There are 3 main UPS technologies – Offline, Line Interactive and Online Double Conversion – and two different types of inverter – square wave (also called pseudo-sine wave or modified sine wave) and sine-wave.

An Offline UPS provides basic levels of power protection. There is usually a degree of surge suppression incorporated and when the input mains voltage goes out of tolerance (that is too high/low or fails) the UPS inverter starts up and provides power to your equipment. There is a break in the mains supply when this occurs in the region of 10-20 thousands of a second which will generally go unnoticed by the majority of equipment. The inverter in offline UPS is nearly almost always a square wave.

A line interactive UPS is similar to an offline UPS but has the added benefit of voltage regulation. This means that it will reduce the mains voltage if it goes too high (called “buck”) or raise the mains voltage when it goes too low (called “boost”). It will do this without reverting to battery and hence conserve battery life. Since a line interactive UPS has an inline transformer, it also provides in-built filtering and hence a higher degree of power protection. Like the offline types, there is also a break during the transition from mains power to battery power. In higher quality line interactive units, this break may be as low as only 2 to 4 thousandths of a second. Line Interactive UPS Systems come with square wave as well as sine wave inverters.

The online double conversion UPS System is considered by many to provide the highest levels of power protection. The inverter is always on, and provides power to the load. This means that there is no deviation in output voltage and no break if the UPS reverts to battery power. The output waveform in an online UPS System will almost always be a sine-wave, generally of very high quality and can enhance the operation of certain equipment. An online double conversion UPS also has a bypass circuit, which allows power to be continually fed to your load even if the UPS develops a fault or is overloaded.

In choosing your technology you need to consider:

1. The power consumption of your load
2. The type of load you have
3. How critical your load is
4. The environment in which the UPS will sit
5. The required runtime
6. Your budget

Your normal UK socket outlet is rated at 13Amps which means the most power you can draw from a UK outlet is 3KVA, or 3KW. (Power Measurement will be covered in a separate paper). Above this level (for specialist equipment, or hardwired installations) most UPS will be online double conversion technology as the economies of scale start making other technologies non cost effective. Below this level, however all technologies are available, although above around 2KVA, line interactive systems start to become heavy and unwieldy due to the size of transformer that is required.

If your load is made up of computer type power supplies, then both square wave and sine wave products will power this equipment adequately. If your load contains motors, transformers, pumps or other inductive components (on the input power supply) then square wave systems are not suitable and you must opt for a sine-wave system.

Any load that is susceptible to mains disturbances such as in analytical equipment or audio applications should also choose a sine-wave system. Where mains distortions affect equipment performance then you need to opt for online double conversion where a pure sine-wave is always present.

If your load is critical for operation then the increased benefit of online double conversion technology should be used. This provides additional security against all power disturbances as well as the comfort of a fall-back bypass in case a fault develops with the UPS. You may wish to parallel together UPS and again, this can only be achieved with online double conversion UPS. (We’ll cover parallel systems and redundancy in a separate paper). If you have a PC where you are looking for simple battery backup to stop annoyance rebooting or tripping then an offline or line interactive unit would suffice.

Offline and line interactive UPS Systems are generally quiet in operation and do not utilise cooling fans in normal operation (usually). This means they are suitable to be placed in an office or home environment. Online Double Conversion UPS systems require forced cooling and can be quite noisy and therefore tend not to be suitable for use in an office environment.

If you are running your UPS in conjunction with an emergency backup generator then the benefits of online double conversion technology come to the fore, as the UPS will cover for any transition to generator operation and also provide a constant unchanging power source during generator start up and any shifts that may occur in frequency and or voltage during load switching.

Offline UPS tend to be the smallest of all technologies so can be useful to provide emergency ride through for areas where space is at a premium.

Long runtimes on UPS are better handled with online UPS Systems. This is because they are designed to operate continually from the inverter. In which case it does not matter if the input power is from the rectifier or the battery – the operation is the same. Offline and Line Interactive units tend not to be designed for this continuous operation. (Battery selection and runtime will be covered in a separate paper.) It is also unwise to power any equipment with a square wave inverter for any prolonged period of time as you could cause degradation to electronic components. Square wave systems are designed to allow basic computer systems to ride through brief power outages or time to shut down a system in the event of a prolonged power cut.

You may have already made up your mind which technology you need, but let’s take a look at relative costs. Offline UPS systems used to be by far the cheapest. However nowadays an offline UPS and a line interactive square wave unit are typically similar prices. Online Double Conversion used to be far more expensive than other technologies, but now is comparable to a high quality sine-wave line interactive UPS.

