Technical Support | Athena UPS

1. General Definitions

Complete loss of power. Some literature considers a voltage drop below about 80V to be a blackout as well since most equipment will not operate below these levels.
Sag or Brownout
Decrease in voltage levels which can last for periods ranging from fractions of a second to hours. This can be caused by heavy equipment coming on line such as shop tools, elevators, compressors, etc. Also occurs when utility companies deliberately do this to cope with peak load times.
A tremendous increase in voltage over a very short period of time often caused by a direct lightning strike on a power line or when power returns after a blackout.
A substantial increase in voltage lasting a small fraction of a second, often caused when high powered appliances such as air conditioners are switched off.
ElectroMagnetic Interference and Radio Frequency Interference. Caused by, inter alia, lightning, generators, radio transmitters, industrial equipment.
Metal Oxide Varistors are added to circuits in order to control spikes.These are common in power strips. If you see more than two, you likely have a fairly decent power strip. They look like large disk capacitors.
Circuitry that converts DC battery power to AC power required by most computer equipment.
Surge Protector
Circuitry consisting of MOVs, capacitors, rod-core inductors, etc. for suppressing surges and spikes usually embedded in a power strip.
Line Conditioner
A transformer that attempts to smooth out fluctuations in input voltage to provide near uniform output voltage or voltage waveform.

2. What is a UPS and how does it work?

Q1: What is a UPS?

A1: An Uninterruptible Power Supply is a device that sits between a power supply (e.g. a wall outlet) and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) from adversely affecting the performance of the device.

Q2: Vendor X says that Product Y is a UPS, but it's different than what you described above. Who's right?

A2: There really is no standard definition of what a UPS is. Anything ranging from a 9 volt battery backup in a clock radio to a building-/compound-wide backup generator has been called a UPS by someone. The majority of this document refers to larger objects that help devices remain temporarily operational when changes to the power they receive would otherwise interrupt their function.

Q3: Can you give me some more information on the UPS industry?

A3: There are basically three different types of devices, all of which are occasionally passed off as UPSes.

  1. Standby Power Supply (SPS). In this type of supply, power is usually derived directly from the power line, until power fails. After power failure, a battery powered inverter turns on to continue supplying power. Batteries are charged, as necessary,when line power is available. This type of supply is sometimes called an "offline" UPS.

    The quality and effectiveness of this class of devices vary considerably; however, they are generally quite a bit cheaper than "true" UPSes. The time required for the inverter to come on line, typically called the switchover time, varies by unit. While some computers may be able to tolerate long switchover times, the mileage may vary. Some articles in the trade press have claimed that their testing shows that modern PCs can withstand transfer times of 100ms or more. Most UPS units claim a transfer time tobattery of about 4ms. Note that even if a computer can stay up for 100ms, it doesn't mean that 100ms switchover is okay. Damage can still be done to a computer or data on it even if it stays up.

    Other features to look for in this class of supplies is line filtering and/or other line conditioners. Since appliances connected to the supply are basically connected directly from the power line, SPSes provide relatively poor protection from line noise, frequency variations, line spikes, and brownouts.

    Some SPSes have surge/spike suppression circuitry as well as transformers to "boost" voltage without switching to the battery if a modest voltage drop occurs. Often, as a "standby" UPS becomes more featureful it is called a "line interactive" UPS.

  2. Hybrid (or Ferroresonant) UPS systems. The theory behind these devices is fairly simple. When normal operating line power is present, the supply conditions power using a ferroresonant transformer. This transformer maintains a constant output voltage even with a varying input voltage and provides good protection against line noise. The transformer also maintains output on its secondary briefly when a total outage occurs.Best claims state that their inverter then goes on line so quickly that it is operating without any interruption in power. Other UPS vendors maintain that the transition is less than seamless.

    [ Note: According to some sources, ferroresonant transformers in a UPS system can interact with ferroresonant transformers in your equipment's power supply and produce unexpected results. On the other hand, ferroresonant UPS systems don't kick off a lot of heat, which is important in some environments. Moral of the story: test equipment to make sure it meets your needs before you buy. ]

  3. What I call "true" UPS systems, the supplies that continuously operate from an inverter. Obviously, there is no switchover time,and these supplies generally provide the best isolation from powerline problems. The disadvantages to these devices are increased cost, increased power consumption, and increased heat generation.Despite the fact that the inverter in a "true" UPS is always on, the reliability of such units does not seem to be affected. In fact, we have seen more failures in cheaper SPS units.

    [ Note: Given the same quality inverter, you'd expect the one that runs least to last longest. These devices are often called "online" UPSes. ]

Q4: How can it help me?

