How do I choose the right UPS System?

Your guide to selecting the right UPS system, also known as an Uninterruptible Power Source or Supply, to back up your computer system and other critical loads.

This very comprehensive article, divided into 7 sections for easy referencing:

  • 1. What a UPS is not
  • 2. A UPS serves a dual purpose
  • 3. Backup-time to Cost ratio
  • 4. UPS and battery care
  • 5. Combining your UPS with a generator for maximum benefit
  • 6. Sizing the right UPS
  • 7. Points to remember

will answer most, if not all, of your questions regarding which UPS system best suits your particular standby power needs. Covering the most practical aspects of UPS functionality, we even offer you a step by step guide to sizing the right UPS for your situation.

What a UPS is not

Many people think that a UPS system is a backup system for when the lights go out as well as a protection device for lightning surges.

In actual fact, a UPS may at times prevent the connected inverter load from being damaged by lightning and sacrifice itself or its battery, by consuming huge amounts of lightning energy to the point where it is permanently damaged.

However, a UPS is not really intended to take the place of a surge device such as lightning protected plugs and surge plug strips, which are cheap by comparison and easily replaced if they get taken out by lightning.

These types of low-cost surge protection devices will, however, never be able to replace a UPS when it comes to a loss in the power supply to your home or small office. The only thing you can use to prevent permanent damage to your computers and other sensitive electronic devices is to purchase a UPS battery backup unit.

A UPS serves a dual purpose

A UPS system will protect your critical loads for all types of power disturbances, from short power dips called brownouts to complete power failures called blackouts. A UPS system will also act as a blocker and power filter for power surges in the mains supply to your house or small office or even a large building/factory.

What the UPS does is make use of a battery backup which can vary greatly in size depending on how much battery backup time you need. Most UPS applications are based on a short battery backup that allows you enough time to switch off your computer and other sensitive loads in a controlled shutdown, and thereby prevent them from crashing and being damaged as well as losing data when the power goes out.

Another important thing to remember is that the higher the percentage inverter loading the shorter the battery backup time you will get from the UPS inverter. It is normally best practice to size the battery backup at full rated inverter load; although most UPS systems never run anywhere near the full inverter capacity or VA rating.

In order for you as a potential UPS user to buy the right product for your personal or small office power supply backup needs, it is essential that you understand how to size a UPS correctly, how to select your battery backup time after the power goes out and save your precious data and how to ensure that you get long term service and product support.

A UPS for damage control

Imagine the frustration you feel when the power goes out and you lose everything you have been working on for the last three days. Your report is due in the morning but your PC has crashed and the data is corrupted and lost, never to be found again. Bad power supply from the AC mains will cause major wear and tear on any data device’s electronics.

Anything that uses electronics, from a PC, server, TV, network switch, and up to the controllers and programming devices of all types of machines from medical, heavy industry and virtually any device that uses electricity, will suffer wear and tear at the mercy of a bad utility power supply grid.

In Africa, the quality of this power supply is one of the worst of all the continents on Earth. Quality of supply refers not only to reliability and uptime but also how much damaging power in terms of noise, peaks, spikes, dips, etc., happen on the supply grid.

In many countries the power supply quality is very good, the same types of loads are connected to these grids but they are very strictly controlled in terms of what they do to the incoming supply waveforms and what they inject back into the mains grid which can cause potential harm to other users on the grid.

This interference is prevented when the manufacturers of equipment design anti-grid wear and tear injection into the front end of their equipment, thereby filtering it out of the equation.

This is not the case in the African grid environment. The only way to prevent bad power from damaging your sensitive load is to put a UPS backup inverter system on the mains supply side to the input of your computer or critical loads.

A UPS for data protection

If you consider a laptop computer which runs on its power supply, like any PC, but the power supply also charges the built-in laptop battery. When the power goes out the laptop will continue to run uninterrupted because the battery immediately takes over from the mains supply. You don’t lose data and your laptop doesn’t crash.

A UPS system works in the same way. It is a “black box” that contains a battery charger and a battery backup. The only difference is that a UPS has a third component that a laptop does not have and that is a built-in inverter.