With regard to running costs, the offline UPS is the most efficient as in normal operation the input power goes straight through to the load, so the only power loss is to keep the battery float charged and power the UPS logic. Line Interactive units are similar in efficiency but experience more losses due to the transformer required for the buck and boost facility. Online Double Conversion however, has the drawback of being the least efficient of the technologies.

In summary, for simple low cost battery backup then the offline or line interactive square wave systems are suitable. For high grade protection in a quiet environment, or where your load type demands it, opt for a sine-wave line interactive unit. However, if you want the best power solution, then we recommend online double conversion technology as it is compatible with all load types and provides the highest degrees of power protection.

The first thing you need to know is that battery discharge is non-linear. For those of you who don’t understand the expression I’ll elaborate. A linear expression is one where, for example, you put two in, you get four out. So it follows that if you put three in, you get six out, or you put five in, you get ten out. EG. whatever you put in  you get twice out. In the non-linear world this doesn’t hold true, for example, you put two in, you get four out, but when you put three in, you get nine out etc.. This non-linearity makes the discharge characteristics very difficult to express mathematically.

Luckily, the battery manufacturers provide us with discharge tables that we can look up, but first we need to know some information about the UPS, the load and about the method.

End of Discharge Point

At what point will the UPS switch off? Your normal 12V lead acid battery contains 6 cells each of a nominal voltage of 2V (so you get 6x2V=12V). In practice the voltage is slightly higher than this and reduces as the battery is discharged. It is important not to allow the battery to become too discharged, so the UPS will monitor the cell voltage and cut off when it gets to a predetermined point. This is usually around 1.7V per cell or 10.2V for 12V battery.

UPS Efficiency?

Well, more precisely, the inverter efficiency. The inverter is used to convert the battery DC power into AC power. There will be losses associated with this. The better the inverter, the lower these losses are. If you’re unsure, use a worst case of say, 80% efficient. This means that for every 100W provided to the load, the batteries will need to provide 125W (simply 100/efficiency).

The Load Power Factor

Is the load purely resistive, or does it have a power factor? We’re only interested in the amount of WATTS that are needed.

Amps or Watts Method?

Firstly, there are two methods for calculating runtime, the Amps method, or the Watts per Cell Method. Generally, Watts per Cell is used for short term discharges and Amps is used for long term discharges.

Process

It’s easier to do this with an example, so let us take a standard server type load with a number of ancillary devices. We know from measurement  that the Ampere draw is 7Amps and we have mains voltage of 235V. Our Watts therefore (assuming unity power factor) is 1645W.

Our UPS has an inverter that is rated at 90% efficient, so the amount of power from the battery required to deliver 1645W is 1645/0.9 = 1828Watts.

Our UPS is a 3KVA, 2.1KW UPS that contains 8 batteries connected in series. Each battery is rated at 12V 7.2 Ah.

 I now need to look up the manufacturers data sheet and I find the following for a 7.2Ah battery:

Constant Power Discharge Method 

Our battery load is 1828W and we have 6×8=48 cells. Therefore our Watts per Cell is 38Wpc.

We know the FV (Final Value or End Of Discharge Point) is 1.7Vpc so looking along the Constant Power Discharge Table, we can see that 52Wpc would give 5 minutes, so we will get more than 5 minutes runtime. We can see that 35.16Wpc would give 10 minutes runtime, so we will get less than this.

So the calculated runtime for this example, based on constant power discharge is between 5 and 10 minutes.

 Constant Current Discharge Method

We have a total of 8x12V batteries in series, giving us a string voltage of 96V. We need to deliver 1828W so our Amperage is 1828/96 (from Power=VoltsxAmps, so Amps=Power/Volts). = 19Amps.

Now looking along the table above in the Constant Current Section, with our FV of 1.7, we see that a 26A discharge would give us 5 minutes, so we will get longer than this. A 17.6Amp discharge would give 10 minutes, so we will get less than this.

Therefore using the constant current discharge method we will get between 5 and 10 minutes runtime.

Working it out in reverse – I want 2 hours runtime – how many batteries do I need?

Using Watts Per Cell Method.

From the table under Constant Power Discharge, we can see that for our FV of 1.7, for a 2 hour runtime we need to have a WPC discharge of no more than 6.05. Our load is 1828W, so we need 1828/6.05 = 302 cells, which is 50.35 batteries. The battery requires a 96V string voltage, based on banks of 8, so we will require 6 banks to get close (that is 48 batteries), or 7 banks to be sure (that is 56 batteries).

Using Constant Current Method.

From the table above under Constant Current Discharge, you can see for our FV of 1.7V we need to have no more than a 3.05A discharge from each of our batteries to achieve a 2 hour runtime.

