A UPS System is designed to provide Uninterrupted Power to your electrical equipment, which we call the load. This 10 Point Guide to UPS System Selection is intended to give you all the options you need to help you choose the correct UPS. It is aimed at single phase applications under 10KW.
1. Is the UPS System compatible with your load?
Lower cost UPS Systems contain “square wave inverters” that are only suitable for Switch Mode Power Supplies 
, as found in computers and similar electronic systems. If your load contains lighting, motors or any non-computer loads then this type of UPS is not suitable, and you will need a Sine Wave UPS System instead. In addition, some modern power factor corrected computer power supplies have been found to be incompatible with square wave UPS. If in doubt always choose a sine wave UPS.
2. Is the UPS System adequately rated for your load?
The power consumption of your load should always be less than the maximum rating of the UPS System. Make sure you know or over-estimate the consumption in Watts of your load, and know the maximum Watts rating of the UPS System as this is usually different from the Volt-Amp (VA) rating – which is often exaggerated for specmanship reasons. If you don’t know the Watts rating of the UPS then don’t buy it.
See Working Out UPS System Power Requirement.
3. How long do you want the UPS to hold up for in the event of a power cut?
Be realistic, as the longer runtime you want to achieve the more batteries (and hence cost, size and weight) is required. Usually power cuts are momentary and so a UPS that provides a few minutes runtime is suitable for most applications. If your application becomes more critical then you may wish to look at extended run UPS Systems to provide a longer degree of back up.
Remember that there is a practical limit to achievable runtime due to factors such as battery capacities, space available, and most importantly the ability of the UPS to recharge a flat battery bank within a reasonable time frame. You should look at getting a 90% recharge within 4 hours.
4. Which UPS technology is right for your application?
UPS technologies including offline and line interactive have a small break when transitioning from mains power to battery backed inverter power. Most equipment will not be affected by this gap as it is only in the region of about 10msec (1/100th of a second). Some systems claim a response of faster than this, in the region of 2-4 msec but usually quote a “typical” figure. In practice it all depends on where the mains power is on its cycle when power is lost, but it is realistic to assume that you will lose at least a half cycle of power (which is 10msec for a 50Hz supply) and it is worth assuming that you may lose a full mains cycle (20msec for 50Hz). If your load cannot withstand a gap of a mains cycle then you need to opt for a no break technology – online double conversion.
If you regularly don’t lose power but suffer from brownouts then line interactive technology can boost the supply without resorting to battery power which of course keeps you running indefinitely and also extends battery life.
You should also consider where the UPS is to be located and if fan noise that is generated by some systems will be unwelcome. Online technology has an always on fan which can be quite loud. You could consider an AC ReGenerator for these applications. Also, some line interactive units have always on fans and others have fans that activate when the UPS is active – either on inverter, or when in “buck” or “boost” mode.
Consideration should be given to what you need to protect against. Is it just power cuts, or voltage swings? Do you have distorted mains power or spikes and noise on the power line causing issues. Practically all UPS Systems will incorporate surge protection, and provide battery backup. Line Interactive technology introduces voltage regulation whereas only online double conversion technology can provide protection against harmonic (distortion) problems.
5. Power Distribution
Have you considered how you are going to get power from the UPS system to your load? What sort of power connections are on the UPS? Can you plug your equipment straight in, or do you require additional adapters or power distribution units? Do you require a more advanced method of connecting your system through bespoke power distribution boxes? Is the system to be hardwired?
Generally, UPS Systems under 3KVA will be pluggable and systems over that will be hardwired, or connected with industrial type couplers. The output of pluggable UPS systems varies, with some providing IEC type outlets and others with national specific sockets (UK Socket outlets here in the British Isles). For IEC outlets, a IEC-IEC cable, also known as a loop-through cable is usually provided, and in some circumstances a mains lead is not provided -the rationale being you remove the IEC lead from your computer, connect this to the UPS, and then connect the IEC lead from the UPS to the computer. UPS with IEC only outlets cannot provide power to devices which utilise “wall wart” type power supplies that require directly plugging in. For such systems you will need an adapter converting from the IEC output to a UK gang socket to plug your ancillary devices into.
6. Load Shedding
In order to increase runtime, certain UPS Systems have the feature of shedding unwanted load. For example in an unattended server room, you may wish to remove power from non-essential equipment such as monitors in order to give the server more time before a shutdown is necessary. Consider if this is a necessary feature and can your UPS System provide this? If not, have you considered a power distribution unit that incorporates a load shedding feature?
