Guide to Uninterruptible Power Supplies (UPS)

Eaton_1500VA_smallThe UPS is a misunderstood beast, so we have written this guide to clear up misconceptions and provide information to help you work out if you need one.

At its core, a UPS is a battery that sits between the mains power supply and your equipment.  When the power drops out, the battery is there to keep your gear running long enough to save your work and shut it down normally.

Avoiding an abrupt power cut to a PC or server is always a good thing.  A power cut can damage your files, OS, and occasionally hardware.  Complex systems and those under load, such as servers, are at particular risk.  For example, if a database write is in progress when the power is cut then your database will likely be damaged.

A UPS with large battery capacity can also be used to keep critical systems running for hours.  Many people assume a long uptime is the main reason to buy a UPS, but in fact that is a relatively uncommon, secondary purpose.  If you need extended runtime, a UPS can be configured for that purpose, but you might also want to look at generators in combination with a UPS.

Another benefit provided by most UPS units is built in power protection, from basic surge protection to more advanced power conditioning.  A UPS does not necessarily replace a good surge protector, and in fact using a surge protector behind a UPS may add a better quality of power protection compared to entry level UPS protection, and will at least protect the UPS itself.

Types of UPS Technology

There are three technologies typically used on consumer and SMB level UPS devices.

Standby UPS for Backup Power

Standby_UPS_CircuitEntry level UPS units use a “standby” design where power flows directly from the mains to your devices when mains power is available.  At the same time, the battery is charged using mains power.

When mains power is cut off, battery power is automatically switched on through an inverter to power your devices.  It takes a small but appreciable period of time to switch to the battery, and that time can cause issues for some sensitive equipment.  For example, some modern active PFC power supplies might pull excessive current for a moment as it comes back online if the switch takes too long, overloading the UPS.

Most of these units employ a basic surge protection in the circuit to protect against surges and brown outs (not shown in diagram).

The signal output tends to be a simulated sine wave, which is fine for most gear but can cause issues with some power supplies.

Line Interactive UPS with Power Conditioning

Line_Interactive_UPS_CircuitA line interactive UPS is often assumed to be an “online” system where the battery is always feeding power directly to your equipment.  Not true.

A line interactive UPS is similar to a standby UPS but adds an additional component able to regulate voltage.  When the mains voltage goes a little above or below an acceptable level then this additional component can adjust the voltage sent to your equipment, and so the battery does not need to be drained to handle it.  The UPS will usually click when this kicks in.  If voltages vary too much, then the battery will take over as the power source, same as for a standby UPS.

The output signal from a line interactive UPS may be a simulated or pure sine wave output, depending on the model.

Line interactive UPSs are relatively cheap and are the best value type of UPS for many circumstances.

Online Double Conversion UPS: Clean and Continuous Power

Double_Conversion_UPS_CircuitDouble conversion UPS models are the high end option in the for consumer and SMB market.

This technology keeps the inverter online at all times so any power interruption does not require switching and the output power quality is consistent, clean, with a true sine wave output at all times.

Because the battery and inverter are always online, additional stress is placed on these components and there is some loss of efficiency with associated waste heat.  Its not a serious problem, but is a reason why you need to ensure the UPS is well ventilated and personally I prefer to stick a decent surge protector behind these units.

A similar but more recent technology called an online delta conversion UPS is very similar to a double conversion unit but adds components to improve efficiency and reduce issues with the traditional online UPS.  This type of technology tends to be found in only large UPS units, over 5kVA but a worthwhile upgrade for higher end commercial demands.

Online double conversion systems are the best choice for critical systems with modest sized installs, but do cost significantly more than line interactive or standby UPS models.

Whats with Simulated vrs Pure Sine Wave??

Mains AC power is supplied in the form of a sine wave that smoothly alternates between positive and negative values.  Recreating that form from a DC source at the other end of a UPS battery can be expensive.  Very basic equipment will produce a square wave where the voltage jumps straight from positive to negative 240V.  Harsh.  Most cheapish UPSs will do a better job with a modified square wave form, also called a simulated sine wave that is closeish to the real thing, but not a smooth curve, just some steps.  Better units including all online UPS units will produce a nice smooth sine wave that works best.

squarewave modsquarewavesinewave

Modern, efficient computer power supplies (anything with the 80+ certification) feature active power factor correction (PFC) and they do not always play well with simulated sine waves, let alone the even more horrible square waves.

If you find yourself with  an active PFC power supply and anything less than a pure sine wave output, you may find your PC randomly rebooting and/or your power supply struggling and dropping efficiency when it switches to battery.  Many PSs will handle it, and as long as it works for the short time you are on battery you can get away with a cheaper unit, but its not ideal.