Our total current draw is 1828/96 = 19Amps (1828Watts load/Battery String Voltage = 96V)

Dividing the 19Amps total current by 3.05 gives us the number of strings needed to achieve 2 hours runtime which is 6.24. Obviously we cannot add in a quarter of a string so we need to round up. In this case we require 7 battery strings, or a total of 56 batteries to achieve a runtime of 2 hours.

Alternatively, you could of course opt for higher capacity batteries, and maintain the same number of batteries. The examples above were using 7.2Ah lead acid batteries but there are other choices available.

There are many factors that also impact your runtime, such as:

1. Ambient Temperature The warmer it is the better your batteries will perform, however this comes at a cost. As temperature increases the battery life expectancy decreases rapidly.
2. Battery Health Batteries have a useful working life. The more they are used, the less able they are to be recharged fully, resulting in reduced runtime.
3. Load Variance A effect known as Peukert’s Law extends runtime if the load is not constant but is low for a period allowing the battery to “catch up”
4. Load Power Factor The higher the load power factor the more power you draw from the batteries. This is also why you may see different runtimes quoted for different UPS systems that use the same batteries due to specifying runtime at different load power factors.
5. Inverter Efficiency The more efficient the inverter the less power is wasted and the more battery power can go into supplying the load.
6. End Of Discharge Point The UPS is programmed to switch off when a certain battery voltage is reached. In many cases this is 1.75Volts per cell, or 10.5V for a 12V battery, however this may be as low as 1.65Vpc. The lower the EOD voltage the more runtime you will achieve, however this is at the detriment of reduced battery life and increased recharge time.

Most UPS Systems use Valve Regulated Sealed Lead Acid Batteries or VRSLA for short. Some may not include the “Sealed” as technically it is not correct – as they have valves – and so you may see VRLA used instead but they are talking about the same technology. The workhorse of the UPS industry for systems from 500VA to up to 10KVA is a battery about two thirds the size of a house-brick and is rated at 12V and 7.2Ampere-Hours or Ah. The Ah rating is a measure of the batteries capacity. A battery rated at 12V 9Ah will give more runtime than one rated at 12V 7.2Ah for example. Except for small UPS systems there is usually several batteries connected in series to give a higher terminal voltage and hence lower current for design reasons. For example, 1KVA UPS usually have 3 batteries, giving a 36V voltage, 2KVA 6 batteries – 72V and 3KVA 8 – 96Volts. 6 & 10KVA systems usually have around 20 giving 240Vdc nominal input, although these figures will vary from product to product. For example, lets say a 2KVA UPS contains 8x7Ah/12V batteries. Upgrading to a 3KVA UPS that uses the same battery will not give you any more runtime, as, like I said above – runtime=batteries, and so a common misconception is that going for a larger UPS will automatically give you more runtime. This need not necessarily be the case.

Some UPS Systems allow the connection of external battery packs in order to prolong runtime. This is a great way to provide guaranteed availability of power, however a point very often overlooked on extended run UPS Systems is the recharge time. Many UPS systems are fitted with a charger rated at no more than 1Amp, which is fine for a battery string rated at anywhere up-to 10Ah. For example, the recommended charge current for VRSLA batteries is between C/10 to C/4 where C is the Ah rating. At currents lower than this the batteries will charge but will take significantly longer to do so.

For example, let us assume we’ve discharged a UPS battery rated at 10Ah. It will take over 4 hours to recharge this battery to 90% of capacity and a further several hours to fully recharge. If we double the battery bank by adding in another battery pack, the charge time will now be 8 hours to get to 90%. Add in another and now it’s 12 hours. So before you go buying a multitude of battery packs, ensure that the charger is up for the job of recharging the batteries after a heavy discharge.

Our long runtime UPS Systems for applications such as emergency lighting are fitted with a 5A charger for exactly this reason.

One other note of caution comes with the use of line interactive UPS Systems for extended run applications. As line interactive units are not “on” all the time, many are not designed with long run times in mind. The best technology for long runtime applications is online double conversion technology. Since this technology is “on” all the time, it does not matter whether power is being taken from the mains or from the battery, it will continue regardless.

Legacy computer systems had a rectified input power supply that takes current in surges, rather than in a smooth sine-wave fashion. The results of this were that the power factor (the ratio of apparent power to true power) of computer power supplies worked out to be 0.7, that is for each 100VA of apparent power, the UPS needed to deliver 70Watts of true power. This is why UPS have traditionally had two ratings – VA and Watts and typically these tended to be different by a factor of, yes that’s right 0.7.

