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PC Power Supplies | what is power efficiency?

Efficiency is the ratio between the useful output of an energy conversion device and the input.*For example, if your computer uses 300W, but pulls 400W from the wall, then the efficiency is 300W/400W, or 75%.

Computer power supplies are typically 75% efficient, especially those units included with computer chassis or units made more than a couple of years ago before power supply efficiency was made such a priority.*The BFG GS, LS, MX and ES power supplies are typically 80% efficient or better.

Why is efficiency important?

Quite simply, if your power supply is more efficient, your computer will use less power.*Depending on how much you pay for power from your utility company and how much power your computer typically uses, you can save anywhere from $1 to $10 per year, per computer? perhaps more!*Furthermore, because any AC power that is NOT converted into DC power is exhausted as heat, a more efficient power supply inherently runs cooler.*Not only does this mean your office is going to be cooler, but also allows the power supply manufacturer to use a slower, quieter fans to cool the power supply.

Aren't higher wattage units less efficient at lower loads?

With all things being equal, yes. But you can't always compare brand A with brand B and assume that because brand B is a higher wattage that it's going to be less efficient at lower loads.

It is true that most power supplies are only at their most efficient when the load on them is 20% or more of their capability. So with conventional power supplies, you pretty much have to throw any kind of green initiative out the window when trying to buy a power supply that will allow for future expansion; like buying a second graphics card for SLI or adding more hard drives to run a RAID array.

Of course, even conventional power supplies vary from unit to unit, and since the initiative to be more efficient is a relatively new concept be aware that even if a modern day computer power supply is only 80% efficient from loads of 20% and up, it may be 77% or 78% efficient at a 10% load and this may still be much more efficient than the power supply you're replacing, even at it's best efficiency!

What is 80 Plus?

For a fee, 80 Plus will test your company's power supply to confirm that is over 80% efficient at 20%, 50% and 100% loads. Recently, 80 Plus expanded to include bronze, silver and gold certifications for power supplies that are over 82%, 85% and 87% respectively. Naturally, because of multiple PSU companies sharing the same platform and the cost of certification, the list is not all inclusive... but it's a heck of a brilliant start: 80 Plus PSU List

Power Supply Efficiency FAQ - AnandTech Forums

Power Factor Correction

What is "power factor"?

Power factor, or ?PF? for short, is the ratio of the real power to the apparent power.

Real power is the capacity of the circuit for performing work in a particular time and is measured in Watts.

Apparent power is the product of the voltage and current (V x A) of the circuit and is measured in volt-amperage (or ?VA?.)

I know that it almost sounds as if Watts and VA are the same thing, and in DC they are (240W DC is equal to 240VA DC, for example) but because energy stored in the load of a device using alternating current (AC) is returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can actually be greater than the real power. This would give you a power factor of less than 1. Power factor below .70 is generally considered poor power factor.

So what if I have a poor power factor?

Most consumers are charged per kWh (kilowatt hour) by their utility company. Fortunately, for the customer, poor power factor does not typically affect how much wattage your computer uses. But poor power factor does has an affect on how much power the utility companies can deliver. This means that the utility companies either have to increase their grid's capacity to compensate for the increased power load, charge per kVA instead of per kWh (some commercial/industrial accounts are charged per kVA while residential customers are still charged per kWh), charge a "power factor penalty charge" (which can be applied to customers with power factors even as high as .95!) or impose a martial law of sorts requiring all appliances sold in the country to have a power factor of .96 or better.

The European Union believed the latter of these to be the best solution, so as of January 1st of 2001 the EN61000-3-2 was put into place imposing limits on the harmonic currents drawn from the mains. In other words, if you're in the EU, you are REQUIRED to have a power supply with power factor correction. Power factor correction is not (yet) a requirement in the U.S.

Poor power factor can also limit how much current you can draw from a circuit. If you?re using a 20A breaker and are drawing a total of 15A in ?real power? and the power factor is only .70, then you are drawing an apparent 21.4A, thus overloading the breaker.

Also, one of the requirements for a computer power supply to be considered "Energy Star" compliant is that it has a power factor of at least .90.

What is power factor correction?

Poor power factor can be corrected by adding some form of power factor correction to the AC input of the power supply. Power Factor Correction comes in two forms: Active Power Factor Correction, or APFC, and Passive Power Factor Correction.

Computer power supplies can create harmonics of the same frequency as the input current, due to the non-linear load caused by the bridge-rectifier doing the AC to DC conversion, and typically have poor power factor (typically 0.55 to 0.65).

Passive Power Factor Correction uses a filter that kills any harmonic current and passes current only at line frequency (typically 60Hz in the U.S.) The filters typically come in the form of large, high-value inductors.

Active Power Factor Correction is done by using a boost converter in between the bridge-rectifier and main input capacitors. The boost converter attempts to maintain a constant output voltage while drawing a current that is always in phase and at the same frequency as the line voltage.