When the power goes out, the battery will immediately “kick in” and supply the inverter, which in turn will supply your desktop PC, office server or other sensitive electronic equipment such as alarm system, CCTV etc. It will not see any power dips or outages, allowing you to continue working for many hours if you have a long backup battery or allow you to save your work and shut down the critical load in a controlled shutdown if your backup battery is only a few minutes and you don’t have a backup generator.

Backup-time to Cost ratio

With long battery backup-time comes extra cost.

A small backup UPS for a home PC or similar emergency load is relatively low in cost in comparison to that of a desktop PC. You can expect to pay a couple of hundred Rand depending on the type of technology used.

This kind of unit usually only has one or two small batteries inside, about the same size you use on an electric gate motor or alarm system. They will only give you a few minutes of backup power (inverter % loading dependent) when the mains fail.

If this is sufficient time for you and you do not need continuous backup power for many hours after a power outage then this is by far the cheapest choice of UPS system.

However, if your business or method of income is dependent on continuous power availability then you would have to consider either a long backup battery UPS or a combination of a UPS system with a short backup and a generator set.

If you choose to be able to run your load for several hours after a power outage or blackout then you will need a much bigger battery which would be housed in a separate battery cabinet or on a battery stand. Sometimes they are kept in a dedicated UPS backup battery room.

A UPS for safe, automatic shutdowns

Most reputable UPS manufacturers, like AROS RIELLO and Pureline, supply the UPS with freely downloadable shutdown and monitoring software. With the use of a cable to interface with your PC, or a special SNMP option which allocates your UPS an IP address on your network, the software can be set up to automatically save work and shut down the UPS inverter’s connected loads safely without any crashing or data loss. You don’t have to be present; no intervention is necessary from a UPS user.

This is a particularly useful tool when the critical UPS inverter load is left running unattended for long periods or even when there is a backup generator set that doesn’t start and the UPS battery starts going flat; at least the inverter load will not crash and be damaged.

UPS and battery care

Battery Water

When a battery charges it gives off gas which also contains water particles, therefore, it will lose water while charging and need to be periodically topped up with battery water. Usually, the batteries used with a UPS are sealed batteries and require minimal maintenance. The sealed VRLA battery that is used in a UPS to supply power to its inverter stops the battery from losing water, so it doesn’t need topping up. It does, however, dry out after a few years and then requires replacement.

Battery Temperature

Another thing to remember is that your battery should be operated in as cool a surrounding as possible. In high temperatures, above 25 degrees Celsius, your battery’s lifetime is very much shortened. You can bank on your sealed VRLA battery life being halved, for every 1 degree Celsius above 30 degrees of UPS battery environment operation.

It is critical to make sure that your UPS backup battery is kept as cool as possible. However, at very low temperatures, below 15 degrees Celsius, a battery will perform worse and will not give full capacity. As long as it is not working below 5 degrees Celsius it will only lose about 10% performance.

Battery backup power

A UPS battery requires periodic replacement depending on the above factors and one other big culprit: the number of times the battery gives off backup power to the UPS inverter to feed the backup load.

Think of it this way, each time you use your battery a little more gets eaten up. The higher the intensity of use of the battery i.e. very long deep discharges of more than about 20 minutes, the shorter the life will be.

If you were to discharge your UPS battery backup due to power failures every day for many hours a day, your typical UPS battery will not last more than 1 year and then it will start giving much shorter power backup and need replacement because it is worn out and used up inside.

So, just like a car’s brake pads; if you drive the brakes hard they only last a short time but if you drive the brakes gently they last longer – a battery’s life expectancy works the same.

If your UPS backup battery is going to be doing long discharges every day, then it is important to tell your Standby Systems consultant so that very special battery sizing techniques are used to ensure that you don’t need to replace it, at a high cost, within 1 year.

Combining your UPS with a generator for maximum benefit

If you want a backup power solution to keep you going 24/7 in the case of, for example, data center backup or small to large computer rooms and critical electronics equipment rooms, the best and most reliable, as well as the cheapest power backup solution by far (in terms of total cost of ownership and long term running costs), is to use a combination of a backup inverter UPS system with 5 to 20 minute battery backup AND a generator set with an automatic mains failure panel for auto-start on a power failure. This combination will guarantee the smooth running of your critical UPS inverter loads 24/7, 365 days a year.

