battery fails, they set off the buzzer. In almost all UPS systems, a buzzer alarm is critical - something is wrong. Some UPS systems also monitor the ground feed continuity and will alarm if the input feed ground starts to float making the UPS and the load unsafe to touch.

    The UPS output waveforms are not all sine wave. Often the double conversion types are sine wave, adding to their cost. The standby UPS systems are usually step wave which is also called quasi-sine which is marketing term for step wave (to confuse the buyer). Most PC loads and monitors work fine with step wave (and are even more efficient on step wave!), although some PFC PSUs have problems. Magnetic loads can have real heartburn with step wave (motors, transformers) due to high losses and non-sinusoid voltage waveform effects.

    Ferroresonant transformers are good voltage regulators, but the way they work is very lossy. A good ferro will only run around 90% efficiency. If your load is attached to a ferro, you are adding another power loss in your system. In my opinion, you are better off spend a few more dollars and getting a UPS (which there are ferro types still out there also).

    There is no mention of oversizing your PSU also. Many HTPC and SOHO/home server needs are on 24/7 so power usage and efficiency are paramount to the cost of use / ownership. If you install an oversized PSU, you are taking a efficiency hit (for most brands) that increases your energy usage. The 80 Plus standards do not test below 20% load, so the efficiency of most PSU designs drop off quickly below 20% load. I have seen several that are below 50% with 10% loading. A good analogy on oversizing that I have used before is thinking about car engines. You cannot get a V8 car engine to run as efficiently as a 4 cylinder due to the physics (more friction/mass, etc.). That same effect occurs in a PSU. Larger magnetics, power devices, and other overhead lowers the efficiency at low power. proper sizing can save a good bit of money. Just don't get it too small, especially thinking about system start up (HDD spin up, fans, CPU local PSUs ramping up, etc.).

    You comment on thermal shock is great, but there are many other factors to consider in reliability. Spinning down any HDD and fan loads reduces bearing wear for those mechanical parts. But keeping the main motherboard PCB powered and some operation continuing also helps with reliability. The minor amount of heat that is generated helps keep the PCB dry (PCB material is hydroscopic!), which one major part of the high voltage area in a PSU failing after a long storage (like right after purchase) causing a DOA. And as others pointed out in the comments, allowing the system to go into a sleep state will also cause a cool down thermal shock. The biggest problem with thermal shock is that it break solder joints and helps break bond wires/connections in ICs. It also speeds up electrolytic cap leaking and shortens the life. Does anyone remember the motherboard cap failure from a few years ago?

    The absolute largest cause of computer failures is caused by ESD damage. The data from companies that keep statistics on this unanimously show this as a fact, but the PC enthusiast industry does not work to educate the end users of this well at all. In the electronics industry as a whole, ESD accounts for nearly 55-60% of all failures! This includes component suppliers, etc. So if you want a great topic for a future article, tackle ESD. It is real and it is very costly when ignored. Ever had a PC part that was DOA, i.e., that just "did not work at all" when powered up the first time and would not work at all? Good chance it was ESD.

    Thanks for the article.
  • 3 Hide
    george21546 , January 11, 2012 3:52 PM
    Buy a power meter kill-o-watt comes to mind. Cost 15-20 and will tell you amps, watts, power factor and cycles per second. Best of all it will measure watts over time so you can check how much your system is using in each of it's states. I like to oversize power supplies by 25% unless upgrades are planned.
  • 0 Hide
    chris maple , January 11, 2012 6:48 PM
    The low ends of the ranges shown are too high. Discrete video cards are available that use less than 10 watts, same for hard drives. Motherboards rarely exceed 25 watts.
    My system has an Intel Core I7-870, discrete video card, 2x2G RAM, 2 1Tbyte hard drives, an SSD and a DVD burner. It usually runs at 70 watts and has never exceeded 200 watts driven hard.
  • 0 Hide
    ethaniel , January 11, 2012 8:49 PM
    I thought it was only -5% tolerance for the -/+12v rail. Good data.
  • 0 Hide
    BlackHawk91 , January 11, 2012 10:22 PM
    Would enabling the S3 sleep mode interfere with OC settings and/or performance?
  • 0 Hide
    hardcore_gamer , January 12, 2012 12:12 PM
    palladin9479Holy cow. Thanks for that Asus PSU link. I now know what's causing my system instability.AMD Phenom II x4 980BE OC'd4 x 4GB DDR3-1600 memory2x NVidia GTX-580 SLI'd4x SATA ......

    You have a serious bottleneck there bro ;) . Time to upgrade the CPU.
  • 0 Hide
    g-unit1111 , January 12, 2012 10:50 PM
    palladin9479Holy cow. Thanks for that Asus PSU link. I now know what's causing my system instability.AMD Phenom II x4 980BE OC'd4 x 4GB DDR3-1600 memory2x NVidia GTX-580 SLI'd4x SATA HDD's1x SATA DVDRW7x FANs (Water cooled system)Comes to 1150W recommended. I have a Corsair HX-1000 1000W PSU.

    Yeah... that floored me as well, mine is 900 minimum.

    1 x AMD Phenom II X6 1055T OC'd
    2 x Geforce GTX 550TI
    4 x 4GB DDR3
    1 x SSD
    2 x HD
    2 x DVD-RW
    5 x CPU fans (double heat sink)

    I know now what's causing most of my heat issues is that I'm running an underpowered PSU (Corsair 750). I will definitely make this my next upgrade.

    And that thing about putting systems to sleep, I'll do that more often.
  • 0 Hide
    palladin9479 , January 12, 2012 11:57 PM
    Remember that ASUS link is calculating the approximate maximum power draw possible on your system. Basically with everything going full blast which doesn't happen too often.

    PSU's in general start to get stressed once their over 80% of their rated output. Prolonged stress can cause components to wear out much earlier then before. This is why a PSU may be fine for awhile but then start to have random issues six months or more after installation. I just didn't think I was burning that much juice, but now it seems I am.
  • 0 Hide
    A Bad Day , January 13, 2012 2:51 AM
    Just a question, is it worth watercooling a PSU? I know it would boost efficiency and allow it to put out higher watt than specified, but is it worth it?
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