Note there are simulated sine waves, and then there are simulated sine waves.  In other words some of the modem UPS units do a pretty good job of producing a nearly accurate simulated curve, others not so much.  You tend to get what you pay for in that regard and closer to a pure sine wave, the better.

In general, try to get a UPS with a pure sine wave output but if the cost is excessive for your use, its likely a simulated wave for good brand name UPS units will be OK.

The much misunderstood “VA” vrs Watts

Once you decide on the UPS technology you want, you need to match it up with the battery specifications.

UPS units are usually quoted with “VA” specification, often in their model name.  VA stands for Volts x Amps, and if you remember your high school science you will know Volt*Amps= Watts so you would reasonably expect that VA figure to represent the output capacity in watts.  But then you might notice that a UPS will also specify maximum output in Watts, and the number will be different, and lower, than the VA number.  So whats up with that?


The reason for the difference is because the VA represents the maximum theoretical output, called apparent output, but available power will be less and convention says we should not use more than 60% of the apparent output, meaning the watt rating will be about 60% of the VA.  A 1000VA UPS should be rated at 600W.

Some manufacturers will use a relatively high maximum wattage quoted, maybe 70% or more of VA.  There may be some justification to do that in some cases, though I think more often it’s the marketing people sticking their nose in.  Personally, I prefer to look at the VA and work out wattage based on 60% of that.

That convention, and the variance it implements, brings up a good point.  Do not overload your UPS.  A reasonable target to run the UPS at about half the rated load, so if your equipment uses around a 300W and may burst up a bit past that, then go for a UPS rated to handle about 600W, or 1/0.6 * 600W = 1000VA.  Its usually OK if you run it up to that 60% figure, and potentially a little past that for short periods, but lower is safer and will give you better runtime.

Another issue many fail to consider is the behaviour of active PFC power with switching UPS units supplies (ie, all new PC power supplies nowadays).  Where the switchover to battery takes a while, the power supply may try to slurp up a lot of power to catch up in the moment that power is switched back on, and that can overload the UPS.  Consider the nominal rating of the power supply, plus a bit, against the UPS rating and aim for a UPS that is rated a little higher than the power supply to avoid this problem.  For example, a 750W PS that might normally use just 150W would probably be OK with a 1000VA line interactive UPS but might fail with a 500VA unit, even though 500VA can supply much more than 150W.

Take into account that a battery performance and capacity will be reduced over time, till they need replacing.  Also consider that the runtime will improve as your reduce load on the battery, so loading it up to near maximum may not give you long enough to shut down for your equipment.

UPS Runtime

The VA figure relates to output at a given point at time.  Many people assume that a high VA means high run time.  In fact that’s not true and the two specifications are not directly related.

runningIt is quite possible for a certain model 1000VA UPS to run for say 10 min at half load, and a different 1000VA model to run for an hour at half load.  The figure that matters for a battery is how much energy in watt hours that it can store.

Most UPS units are built to allow time for shutdown but not much more than that, so if you need a long runtime after the power goes out, you need to look at long runtime batteries, add additional batteries, or run a UPS at a fraction of its maximum output.

UPS specification normally quote expected runtime at half load.  If you run it at full load, then roughly halve that time.  If you run at a quarter load, double it, and so on.  Also again consider that those numbers will tend to reduce as the battery ages and you always want to factor in some extra buffer time.

Beware of outlets with weird plugs and no battery protection

Many entry level UPS units now have standard wall power point style plugs available to make it easier to plug in any gear you want.

Eaton_550VABe aware that some may be only wired up to surge protection and not battery backup.  For example, you might have a PC plugged into one plug and a laser printer into the other for convenience.  You don’t want the laser printer to switch to battery; just make sure you know which plug is which!

You can plug in a double adapter or power board into a UPS outlet, but take care not to overload it with excessive demand.  It is not the number of devices that matter, but their total load.

UPS_CableOlder and larger UPS units will have a number of three prong power leads, or “jug leads” as I call them.  That’s the standard power lead you see running into your PC.  They are fine when plugging into a PC power supply, but can be a pain if you want to plug in other gear and you may need to buy separate cabled that convert to a wall plug.  Just make sure when you buy at UPS that you end up with the cables you need to plug in your gear.

Equipment you should never plug into a UPS

Some equipment can draw a heavy load of power for a short time, and these can damage the UPS and any other equipment attached.

Laser printers are notorious for it.  Don’t plug in anything that needs that big pulse of power that spike up beyond what your UPS can handle.