A4: A UPS has internal batteries to guarantee that continuous power is provided to the equipment even if the power source stops providing power. Of course the UPS can only provide power for a while, typically a few minutes, but that is often enough to ride out power company glitches or short outages. Even if the outage is longer than the battery lifetime of the UPS, this provides the opportunity to execute an orderly shutdown of the equipment.


  1. Computer jobs don't stop because the power fails.
  2. Users not inconvenienced by computer shutting down.
  3. Equipment does not incur the stress of another (hard) power cycle.
  4. Data isn't lost because a machine shut down without doing a "sync" or equivalent to flush cached or real time data.

Q5: What sort of stuff does a UPS do?

A5: A UPS traditionally can perform the following functions:

  1. Absorb relatively small power surges.
  2. Smooth out noisy power sources.
  3. Continue to provide power to equipment during line sags.
  4. Provide power for some time after a blackout has occurred.
In addition, some UPS or UPS/software combinations provide the following functions:
  1. Automatic shutdown of equipment during long power outages.
  2. Monitoring and logging of the status of the power supply.
  3. Display the voltage/current draw of the equipment.
  4. Restart equipment after a long power outage.
  5. Display the voltage currently on the line.
  6. Provide alarms on certain error conditions.
  7. Provide short circuit protection.

Q6: How long can equipment on a UPS keep running after the power goes?

A6: That depends on how big a UPS you have and what kind of equipment it protects. For most typical computer workstations, one might have a UPS that was rated to keep the machine alive through a 15 minute power loss. If it is important for a machine to survive hours without power, one should probably look at a more robust power backup solution that includes a generator and other components.Even if a UPS powers a very small load, it must still operate its DC (battery) to AC converter (the inverter),which costs power. A rough extrapolation from APC's documentation, leads me to guess that its 2000 VA UPS can operate its own inverter (with no extra load) for just over 8 hours. A 1250 VA UPS could run its converter for about 5. These are very rough guesses based on information provided by one vendor for another.

Q7: Given the same vendor's claims, how can I tell a "good" quality UPS from a "poor" quality UPS?

A7: Testing, testing, testing. This cannot be emphasized enough. There are many good and bad units out there that call themselves UPSes. There are many good units that are wrong for your situation.

Some properties you might look for include:

  1. Sinusoidal Power Output. In general, the closer the AC output of the UPS is to a sine wave, the better it is for your equipment. Many UPS units, especially the cheaper ones, deviate a great deal from a sinusoidal output. Some of them generate square waves. Waveform effects are dealt with in section 2.12 of this document.
  2. Does the UPS have a manual bypass switch? If the UPS is broken or is being serviced, can you pass power through it to your equipment? The last thing you want is for a broken UPS to be the cause of extra downtime.
  3. The more information about a UPS's operation you can get from watching the unit itself, the better. How much power (or percentage load) the equipment is drawing, how much battery life is left and indications of the input power quality are all very useful.
  4. Some UPSes can communicate with their monitoring software via a network connection and SNMP. This is wonderful if your network is on a UPS.

If you do have a UPS that does not output a sinusoidal waveform, some manufacturers strongly urge you to not put a surge protector between the UPS and the computer. The surge protector might mistake the non-sine waveform as a power surge and try to send it to ground. This could be bad for your UPS, not to mention your equipment. I don't know if this has happened or not, but I wouldn't chance it.

Q8: Should I make sure I have a support/maintenance contract for my UPS systems?

A8: Some people strongly recommend this, some don't. It depends on the situation. There are things that can go wrong with UPSes, and they require periodic maintenance. As with all support contracts, you're generally spending a little extra money to reduce risk. Whether this is worthwhile is up to you.

While the electronics in a UPS are likely to last for quite a while, the batteries will periodically need to be replaced. This will happen more frequently the more (and deeper) the batteries are cycled. Replacing the batteries every three years is a pretty typical vendor recommendation (but read the product details for authoritative information). Any UPS battery that has been in continuous service for five years probably should be considered suspect until proven otherwise.

Like any other electronic device, a UPS can fail. You need to have a plan for this. If you don't want to risk having to replace a failed unit at an inconvenient time, you might want to look into a support contract.

Q9: What sort of maintenance can I perform myself?

A9: One good thing you might want to do is periodically test the UPSes and their failure modes. A good time to do this might be right after a periodic level 0 backup. Nobody is logged in and you've got full backups of the machines. Throw the circuit breaker with the UPS on it to simulate and outage and see how the transition goes. Note that in general testing a UPS by pulling the plug from the wall is not a good idea. Electronics like to always have a good ground reference.If you unplug a UPS, it's still powered but now has what electricians call a "floating ground". Not only can this be bad for electronics, but it can be quite dangerous as well. It is likely that unplugging just about any UPS for a short amount of time isn't likely to result in disaster (don't take my word for it, though!), but in all cases, throwing a circuit breaker would be a better thing to do.