Power factor correction won?t make your power supply more efficient (convert more DC output power with less AC input power), but can allow for more devices to be plugged into the same circuit. If you have a number of PCs on the same circuit, say in the event of a LAN party where a number of computers are plugged into a single power strip, it is easier to overload that circuit if a number of the PCs have poor power factor. Say for example you have a 20A breaker and there are five PCs plugged into the outlets on this breaker. Let?s assume the PCs are each drawing 115V at 3A from the wall, or 345W each, for a total of 1725W. This isn?t a lot of power and something the breaker should be able to handle without problem, but if the computers in question lack power factor correction, the ?apparent? current draw could be as high as 27A (assuming a power factor of .55)! This will easily trip the breaker.

So I'm in the U.S. PFC is a non-issue, right?

Well... yes and no.

You may not NEED power factor correction, but "green" is more marketable and it costs a PSU factory less to make a bunch of the same platform, even if it includes PFC, then split production up between non-PFC and PFC designs.

These days, PFC is typically integrated into the design of the platform. So much so that the PSU manufacturers couldn't even really REMOVE the PFC in an effort to cut costs. That's ok because they're making up for it in larger quantities being able to sell their product around the globe. It also reduces returns because with active PFC it's impossible to plug the PSU into 230V while the switch is set to 115V (BOOM!)

Power Factor Correction FAQ - AnandTech Forums

Information about Multiple or Single 12V Rails.

What is "multiple +12V rails", really?

In most cases, multiple +12V rails are actually just a single +12V source just split up into multiple +12V outputs each with a limited output capability.

There are a few units that actually have two +12V sources, but these are typically very high output power supplies. And in most cases these multiple +12V outputs are split up again to form a total of four, five or six +12V rails for even better safety. To be clear: These REAL multiple +12V rail units are very rare and are all 1000W+ units (Enermax Galaxy, Topower/Tagan "Dual Engine", Thermaltake Tough Power 1000W & 1200W, for example.)

In some cases, the two +12V rail outputs are actually combined to create one large +12V output (Ultra X3 1000W, PC Power & Cooling Turbo Cool 1000W, for example.)

So why do they do they split up +12V rails??

Short circuit protection only works if there's minimal to no resistance in the short (like two wires touching or a hot lead touching a ground like the chassis wall, etc.) If the short occurs on a PCB, in a motor, etc. the resistance in this circuit will typically NOT trip short circuit protection. What does happen is the short essentially creates a load. Without an OCP the load just increases and increases until the wire heats up and the insulation melts off and there's a molten pile of flaming plastic at the bottom of the chassis. This is why rails are split up and "capped off" in most power supplies; there is a safety concern.

Is it true that some PSU's that claim to be multiple +12V rails don't have the +12V rail split at all?

Yes, this is true. But it's the exception and not the norm. It's typically seen in Seasonic built units (like the Corsair HX and Antec True Power Trio.) It's actually cheaper to make a single +12V rail PSU because you forego all of the components used in splitting up and limiting each rail and this may be one reason some OEM's will not split the rails, but say they are split. Some system builders adhere very closely to ATX12V specification for liability reasons, so a company that wants to get that business but also save money and reduce R&D costs will often "fib" and say the PSU has it's +12V split when it does not.

Why don't those PSU companies get in trouble? Because Intel actually lifted the split +12V rail requirement from spec, but they didn't actually "announce" it. They just changed the verbiage from "required" to "recommended" leaving system builders a bit confused as to what the specification really is.

So does splitting the +12V rails provide "cleaner and more stable voltages" like I've been told in the past?

It is true that marketing folks have told us that multiple +12V rails provides "cleaner and more stable voltages", but this is usually a falsehood. Quite frankly, the use this explaination because "offers stability and cleaner power" sounds much more palletable than "won't necessarily catch fire". Like I said before, typically there is only one +12V source and there is typically no additional filtering stage added when the rails are split off that makes the rails any more stable or cleaner than if they weren't split at all.

Why do some people FUD that single is better?

Because there are a few examples of companies that have produced power supplies with four +12V rails, something that in theory should provide MORE than ample power to a high end gaming rig, and screwed up. These PSU companies followed EPS12V specifications, which is for servers, not "gamers". they put ALL of the PCIe connectors on one of the +12V rails instead of a separate +12V rail. The +12V rail was easily overloaded and caused the PSU to shut down. Instead of correcting the problem, they just did away with the splitting of +12V rails altogether. Multiple +12V rail "enthusiast" PSU's today have a +12V rail just for PCIe connectors or may even split four or six PCIe connectors up across two different +12V rails. The rails themselves are capable of far more power output than any PCIe graphics card would ever need. In fact, Nvidia SLI certification these days REQUIRE that the PCIe connectors be on their own +12V rail to avoid any problems from running high end graphics cards on split +12V rail PSU's.

There's less components and less engineering to make a PSU that DOES NOT have the +12V rail split up, so it's cheaper to manufacturer (about $1.50 less on the BOM, $2 to $3 at retail) and typically this cost savings is NOT handed down to the consumer, so it actually behooves marketing to convince you that you only need single +12V rails.