These power surges caused by computer power supplies can play havoc with the utility supply which is why standards have been introduced to make computer power supplies more utility friendly, and they do this by incorporating circuits to have what is called power factor correction, raising the power factor from the traditional 0.7 to a level approaching 1.

The effect of this on Uninterruptible Power Supply Sizing is clear. On your legacy computers you could add up the VA ratings and your UPS would be practically guaranteed to be sized correctly. However, systems with modern compliant power supplies are different, and you need to make sure you don’t overload the WATTS rating of the UPS.

For example, 4x250VA legacy systems could safely be powered from a 1000VA/700W UPS. Now, you would need to ensure that the 4x250VA systems were powered by a UPS System rated at at least 1000W – about 1500VA (for a traditionally rated UPS). The Eaton 9130 range of Online Double Conversion UPS Systems go some way to overcoming this dilemma by having their systems rated at an impressive 0.9pf, which means that for every 1000VA, the UPS can supply 900W.

If your power supply doesn’t say or if you are unsure, the safe bet is to take the VA rating you have and multiply it by 1.4, in this instance your UPS will in the majority of cases be appropriately rated.

Take a single phase power system. You can calculate the power you need easily, by multiplying the voltage (230V in Europe) by the current draw from your load (in Amps) and you’ve got the VA rating. If you are unsure of power factor (as most of us will be without special equipment) multiply this number by about 1.4 and choose an Uninterruptible Power Supply above this value. You’ll have no problems. But what about a three phase system?

A three phase UPS is, in its simplest form, three identical single phase UPS Systems stuck together, and you cannot overload any one of these systems.

Imagine you have a three phase load, of which you have split into several load sections – for example, the lighting circuit, the electrical distribution circuit and the cooling circuit. Each of which is a single phase load. You switch everything on, take your ammeter and measure the current flowing in each phase, and get, for example, 15A, 25A, 40A. So what size UPS do you need?

You could say that our current draw is 15+25+40 which is 80 and multiply this by 230 which gives us 18,400VA. So we need a three phase UPS rated at, say 20KVA. This is wrong, and it is wrong because you need to remember the three phase UPS is three – identically rated – single phase UPS Systems.

What you need to do is to take the maximum current draw, which in this case is 40A, and work this out as a single phase UPS. So we get 40×230 = 9200VA. Then multiply this by 3. The actual size of UPS we need is not 20KVA but actually 30KVA (9200×3=27,600).

Modern three phase UPS Systems can cope with 100% unbalanced loads, that is one phase is producing all the power and the others are supplying zero, but they cannot borrow power from one phase to the other.

Firstly – expandability. Let us suppose you are developing a data room. The plan is to eventually have, for example, 25 cabinets, each with a power consumption of 3KVA = 75KVA total load. However, at present you only need power for 5 (15KVA), with the remainder being added over the next few years or so.

The sensible approach using the standard Uninterruptible Power Supply would be to fit an 80KVA model. However in the early days it would only be operating at less than 20% capacity. So you’ve shelled out for an 80KVA system that wont be at capacity for a couple of years. For an 80KVA system (excluding battery and installation) you’d be looking at a cost in the region of £8,000, depending on options.

With the Modular UPS, you would fit a 100KVA carrier, and 2x10KVA Power Modules at a cost of around £6,000. You can then add the additional 10KVA power modules as and when required at around £1,500 each.

The benefit here is that the initial outlay is lower, however the total cost will be higher, as you need to add in another 6x 10KVA Power Modules units, making the total cost £15,000 as opposed to £8,000 for the standard Uninterruptible Power Supply.

However, let us now suppose that we want a n+1 redundant solution. So with our standard Uninterruptible Power Supply model, we would put in 2x80KVA UPS Systems, at an upfront cost of £16,000. With the Modular UPS we can put in the 1 extra power module that we need, so our initial upfront cost is 1x 100KVA carrier, and 3x 10KVA Power Modules at a cost of around £7,500.

However, the real benefit is to do with the fact that to achieve n+1 we only need 90KVA of UPS power, as opposed to 160KVA in the configuration above. When the data centre is fully operational we would require 1x 100KVA carrier, and 9x 10KVA Power Modules at a cost of around £16,500. So, slightly more expensive but in an equivalent ball park, however other important factors are that the Modular UPS is in one cabinet with a small footprint, occupying probably half the space of the 2x 80KVA Standard UPS Systems and the fact that the power modules can be easily swapped in the event of a fault – thereby improving on availability figures.

