So, you need an uninterruptible power supply (UPS) system to run your electrical equipment when the lights go out? And let’s face it, the lights go out frequently in South Africa. The scale of the system you require depends on your needs. Perhaps you just want enough power for you to have enough time to shut down your equipment properly and avoid harmful “hard” shutdowns. Perhaps you want enough to keep your operations running for the entire time the power is out.
Handily, your UPS is also a power gatekeeper, making sure your equipment is protected from power disturbances like surges, blackouts, brownouts, over voltages or voltage sags, harmonic distortions, line noise, and frequency variations. Typically, a standby UPS will take care of surges, blackouts, and brownouts. If you opt for a line interactive UPS, you can add under and over voltages to that list. With a double-conversion system, all the problem areas are addressed.
Choosing how powerful your UPS should be sounds simple enough if you understand the mathematics that go into making the right calculation. A UPS’s output is usually expressed in either kW or kVA. What this means is that, if you want to run 1000 W – or 1 kW – off your UPS, you need a UPS with an output of 1 kVA, right? Well, no, it’s not quite that simple, for two reasons.
First, there is no such thing as a 100% efficient UPS. Any UPS – even the best available – will have some losses incurred in its circuitry and transformers, called reactance. Reactance reduces the usable power that is actually available from the apparent power. This means that a 1 kVA UPS will not deliver 1 kW; it might only be able to deliver 800 or 900 W. This is called the power factor and is an important value to know. A 1-kVA UPS with a power factor of 0,8 can only deliver a maximum of 800 W. However, neither does this mean that a 1-kVA UPS with a 0,8 power factor is all you need if you only need 800 W. The UPS might be able to do it, but it will require it to run at 100% capacity to provide 800 W and if this value is exceeded, it may short-circuit or fail.
And this brings us to the second reason: the spare capacity required in order for the UPS to provide the power you need, but still be able to do so reliably and for long enough to give you adequate UPS runtime while the power is out. So, you will have to calculate how much wattage you need. The easiest way to do this is to look at the power requirements of each device you want to run off your UPS. Each will have a maximum wattage rating and if you add up all these maximum wattages, you arrive at the total you require. Then we have to include at least an additional 20% margin so that, even at maximum power draw, your UPS does not exceed 80% capacity. This allows you sufficient peak operating margin at maximum capacity.
Let’s apply these calculations to our hypothetical 1 kVA UPS and see how much of its capacity is left if we assume it has a 0,8 power factor and if we can only use 80% of what is left. This brings us to 1000 W x 0,8 power factor x 0,8 (80%). The answer is 640 W and this value is the maximum the peak wattages of all your devices together may be for this specific UPS. Do not even consider anything lower than that 20% buffer – and a higher percentage would always be better, especially to cope with those spike loads of electrical appliances starting up, when they use more power than they do while they are up and running normally. So, if you have calculated that you are going to need 1800 W, having a 4 kVA UPS with a 0,8 power factor running at 60% capacity is far better than taxing a smaller UPS to the limit and it will give you far longer UPS runtime when the power goes out, too.
And, since we have mentioned battery runtime, this is the next question. How long do you need the UPS to be able to run when the power goes out? If you have equipment elsewhere in a building – or even off-site – that has to run off the UPS, you will require more runtime. If you have a generator ready and waiting to kick in the moment the power goes off and you can rely on it, you need far less battery runtime. Running 1800 W off a 4 kVA UPS at less than 60% load might be able to provide you with 20 minutes of battery runtime. The more excess capacity the UPS has, the longer it can run before the batteries are depleted. This might be a very important consideration if you first have to travel to a site once the mains power fails.
Whereas, in years gone by, a power factor of 0,8 was typical, times have changes and modern UPSs are so efficient that most of the quality products available now have a power factor of better than 0,9. However, keep in mind that no UPS will ever be 100% efficient. To keep an eye on how much you are taxing the UPS, look at its display indicating what percentage of maximum capacity you are using.
However, this can also be misleading, especially in the case of large, 3-phase UPS systems. This is because, in South Africa, you get 240 V between any phase and neutral. No connections are made between phases and, unless the loads on all three phases are more or less the same, the load reading on the display will be some way off. For example, if a 200 kVA UPS with a power factor of 0,9 (therefore capable of 180 kW) has phase A loaded to 65%, phase B to 90%, and phase C to 25%, it will still have 80 kVA (or 72 kW) unused, which is 40% of its capacity. However, the display will read 90%.
You may not exceed either the rated kVA or kW numbers so the kW number will always be the one to keep your eye on. But nowadays, there are UPS setups available that are power-factor-corrected, which means that their kW and kVA values will be the same.
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