But some people claim they can overclock better, etc. with a single +12V rail PSU

B.S. It's a placebo effect. The reality is that their previous PSU was defective or just wasn't as good as their current unit. If the old PSU was a cheap-o unit with four +12V rails and the new one is a PCP&C with one +12V rail, the new one isn't overclocking better because it's a single +12V rail unit. It's overclocking better because the old PSU was crap. It's only coincidental if the old PSU had multiple +12V rails and the current one has just one.

The only "problem" the occurs with multiple +12V rails is that when a +12V rail is overloaded (for example: more than 20A is being demanded from a rail set to only deliver up to 20A), the PSU shuts down. Since there are no "limits" on single +12V rail PSU's, you can not overload the rails and cause them to shut down..... unless you're using a "too-small" PSU in the first place. Single +12V rails do not have better voltage regulation, do not have better ripple filtering, etc. unless the PSU is better to begin with.

Ok... What's the bottom line?

The bottom line is, for 99% of the folks out there single vs. multiple +12V rails is a NON ISSUE. It's something that has been hyped up by marketing folks on BOTH SIDES of the fence. Too often we see mis-prioritized requests for PSU advice: Asking "what single +12V rail PSU should I get" when the person isn't even running SLI! Unless you're running a plethora of Peltiers in your machine, it should be a non-issue assuming that the PSU has all of the connectors your machine requires and there are no need for "splitters" (see Example 1 in the previous bullet point).

The splitting of the +12V rail - AnandTech Forums

On Warranties

One can tell a lot about the quality and reputation of a company and their power supply products by the warranty offered. Most companies will only offer a one year warranty with their product. In comparison some companies like Seasonic, Silverstone, Corsair and PC Power and Cooling will offer, three, five and seven year warranties. Clearly a company that offers a longer warranty has more confidence in the design and quality of their product compared to companies that only offer one year warranties. It is my recommendation that you purchase a power supply unit with at least a three year warranty; this is especially true of very expensive computers. It is true that you will pay more for this, however it is well worth the extra cost to have the heart of your computer backed by the company.

Continuous vs. Peak wattage

There are some companies such as PC Power and Cooling that will advertise the continuous wattage of their product, where as most companies advertise the peak output of their product. The difference between the two is the peak output is the maximum wattage that power supply can output. It cannot maintain this level for long periods of time, but can produce the output if demanded. Continuous output means the power supply is able to output that specific wattage for an indefinite amount of time (Under specific temperatures). The idea here is that if you see two power supplies where one says peak wattage output of 500 watts and another advertised as 500 continuous watts, the one that is rated for a continuous output of 500 watts actually has a much higher peak ratting. Continuous output is more important than peak, it is not safe or wise to have a power supply running at or near it’s peak output, as this will cause more inconsistent voltages, amperages, decreases in efficiency and increases risk of failure. It is recommended to purchase a power supply that is able to output more than your system requires.

How many watts do I need?

The easiest way to determine how powerful a power supply one should buy is by how much you spend on the computer as a whole. Clearly the higher end components will require more power where cheaper ones will require less. The more fans, optical drives, hard drives, video cards and enthusiast cooling methods one has the more wattage a power supply should have.

For example: Lets say one is building a cheap internet machine, that has the cheapest quad core processor, two GB of DDR3 RAM, onboard video, one hard drive, one optical drive and two 80mm case fans. The power supply that should be bought for this machine should have a maximum output of around 350 to 400 watts. If the parts are even lower end than the examples, consider a power supply between 300 and 350 watts. You should expect to pay between twenty and sixty USD for this type of power supply.

Example2: This computer will be a mid range gaming machine. It will have, a middle of the line AM3 quad core, four GB of DDR3, a single ATI 4870, four 120 MM fans, two optical drives and two hard drives. Because this is a middle of the line gamming computer it will require a bit more wattage than the previous example. A computer similar to this should have a power supply between 450 to 550 watts. Expect to pay between forty and eighty USD for this type of power supply.

Example3: This computer will be a high end gaming machine with an over clocked i7 quad core, four GB of DDR3, a single GTX 295, two SSD’s in RAID 0, two blue ray burners and a terabyte back up drive. This type of computer should have a power supply between 600 and 800 watts, expect to pay between one hundred and three hundred USD for this type of power supply.

Example4: This is an ultra enthusiast gaming computer. Its components are a core i7 over clocked to 4 GHz, twelve GB of DDR3, two 5970’s in crossfire, four hard drives, two blue ray burners and water cooling. This type of computer should have a power supply of at least 850 continuous watts, with a strong recommendation for one having a peak output of at least 1000 watts. You can expect to pay between one hundred fifty and five hundred USD for this type of high quality unit.

A general rule of thumb: The more expensive the computer the more expensive the power supply.

Power supply calculators can be used to get a general idea of ones requirements:

eXtreme Power Supply Calculator
Antec Power Supply Calculator
The Original - Power Supply Calculator Offered by Journey Systems. LLC. Providing Custom PCs, Servers, Laptops, Workstations, Gaming PCs, Quality Custom Computers with 3 Year Warranty

I myself do not use calculators; I just give an estimate based on other components being purchased.
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