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Power-Protection Systems: Backup Power Options

Power Supply Reference: Consumption, Savings, And More

Backup Power

The next level of power protection includes backup power-protection devices. These units can provide power in case of a complete blackout, thereby providing the time necessary for an orderly system shutdown. Two types are available: the SPS and the uninterruptible power supply (UPS). The UPS is a special device because it does much more than just provide backup power; it is also the best kind of line conditioner you can buy.

Standby Power Supplies

A standby power supply is known as an offline device: It functions only when normal power is disrupted. An SPS system uses a special circuit that can sense the AC line current. If the sensor detects a loss of power on the line, the system quickly switches over to a standby battery and power inverter. The power inverter converts the battery power to 120 V AC power, which is then supplied to the system.

For a stand-by (switching) type UPS to work, the hold-up time of the power supply has to be longer than the switching time of the UPS. For example, I have a TrippLite SMART1000LCD, which is a stand-by type UPS with a 4ms switching time. Because this is well below the 16ms hold-up time called for in the official Power Supply Design Guide, it should switch well before any properly designed and functioning power supply allows the system to reset. Unfortunately if the power supply in a PC is poorly designed or overloaded, it may be disrupted by even a 4ms switch, causing the system to shut down or reset anyway, and all unsaved work to be lost.

A truly outstanding SPS adds to the circuit a ferroresonant transformer, which is a large transformer with the capability to store a small amount of power and deliver it during the switch time. This device functions as a buffer on the power line, giving the SPS almost uninterruptible capability.

Tip: Look for SPS systems with a switch-over time of less than 10 milliseconds (ms). This is shorter than the hold-up time of typical power supplies.

SPS units also might have internal line conditioning of their own. Under normal circumstances, most cheaper units place your system directly on the regular power line and offer no conditioning. The addition of a ferroresonant transformer to an SPS gives it extra regulation and protection capabilities because of the buffer effect of the transformer. SPS devices without the ferroresonant transformer still require the use of a line conditioner for full protection. SPS systems usually cost between a hundred and several thousand dollars, depending on the quality and power-output capacity.


Perhaps the best overall solution to any power problem is to provide a power source that is conditioned and that can’t be interrupted—which is the definition of an uninterruptible power supply. UPSs are known as online systems because they continuously function and supply power to your computer systems. Because some companies advertise ferroresonant SPS devices as though they were UPS devices, many now use the term true UPS to describe a truly online system. A true UPS system is constructed in much the same way as an SPS system; however, because the computer is always operating from the battery, there is no switching circuit.

In a true UPS, your system always operates from the battery. A voltage inverter converts from +12 V DC to 120 V AC. You essentially have your own private power system that generates power independently of the AC line. A battery charger connected to the line or wall current keeps the battery charged at a rate equal to or greater than the rate at which power is consumed.

When the AC current supplying the battery charger fails, a true UPS continues functioning undisturbed because the battery-charging function is all that is lost. Because the computer was already running off the battery, no switch takes place and no power disruption is possible. The battery begins discharging at a rate dictated by the amount of load your system places on the unit, which (based on the size of the battery) gives you plenty of time to execute an orderly system shutdown. Based on an appropriately scaled storage battery, the UPS functions continuously, generating power and preventing unpleasant surprises. When the line power returns, the battery charger begins recharging the battery, again with no interruption.

Note: Occasionally, a UPS can accumulate too much storage and not enough discharge. When this occurs, the UPS emits a loud alarm, alerting you that it’s full. Simply unplugging the unit from the AC power source for a while can discharge the excess storage (as it powers your computer) and drain the UPS of the excess.

Many SPS systems are advertised as though they are true UPS systems. You can tell a Standby Power Supply from a true UPS by the unit’s switch time. If a specification for switch time exists, the unit can’t be a true UPS because UPS units never switch. However, true UPS systems are very expensive, and a good SPS with a ferroresonant transformer can virtually equal the performance of a true UPS at a much lower cost.

Note: Many UPSs and SPSs today come equipped with a cable and software that enables the protected computer to shut down in an orderly manner on receipt of a signal from the UPS. This way, the system can shut down properly even if the computer is unattended. Some OSs designed for server environments contain their own UPS software components.

UPS cost is a direct function of both the length of time it can continue to provide power after a line current failure and how much power it can provide. You therefore should purchase a UPS that provides enough power to run your system and peripherals and enough time to close files and provide an orderly shutdown. Remember, however, to manually perform a system shutdown procedure during a power outage. You will probably need your monitor plugged into the UPS and the computer. Be sure the UPS you purchase can provide sufficient power for all the devices you must connect to it.

Because of a true UPS’s almost total isolation from the line current, it is unmatched as a line conditioner and surge suppressor. The best UPS systems add a ferroresonant transformer for even greater power conditioning and protection capability. This type of UPS is the best form of power protection available. The price, however, can be high. To find out just how much power your computer system requires, look at the UL sticker on the back of the unit. This sticker lists the maximum power draw in watts, or sometimes in just volts and amperes. If only voltage and amperage are listed, multiply the two figures to calculate the wattage.

As an example, if the documentation for a system indicates that the computer can require as much as 120 V at a maximum current draw of five amps, the maximum power the system can draw is about 550 watts. The system should never draw any more power than that; if it does, a five-amp fuse in the power supply will blow. This type of system usually draws an average of 200 to 300 watts. However, to be safe when you make calculations for UPS capacity, be conservative; use the 550-watt figure. Adding an LCD monitor that draws 50 watts brings the total to 600 watts or more. Therefore, to run two fully loaded systems (including monitors), you’d need a 1200-watt UPS. A UPS of that capacity or greater normally costs several hundred dollars. Unfortunately, that is what the best level of protection costs. Most companies can justify this type of expense only for critical-use PCs, such as network servers.

Note: The highest-capacity UPS sold for use with a conventional 15-amp outlet is about 1400 watts. If it’s any higher, you risk tripping a 15-amp circuit when the battery is charging heavily and the inverter is drawing maximum current.

In addition to the total available output power (wattage), several other factors can distinguish one UPS from another. The addition of a ferroresonant transformer improves a unit’s power conditioning and buffering capabilities. Good units also have an inverter that produces a true sine wave output; the cheaper ones might generate a square wave. A square wave is an approximation of a sine wave with abrupt up-and-down voltage transitions. The abrupt transitions of a square wave are not compatible with some computer equipment power supplies. Be sure that the UPS you purchase produces power that is compatible with your computer equipment. Every unit has a specification for how long it can sustain output at the rated level. If your systems draw less than the rated level, you have some additional time.

Caution: Be careful! Most UPS systems are not designed for you to sit and compute for hours through an electrical blackout. They are designed to provide power only to essential components and to remain operating long enough to allow for an orderly shutdown. You pay a large amount for units that provide power for more than 15 minutes or so. At some point, it becomes more cost-effective to buy a generator than to keep investing in extended life for a UPS.

Some of the many sources of power protection equipment include American Power Conversion (APC) and Tripp Lite. These companies sell a variety of UPS, SPS, line protector, and surge protector products.

Caution: Don’t connect a laser printer to a backed-up socket in any SPS or UPS unit. Such printers are electrically noisy and have widely varying current draws. This can be hard on the inverter in an SPS or a UPS and frequently cause the inverter to fail or detect an overload and shut down. Either case means that your system will lose power, too.

Printers are normally noncritical because whatever is being printed can be reprinted. Don’t connect them to a UPS unless there’s a good business need to do so.

Some UPSs and SPSs have sockets that are conditioned but not backed up—that is, they do not draw power from the battery. In cases such as this, you can safely plug printers and other peripherals into these sockets.

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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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