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UPDATED OCTOBER 10, 2023
On the surface, sizing a three phase Uninterruptible Power Supply (UPS) sounds like it should be an easy task, right? I know my total load and what battery runtime I want and that's it - I'm done.
In the simplest of terms, that is all that is needed. But you also need to consider other variables to ensure that the UPS is properly sized for the intended load. Correctly sizing the UPS is critical: under sizing the UPS can lead to dropping your load and oversizing can lead to lower efficiency, increased utility costs, wasted floor space, increased peripheral equipment cost, and more.
There are several key considerations when sizing a UPS. Below, I walk you through just some of the basic steps to teach you how to size a UPS and determine the appropriate uninterruptible power supply size to support your equipment.
Not all equipment needs to be supported, so reviewing all your equipment and breaking it down into a list of either critical load or non-critical load can help make sure your final uninterruptible power supply size requirement is not drastically oversized. Non-critical can be considered any piece of equipment that can be allowed to fail when utility power is lost.
After determining which equipment needs to be supported, the next step is to calculate the total amount of power that equipment consumes. Understanding your equipment's power consumption is critical when sizing a UPS. You can usually get this information from the label on the equipment, technical data sheets or specifications, contacting the manufacturer, or you can apply a meter* and read the total power draw.
The power consumption of the equipment will typically be listed in either Watts (W) or Volt-Amperes (VA). Since most UPS systems are rated in Kilovolt-Amperes, you may need to convert the Watts to VA by dividing the Watts by the power factor of the equipment.
Once you have all the equipment power consumption numbers, add them all together and this will be the base starting value for the UPS. It is recommended practice to never run a UPS at 100% load capacity, so you will want to take your total number and multiple it by 1.2 to 1.25 for additional headroom. This will ensure more reliable system operation.
It should be noted that this number does not factor in any future expansion or additional loads unless you have already added those power consumption numbers into your total, so planning for future state is something to think about.
*If you are using meter readings to get the power consumption, all the equipment should be powered on and the readings should be taken during peak operation.
With this information in hand, you are now done and can move on to your other decision-making points such as battery run time or redundancy, right? Not necessarily. Some types of equipment or loads have dynamic properties that must be considered, so the next step is to review the type of equipment you previously identified as being supported by the UPS.
Equipment like transformers, PDUs, motors, compressors, and some medical devices have a high inrush of current during start-up compared to steady state operation. Whether the UPS needs to be able to support this inrush current depends on the sequence of operation of the equipment.
With these considerations in mind, choosing the correct uninterruptible power supply size for your operations prevents your business from going dark.
Now that you figured out what equipment needs to be supported, the power consumption of that equipment, any dynamic load concerns, and the additional headroom recommended, you have the basic starting point of how to size a UPS. From here, the discussion would move to answer questions, such as:
These topics, and many more, are for future discussion. Stay tuned!
Edited by Nicole Kristof, Digital Marketing Specialist