It might be useful to install a GFI (Ground Fault Interrupter) on your UPS-covered outlets to facilitate this testing without having to throw a breaker, especially if you don't have your UPS protected machines on an isolated circuit (which you probably should). These are the sockets found in most modern kitchens and bathrooms with a red and a black button. You push the latter to cut power and the former to restore power.

Almost all UPSes use lead-acid batteries, like most car batteries. Unlike, say, NiCad (Nickel-Cadmium) batteries, lead-acid batteries do not have "battery memory." Each "deep cycle" (running the batteries to very low or even drained levels) will decrease a lead-acid battery's effectiveness, so this should be avoided. Of course, handling these situations is the reason you've bought a UPS, but one should not run a UPS down when doing so isn't necessary.

As a UPS gets older, its battery life will become shorter. Of course there's no way to reliably test how long it is without running the battery down and you don't want to do that because they have lead acid batteries. All of these are very good reasons to get a support contract for them that includes periodic battery replacement. At the very least, you can figure that under a normal workload the batteries will usually still be reasonably good at the end of the UPS warranty figure, so that's a good place to start guesswork.

Q10: Isn't a UPS just a glorified power strip/surge protector with some batteries and a little power conditioning thrown in?

A10: Basically. It's also got a power inverter and some other circuitry. It may also have a timer, thermometer or other gadgets.

Q11: How important is the UPS output waveform?

A11: That's a good question worthy of some debate. One school of thought holds that one should always run equipment on the best approximation of sinusoidal input that one can, and that deviations produce harmonics which may either be interpreted as signal if they get through a power supply, or may actually damage the equipment. Another school holds that since almost all computers use switching-type power supplies, which only draw power at or near the peaks of the waveforms, the shape of the input power waveform is not important. Who's right? I don'tknow. My opinion is that sinusoidal output is worth the extra money, especially for on-line UPS systems that continually provide their waveform to the computer. Also, if you don't know that your equipment has a switching-type power supply, you might want to think twice before buying a low quality UPS.

Q12: Can I really count on a UPS protecting my equipment?

A12: This is a tough question. While most UPS systems that you would probably buy in a store or computer catalog are likely to help your uptime more than hurt it, these are not intended for safety or life-critical equipment.

Basically, these devices should be considered to be pieces of consumer electronics. The number one basis on which most of these devices compete with each other is on price, not quality. If your life depends on computer uptime, you need a special purpose, online, big, redundant, expensive system. When you buy a UPS at your local computer store, you are not buying this sort of system.

This is not to say that these things are bad or a waste of money, it's just that they're not a panacea. In most locations I have worked with most decent UPSes, my equipment statistically has suffered less downtime and lower hardware failure rates when it's protected by a UPS than when it's not. But these devices are not infallible. When you add one to the mix, technically it's one more thing that can and sometimes will go wrong. These devices age and occasionally break. A bad one occasionally slips through quality control. Consider it two steps forward and one step back. That's still progress.

There are some things you can do to decrease the likelihood that a UPS will trip you up. Here are some suggestions. This is not an exhaustive list:

  • Perform regular maintenance on your UPS. This includes changing the batteries periodically. Don't ever run on bad batteries. Unless your UPS has a bypass feature which allows you to change batteries withoutdisconnecting the UPS, this means shutting your important server down on occasion.
  • Don't deep cycle the batteries any more than is necessary.
  • Make sure the UPS keeps in contact with its electrical ground at all times.
  • Don't subject the UPS to temperature or humidity extremes, water, excessive dust, or excessive static electricity. Keep the area around the UPS clean and dry.
  • Don't overload the UPS.
  • If the UPS shows signs of misbehavior or malfunction, remove it from service at the earliest possible opportunity. Don't put it back into service until it has been examined and recertified by qualified support personnel.

Q13: I think I'd like to build/refurbish/upgrade my own UPS myself. Is this a good idea?

A13: My short answer: No, it's not a good idea.

Just as with any other electronics project, it's possible to build one yourself if you know what you're doing. In the case of a UPS, though, the tolerances are very tight, and the consequences of building it wrong can be severe. You're working with energies sufficient to kill a person or start a major fire, the batteries contain hazardous materials, and the serious possibility exists that something can be hooked up wrong with disastrous results. If the wrong types of batteries are installed in a UPS very bad things will happen. Unless you really know what you're doing, you're much better off sticking with equipment that others have certified rather than trying to save just a few bucks by doing it yourself. Of course, if you do really know what you're doing, then you don't need my advice. Definitely leave this project for the professionals.