Sizing the right UPS

When sizing a UPS it is important to decide on the following:

  • 1. What loads are critical to me for clean backup power supply protection?
  • 2. How long must these loads stay on after a power failure?
  • 3. Is the cost of the UPS worth it? If you face regular, 4-hour long blackouts, will they cripple your company due to no productivity during these power failures?
  • 4. What does it cost you in revenue and market share loss for every 4-hour power outage?
  • 5. If you want to operate for extended long battery back up runtimes, do you have the funds available every 2 years or so to replace the battery backup to your UPS inverter?
  • 6. Is a backup diesel generator with an auto-start (AMF) panel a viable option to use in conjunction with a UPS backup system?

Short vs Long backup runtime

If you only need time to shut down when the power fails, then a UPS unit with between 5 to 20 minutes backup battery is more than enough. It is also the cheapest backup UPS option you can make use of. All you need to do now is size up all the loads you want on UPS and call your local Standby Systems dealer for a quotation. If you are not sure of your sizing, write down a list of all the equipment you want to put on your UPS backup system and Standby Systems technical salespeople will gladly assist with helping you with the correct sizing.

If you need to keep your equipment up and running 24/7, 365, then you need to decide on a long runtime battery backup with a UPS that has a battery charger big enough to charge that battery or a diesel generator, which is by far the best option in terms of cost and reliability in the medium to long term life of your UPS backup plant.

Sizing the right UPS for the job

Remember that you need to look at all your critical loads connected to your UPS inverter output as well as their support infrastructure. It doesn’t help if the server is kept running for hours but the workstations cannot operate as well. Similarly, if your telephone system or backbone goes off on a power failure you need some kind of reliable link between your IT system and the outside world. It doesn’t help your PABX is off or your service providers backbone also goes down.

To size your UPS you will need to calculate the power draw of all the critical loads you want to put onto your UPS inverter backup system. Generally speaking, avoid all types of heating and cooling loads and anything with a motor-driven device, such as a pump. For heating and cooling, there are many other alternative ways to solve these problems. For instance, gas or solar water systems for heating and a chest freezer which takes a whole day to defrost when the power goes off, so a few hours is of no consequence.

When it comes to lighting, select only a few individual lights and ensure all your lighting is LED for maximum efficiency.

All electrical devices that you may want to put onto a UPS system will have a rating label, either on the device itself or on the power supply that is supplying the device.

A step-by-step guide to sizing your critical load

Sizing is easy; all you do is write down ONE of the following from the rating plate of each device, with the first prize being number 1 in the following list:

  • 1. Watt or W rating
  • 2. VA rating
  • 3. Volts and Amps rating (Some devices will have the AC volts and current A or amps rating as well as the DC volts and DC Amps rating). You may use either both AC ratings of volts and amps or both DC ratings, it makes no difference.


  • 1. Add up all the Watt or W ratings to a total W or Watts.
  • 2. Divide the total Watts by 0,6; which will give you the equivalent VA rating.
  • 3. Now add up all the VA ratings
  • 4. For devices not displaying VA or W, multiply the Amps rating by the Voltage rating for AC or DC as per point 3 in the above paragraph, this will give you the VA rating. Add up all the values for a total VA.
  • 5. Add up all the VA values and allow an additional 20% on top of that for load growth.

Well done; you have worked out the backup system UPS inverter VA size that you need! All you have to do now is decide on battery backup time.

If you are unsure of your sizing then visit the sizing calculator on the Standby Systems website for more assistance, or contact Standby Systems via the Contact form or telephone numbers, and one of our skilled UPS sales engineers will be able to assist you to make the right UPS choice.

Points to remember

  • The longer the battery backup time on your UPS the higher the cost.
  • The bigger the battery the bigger the running costs each time it is replaced.
  • Standby backup generator sets don’t need replacement every two or three years.
  • The best backup power solution is a UPS with a < 20-minute battery backup and a Standby Generator that automatically starts after a mains fail.
  • The UPS prevents the load from going down in the few minutes that it takes the generator to start and then supply power to your UPS input.
  • A UPS also filters power; if you buy the right one.
  • A UPS prevents bad power from damaging your load; if you buy the right one.
  • A UPS is not a lightning protection device.