Set up Automatic Shutdown

For most buyers, the purpose of a UPS is allow for a clean shutdown of your equipment during a power outage.  To meet this goal, don’t forgot to install and setup compatible UPS management software on your devices so they shut down automatically when unattended.

LansafeThe UPS will be typically connected to your server or PC with a  serial or USB cable, with the option to use a network cable in higher end models.  Those cables are essential to let the UPS talk to your gear and let them know the power just went out.

I personally prefer a simple serial cable when available and in small scale deployments, as they tend to have less issues with drivers and avoid the situation using a network cable where if the switch fails or loses power, signal will be lost (so if using the network cable, make sure the switch/s are plugged into a UPS!).  USB cables will do the job if that’s your only option, but in my experience, tend to be a little less reliable.

I personally don’t trust the automatic setting with the software.  Take a look (and test) the runtime you are getting out of your UPS, and gain an estimate of how long your machine takes to shut down when the shutdown signal is sent.  The software will let you set a delay after the power goes out to start shutting down your server/PC. Hopefully during that time it will come back up, if not, the signal is sent and then your PC will start shutting down.  You may also have the option to turning the UPS itself off before the battery is entirely drained, using that function or not is a matter of design and personal choice.

Leave plenty of leeway.  If your UPS is estimating 30 mins of runtime and your critical server might take 5-10 mins to shutdown once the signal is sent, you might send the shutdown signal after just 5 mins to give it plenty of time to properly shutdown.  You do not want the battery to run or UPS turning off out during the shutdown process!

How to Choose Surge Protection

Lightning2Every year, as Christmas approaches, we see an influx of PCs, modems, and other equipment killed by power surges.  It is that time of year again, so to head off some of the issues, I thought a timely reminder in order.

When a burst of energy is dumped into the grid, a surge results and the voltage at your power point will jump up above the normal 240V.  When the voltage moves above the level that your equipment is designed to handle, damage results. 

Small surges cause cumulative damage to electronics and you won’t notice it happening.  When your computer perhaps reboots, or later just dies, you won’t know that your mains power is the cause. The most obvious impact of surges is when you get a big surge, or spike, and that may immediately kill your equipment.

A surge is just one of a range of power issues you will see described with terms like spike (same thing, but shorter duration), sag (voltage drop), transient fluctuation, interruption, line noise, and others. These are all situations where supplied power moves outside the range of “normal” and is generally called “dirty” power.

What causes surges?

The most obvious source in Brisbane are lightning strikes.  Lightning is a big spark, triggered when the potential between two locations, normally charged clouds and the earth, grows big enough to cause a mighty spark to arc across the air.  Air is a poor conductor and the lightning will travel along the easiest path, so as it reaches the ground, it will jump to any tall structure that is more conductive than the air, and then follow a path of least resistance to the ground.


Lightning does not have to strike a building, or even a nearby power pole, to cause damage.  When lightning strikes, mutual induction results in a boost of up to thousands of volts in nearby cables, including phone and cable internet wiring.  Induction can result in a surge many kilometres from the strike.

The most common damage caused from lightning originates from distant strikes and results cumulative damage though moderate surges.  As strikes get closer, damage increases to the point that a surge might take out your PC with a single hit.  Surge protectors can still help at that level.  A strike that is very close, say on the power pole out the front, will create such a huge surge that nothing short of unplugging your power and other cables will protect you.

If a surge protector is a nice little spillway alongside a small and well behaved creek, maybe diverting a bit of water when the creeks flow gets a bit excitable, then a nearby lightning strike is a tidal wave smashing over the creek and spillway, drowning the whole region.  It will overwhelm everything to hit your equipment.  This is why you should unplug before a storm!

Other sources of power surges and related issues can include utility based causes such as switching generators or capacitors on and off, and local issues such as the use of heavy equipment, or even household equipment such as refrigerators, air conditioners, or fluro lights.  Ever hear the click though your Hi Fi when the fridge turns on and off?  Yeah, that’s not good!

How surges reach your gear

Network_Cable_SurgeWe see a lot of people who conscientiously unplug their equipment every storm, to only find their computer and modem not working the next day.

In addition to mains power, any other cable coming into the house can also carry a charge.  Phone lines, Foxtel, your roof aerial, and cable internet connections can all damage attached equipment.

In extreme cases the surge may pass through a chain of attached electronics.  A nearby lightning strike might fry your modem though the phone line, then run through the network cable to your PCs and kill them as well.

We see this sort of issue as burned traces and components on the mainboard, originating from the network port.

When it wasn’t a lightning strike

Electronic_RustCumulative damage can be caused when power fluctuations are not severe but are still outside the design range for the electronics.