7. Communicating With the UPS.
Do you need a communication interface with the UPS? For some applications you may not require this. For example if the UPS is providing power to a non-computer related load, however even in such circumstances there may need to be a method of knowing mains power has failed and the UPS is running on battery power. Some UPS contain, or can have fitted, a relay, or dry contact card, that you can connect to external alarms to advise of power failure. A common application for this is in Building Management Systems.
Typically though, you will have a computer system connected and you want to have options available to you when the power fails. If you want to actively know things about the UPS status, such as battery level, load level etc., then you will need a UPS that has a serial communication interface and software that can read this. Some UPS only contain a simple contact interface that can only tell if a UPS is in certain modes, such as on battery or low battery and can only react on these alarms.
Make sure you are clear on what you want the software to achieve for you, if the software available is compatible with your operating system, and if the UPS has the hardware to match. Most UPS are fitted with a USB interface, and perhaps an RS232 port which is used to communicate with the host computer. Note that the UPS can only communicate with one computer, so if you require multiple computer shutdowns you need to figure out a way of achieving this from the host.
Some UPS also contain HID compliant USB which allows the UPS to interface directly with the power management on the computer O/S.
Another way of communicating with the UPS is by means of SNMP. You can fit a card to the UPS (or provide an external device) that connects to a network. You then can monitor UPS status over a web browser and have the server operating system react to SNMP traps. Ensure you have the expertise to do this as it is generally outside the remit of the UPS vendor.
8. Ongoing Maintenance
Have you considered ongoing maintenance for the UPS? For smaller systems this is probably no more than wiping away some dust and checking the battery still works. However for more critical applications you need to ensure the UPS is in tip top condition. You may want to employ professional UPS maintainers, however this is not really feasible especially at the lower power end of the market. A better option may be to check if your UPS has a test facility and organise a regular routine for using this. The UPS software may also be able to instigate routine testing.
If you need to exchange the UPS or remove it from circuit you will need to remove power to your load, or alternatively use a Maintenance Bypass Switch. These allow the UPS to be removed from circuit while maintaining power to the load (without UPS support of course). The UPS can be removed, checked or replaced. Note, however that there is a potential for damage with bypass capabilities if not done properly. Maintenance Bypass switches are commonly used on hardwired installations, but can also be incorporated for pluggable systems and also provide power distribution.
UPS batteries are typically sealed lead acid types and have a typical useful life of about 3 years. The actual life depends upon the design life of the battery itself, the number of discharges it has been subjected to, how deep those discharges were, the capabilities of the UPS charger and the temperature to which the batteries have been subjected. In short, if the batteries are regularly used in a hot environment they may only last a few months. If they are cool and hardly ever used then they may last 5 years or so, or longer for higher design life types. When it comes to changing your UPS batteries have you a plan on how to do this?
For low cost systems, it may be simpler to purchase a replacement and recycle your old system, or the UPS vendor may offer a battery swap programme. Check the UPS vendor will take your old system back, or provide an upgrade. Some UPS Systems contains battery cartridges that can be swapped by the user, making it easier to do battery exchange without downtime or risk. Other systems contain internal batteries that need to be opened to change. In the latter case, confirm you are able to undertake this, as batteries can be extremely dangerous if not handled correctly.
9. Expandability
Let’s face it, things tend to get added rather than taken away, and it goes with the UPS system load. You are more likely to add another component than remove it. To this end, it is worthwhile ensuring you have a bit more headroom to allow for future load expansion.
There is also a rule of thumb that suggests not to overload the UPS by more than 80%. Since all UPS systems should be able to function in all modes at 100% load there is no real reason for this, but never the less it is a good rule to adopt as it will give you more headroom, allow for fluctuations in UPS load, and give you a little more runtime.
10. Increasing reliability of power by introducing redundancy
In critical power applications such as data centres, it is common to build redundancy into systems – that is, allow for a backup that will take over should one system fail. The requirements of an industrial data centre may be no different to a computer server or computer room essential for small and medium enterprises.
One such means of achieving power redundancy is to parallel two or more systems together and ensure that the UPS loading is less than the combined UPS capacity minus one unit. For example, a small computer room with a total power draw of 15KW, can be powered by three 10KVA/9KW UPS systems connected in parallel, as 3x9KW = 27KW, 27-9=18KW, which is greater than 15KW. This is known as “n+1″ redundancy.
Smaller systems tend not to have the ability to parallel which requires another solution. An automatic transfer switch is a good option for this. The transfer switch has two inputs, a master and a slave. If the master fails, then the transfer switch will automatically revert to the slave. In such situations redundancy can be achieved by connecting two UPS systems to the transfer switch, one acting as master and the other as slave. Ideal for a resilient UPS system protecting a essential server for example.