It would be remiss of me however, not to include a third scenario. N+1 Redundancy is achieved by having one more Uninterruptible Power Supply than is needed to do the job. Therefore, it is possible to use, for example 3x40KVA UPS Systems, or 4x30KVA UPS Systems, that too, can grow with demand. If we take the latter, we would need initially 2x30KVA UPS Systems at a £6,000 outlay. You can add another for another £3,000, and then finally have the last in, at a total cost of £12,000. Of course, this price excludes batteries and installation. However, in this instance you need to have room for 4 UPS Systems!

I have also not included the additional costs of switch gear needed for the standard Uninterruptible Power Supply Solution. So, taking this into account, along with the additional floor space needed, you would have to argue that the Modular UPS would be a good solution.

There is another factor that gives the Modular UPS a wholesale advantage over other methods and that is efficiency. Let us assume for a moment, that the Modular UPS and the Standard Uninterruptible Power Supply, all share the same efficiency at full load. It is clear that UPS systems operating at half load or less will be less efficient. With 2x80KVA UPS Systems on a 75KVA load, each UPS will be operating at 47% load, whereas the Modular UPS with 90KVA of power available, will be operating at 83% load. So there is probably some running cost calculation that you could also take into account.

Money makes the world go round as they say, so if I were looking for simple UPS support, I’d opt for the standard Uninterrupibtle Power Supply, however if I was needing to include some redundancy in there, the Modular UPS is starting to look like a great contender.

Now UPS Systems are designed to increase the reliability of connected systems, by providing power protection and back up power. If the UPS System should fail then it should revert to bypass and allow utility mains power through (this is for online double conversion Uninterruptible Power Supplies and particularly three phase systems). It’s unclear what happened in this case, but whatever the outcome the critical systems were left without power and the UPS System is perceived not to have done its job.

Here’s where redundancy is required. Basically, you have one more system than you actually need, therefore if one system should fail, the other can take over without any loss of UPS System support. Should the utility fail for an extended period, then you either need batteries to keep you going (or shut down gracefully) or an external generator to kick in and simulate utility power.

Redundancy is usually expressed as ‘n+1′, which means that if you need ‘n’ UPS Systems to power your load, then you install ‘n+1′. For example, if you have a 100KVA load, you can achieve this with 1x100KVA UPS System. If you want redundancy you will need to use n+1 = 1+1 =2 UPS Systems, i.e. 2x 100KVA UPS Systems. Alternatively, you may have achieved your load by using 2x50KVA UPS Systems in parallel, and redundancy can be achieved by installing an additional 50KVA UPS System i.e. 3x50KVA UPS Systems, which may be more cost effective.

I’ll blog more about this later, as the citizens of Taipei walk to work. 

You can read about the Taipei metro power cut here

Simply select what load you have and add your own items and the tool will calculate your total power requirement. Hit “Select My UPS System” and the tool integrates with the existing bo-selector tool to enable you to make further choices such as run time, technology, form factor, manufacturer to give you the Uninterruptible Power Supply that meets your requirements.

It’s a truly simplified process that makes selecting your ideal system easy. Give it a go here: Select Uninterruptible Power Supply By Device

It’s important in any business to keep costs under control, but this does not mean spending no money at all. In many cases an Uninterruptible Power Supply (UPS) may seem like an unnecessary expense, you rack your brains trying to think of the last time you had a power cut and can’t remember. So why bother?

Well, a power cut is only part of the power quality spectrum for a start. What about a surge? Or transients? Dips, noise, harmonics? There are many phenomena that can cause problems with your electronic systems (computers et al) which you should need to protect against. The cost to your business can be huge.

Imagine working on your spreadsheet, or presentation, or important tender document and out go the lights. You’ve lost everything! Or maybe not, you sensibly had the auto-save function on, so in fact you’ve only lost 10 minutes. Phew -not so bad. But then when the power comes back on, your PC no longer starts up. The sudden power loss has caused your hard drive to fail. Not only now do you not have access to anything you have done since your last backup (you did backup didn’t you?), you now have the expense of having your PC repaired, the data extracted, and not to mention the hassle of it all. I bet it’s cost you a damn site more than the UPS would ever have done.

We have a great range of UPS systems at competitive prices for people just like you. Depending upon your level of risk we have a suitable solution. If you want to protect against simple surges, black outs or brown outs, try our VIX range. You can pick up a UPS for under £30 which will protect your PC and shut it down safely if required. At this price it seems crazy not to!

For your more high powered and critical servers, go online double conversion. This way you protect against almost everything poor power quality can throw at you.

What we need is a demonstration, where you can see the effect of each of the different types of UPS. OK simple, what you need is an oscilloscope, some decoupling device, a variac, a multimeter. Oh, and of course one of each type of UPS.

Or you can have a look at this flash file on UPS operation. It’s cool!

UPS Operation Screenshot