If these types of issues happen frequently, they will cause ongoing damage to your system components until they fail.  This type of damage is called “electronic rust” and you can see the result through a microscope.

Many failed parts returned to us do not fail because of normal wear and tear, but rather by excessive wear though dirty power.

Surge Protection Standards

There are some near worthless products on the market labelled as surge protectors.  While we are somewhat protected from false statements on products in Australia, it is still very easy to massage figures for surge protectors.  The best way to compare products is to go with trusted brands and to take a look at specifications that reference standards for measurement.

UL_StandardsThe most commonly used standards for surge protectors are those developed by Underwriters Laboratories (UL) in the US, and specifically UL1449.  This standard specifies the waveforms to be used in testing, defines terminology and test procedures, and categorises the type of protector.  It’s a useful reference when comparing specification of surge protectors, as otherwise the same unit might quote specifications which vary widely depending on how tests are developed.  Better quality surge protectors tend to quote this standard in their specifications.

Others standards are published by the Institute Of Electrical And Electronics Engineers (IEEE) and other professional bodies around the world.

It is unwise to trust specifications that don’t refer to a respected standard.  Don’t take unrealistic numbers on the box at face value!

How Surge Protectors Work

There are three basic types of surge protection: SPDs (surge protective devices), line conditioning/filtering units, and data/signal line devices.  Each provides a certain type of protection but what they have in common is that their job is to manage dirty energy.  Understand they cannot create or destroy energy, only work with what comes out of the plug by modifying, diverting, or dissipating energy.

Failed_MOVDomestic surge protectors incorporate a substance that can burn away to dissipate excess energy as heat to handle surges and spikes.  When the voltage rises too high, current is diverted through this component to ground, usually a metal oxide varistor (MOV).  The energy consumed by the MOV allows the main line to come back to normal voltage for your gear, but at the same time the energy will burn away the MOV.  This means that once the sacrificial MOV is used up, the surge protector won’t be able to reduce the voltage to your gear any longer and will stop working as a surge protector, but will probably keep working as a power board.

Some surge protectors incorporate a fuse, so that if the MOV can’t handle the surge, the fuse will burn out on the line to your equipment, cutting power instead of letting a surge through.  This will only work once and then surge protector will stop working even as a power board, so they are not common in domestic protectors.

More advanced surge protectors incorporate components designed to massage the power signal into a perfect form in order to keep electronic gear happy.  There are many ways to do this and the most effective can be quite expensive to build.  Fortunately while this feature is a useful it is less important for most household electronics than the basic surge protection.  It can be of significant benefit to some equipment, but buy a high quality power supply in your PC, and you can do without in a typical Brisbane house.


Specifications that Matter

Energy Absorption Rating:  An indication of how much energy the unit can absorb before it stops working, as measures in Joules.  This number represents a consumable in the form of a metal oxide that is used up by many small surges, or potentially by one big surge.  The bigger the number, the longer the board will last and the bigger maximum surge it can handle.

It is important to read the fine print and check the rating based on the UL1449 standard as advertised numbers are normally much higher than the numbers based on standards and not useful when comparing products.  You want to see a number of over 1000 Joules based on UL1449.

Indicator Lights and Fuses:  When the sacrificial component used to dissipate energy is gone, or in other words when the energy absorption has exceeded the units rating, the protector no longer works to remove surges but may keep working as a power board.  There is no obvious way to tell if surge protection has failed, so some manufacturers add an indicator light to show when the surge protector needs replacing.

A surge protector may also or alternatively incorporate a fuse designed to burn out when a surge comes along that the sacrificial component can’t handle.  If the fuse goes, the board will stop working entirely.

Clamping Voltage:  The voltage where the protector will kick in.  If it doesn’t kick in till voltage is too high, the surge may damage your gear before the surge protector starts working.  A number to aim for in Australia is 275V (mains power fluctuates around 240V).  Cheap units will tend to clamp at 400V or higher.  Note the lower the clamping voltage, the more energy will be diverted to the sacrificial component over time so the protector will tend to wear out faster, but much better the protector wear out than your electronics it is protecting!

Response Time:  Indicates how long the protectors needs to start working after the voltage goes into the red zone.  If it is too slow, your gear is damaged before it kicks in.  A good quality protector might have a response time at 1 nano second or less.  Cheaper units tend to be slower and may allow significant damage to occur before blocking the surge.  Don’t confuse detection time with response time, detection doesn’t matter, response matters.

Maximum Transient Spike: How much current the device can handle when a large burst of energy comes through, such as with a nearby lightning strike.  Again look for the UL1449 rated value and you want to see big numbers, above 30,000 amps based on UL1449 testing is good.