3. What type of software can I get with a UPS?

Q1: If the power is out for a long time, I would like to have my computer automatically shut itself down gracefully before the UPS batteries die. Can I do this?

A1: Yes. UPS manufacturers support software that will do this for some UPSes on at least some platforms. Ask your UPS vendor for details.

Q2: Okay, how about restarting the system for me once power returns?

A2: Not all UPS software products do this, but many do. Ask about the specific software if you have doubts.

Q3: What other software is out there?

A3: Software packages for UPS machines are getting more sophisticated. Most provide some level of power and status monitoring, but lately there are more GUI's, more interactive packages, SNMP support, and even call-out paging.

4. What size UPS do I need?

Q1: How are the "sizes" of UPSes determined?

A1: Typically, a UPS has a VA rating. The VA rating is the maximum number of Volts * Amps it can deliver. The VA rating is not the same as the power drain (in Watts) of the equipment. (This would be true if the load were only resistive or the circuit were DC, not AC). Computers are notoriously non-resistive. A typical PF (power factor:Watts/VA) for some computers may be as low as 0.6, which means that if you record a drain of 100 Watts, you need a power source with a VA rating of 167. Some suggest that 0.7 may be a good conversion factor, but this will depend heavily on the specific equipment. Moreover, there's really no way to determine these numbers besides measuring them.

[ Note: Some UPSes can continue to deliver power if the VA rating is exceeded, theymerely can't provide above their VA rating if the power goes. Some can't provide power above their VA rating at all. Some may do something really nasty if you try. In any case, I strongly recommend not doing this under any circumstances. Generally, the rule of thumb seems to be never drawing more VA from an UPS than about 75% of its rating. ]

Q2: How can I tell what VA rating I need for my equipment?

A2: First, when possible, get VA rather than wattage ratings. There are a couple of ways to evaluate your electrical load:

  1. Direct measurement. You can get equipment to measure the current draw of your equipment directly. You may or may not have access to this. If you are part of an organization that has its own facilities/electrical type people, they're likely to be able to do this. They might help you out if you ask nicely. If you're on a budget and don't want to shell out for a high-quality ammeter, you might want to try a device called the "Kill A Watt" electric usage monitor made by P3 International.
  2. Compare notes. If you know someone with the same setup you're using, ask them what they use and how close they are to the maximum VA rating.
  3. Use a chart. Most vendors can help you out for common equipment. If you have an unusual setup, or mix equipment a lot, this may be more difficult.
  4. Use the equipment rating. Most pieces of computer equipment have a power rating on some back panel near where power cord enters the chassis. This number is usually very conservative, as it is necessary for the manufacturer to play it safe or they'll get sued. Also, these numbers generally represent a conservative estimate of total draw of the equipment when it is in its most power-hungry configuration. Typical device configurations may be less demanding.

[Note: Method 1 is by far the best, method 2 and 3 are secondary, method 4 is usually overkill, but pretty safe. In a pinch, obtaining a UPS whose VA rating is equal or greater than the sum of all listed electrical load ratings is pretty safe. Don't forget to include headroom for expansion!

Q3: Hmmm... seems like this can be a tough thing to determine.

A3: Yeah, it can be. It's also very important. Remember, if you get a UPS that's too big, then you've overpaid, but your equipment can survive a longer outage. If you get a UPS that's too small, your equipment might not be protected. Therefore, I recommend that you be conservative in buying these things.

Q4: What else should I consider?

A4: It would be nice to know how long your site's typical power outages are. In some places, with nice weather and a flaky power grid, the power is almost never out for more than 5 minutes, but this could happen quite frequently. In this case, you may as well use a UPS with a VA rating close to your equipment rating with no extra batteries. If your area has longer outages, in the half hour or hour range, as is often the case in thunderstorm country, you can either buy UPSes with multiples of the VA rating of the equipment, since oversizing a VA rating for a UPS has the effect of lengthening the amount of time your equipment can stay up in case of a power outage, or you can buy additional battery units for a smaller UPS. You can probably get away with doing simple math to determine how much longer a larger UPS will keep your equipment running, but I recommend running a few tests before committing to a large purchase order. Also, your UPS vendor will almost certainly be glad to help you size the equipment you need. If all else fails and you guess wrong, or move equipment to a location with different power status, you may be really, really glad if you bought a UPS that can be expanded with additional battery units.