Power Filtering / Line Conditioning:  Aims to provide clean AC power by reducing high and low voltage electrical line noise.  There are various ways to design filters and the specs here can be misleading.  Normally more components and more cascading circuits are a good thing and active tracking is a premium feature to look for.  Filtering is only on high end models.  If it is cheap and says it’s a filter, it is probably not a very good filter.

Circuit Isolation:  Some models in a power board configuration will provide isolated circuits for arrays of plugs.  Frequency isolation is less effective than circuit isolation.  This feature can be handy when you are going to plug in electrically noisy equipment into one of the banks so it doesn’t interfere with equipment in the other banks.  Particularly useful for Hi Fi gear.

RJ 45 Protection: This means there is a plug for a network cable which can stop a surge getting through from your modem/router to the PC it is connected to.

AV/TV and Cable TV Protection:   This provides a pass-through to handle surges through your aerial and/or Cable TV Coaxial cables.

Insurance:  Most better brands will back their protectors with insurance, where they will pay damages if a surge gets through one of their protectors.  In fact a close lightning strike has so much power behind that it can strike though a normal protector, and in most cases I have heard of, the quality manufacturers will still pay up.  Insurance is a nice bonus.

Warranty:  Protectors will wear out over time as they intercept surges.  The boards I have tend to last years, but that’s with pretty good normal power and high end surge protectors.   Other staff here tell me they get 1-2 years in more outlying areas where the power is not great.  Again, the quality manufacturers will tend to replace the product even when it stops because its capacity has been exceeded (I know Thor will, don’t have personal experience trying to claim with other brands).

When to use a Surge Protector?

From the Insurance Council of Australia:  It is advisable to use surge protection units, designed to minimise the effects of power surges, on all ‘big ticket’ items in the home including the fridge, television, stereo and computers.

In my view, where surge protection helps best is with sensitive electronics such as modem/routers, PCs, Hi-Fi gear and so forth.  Sensitive electronics is not just limited to what you would expect nowadays…

The last washing machine I bought stopped a working a couple of weeks later.  Turned out it was the control board and while I doubt it was a surge that time, watching the tech replace the board reminded me how electronic components are so much part of our general equipment nowadays.

The tech was near retirement and I had a good chat with him.  He was telling me that the old washing machines of the era I was replacing lasted much longer than the modern ones.  He had a bit of a conspiracy theory where he believed that the boards in them were designed only to last so long before burning out.  He knows what he has seen over the years, and that was his interpretation.  Mine was a little different, got me wondering if the issue was surges over time that would not have been a problem for older models and their simple control systems (and maybe manufacturers keeping costs down with components, so just perhaps they don’t handle normal surges as well as they could!).  I don’t have a surge protector attached to my washing machine, but instead turn it off at the wall when not in use, but it does get you thinking about equipment you do leave on.

On average, you will tend to increase the life of your electronic gear if you run it attached to a quality surge protector.

What about a UPS?

A UPS, or Uninterruptable Power Supply is often confused with surge protectors.  Some people assume they are a better type of surge protector; not true.

Eaton UPSA UPS is a battery that switches in if the mains power is interrupted.  We sell them with all server equipment so the servers have enough time to shut down without damage to files and databases if there is a power outage.  They are also used to keep critical services up for a while during power loss, such as phone systems, servers and so forth.

A UPS does not necessarily protect from surges.  Most will, but it is better to think of them as a battery backup with incorporated power protection.  Power protection in entry level UPS models is poor.

A UPS does not necessarily sit above surge protectors in a power protection range, in fact there are cases where we have sold top quality surge protectors to clients using cheap UPSs to prevent surges getting to, and through, the UPS.  It can also be worthwhile using a surge protector behind even a top quality UPS to help protect the UPS.

What are the best value brands?

Surge protectors from most major manufacturers will do at least a basic job.  You can pretty much rate them by cost, a $20 surge protector is possibly better than nothing, but it is not going to last long or do a very good job.  Think of them as a power board that might have some other benefits.

We have sold the Thor brand over the years in the high end with good results.  Reports from customers have indicated that they do their job well, and I know they look after people with warranty and the rare insurance claim.  Most of my protectors at home are Thor.

In the entry level, pretty much any major brand around the $50 mark will get you a decent if limited surge protector, appropriate to protect lower priced equipment.  We sell some Belkin surge protectors and various other brands which are all adequate for basic protection.

Bottom line is, get something for any equipment that you care about, and for expensive gear, get an appropriate high end board.  Remember to replace your protectors regularly once the indicator tells you their protection has worn out, and when you hear a storm coming, unplug!