What Size UPS do I Need?

The Basics of Sizing a UPS



by Michael Ruhe, Product Manager Engineered Solutions

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 need to consider other variables to ensure that the UPS is properly sized for the intended load. Correctly sizing the UPS is critical: undersizing 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.


Below, I walk you through some key considerations on how to size a UPS. These are just some of the basic starting points to help determine the appropriate size.


Identify What Equipment Will Be Supported By the UPS


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 UPS 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.   


Calculate the Total Amount of Power Your Equipment Consumes


After determining which equipment needs to be supported, the next step is to calculate the total amount of power that equipment consumes. 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 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 take into consideration 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.


Review Equipment You Are Supporting


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 equpiment or loads have dynamic properties that also must be taken into account, 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.


For example, it is recommended practice to turn on PDUs with large transformers with the UPS on bypass, and once the load reaches steady state, to transfer back to normal UPS operation. In this case, the UPS sizing does not need to take that into consideration.


An example the other way is if the UPS is supporting something like an MRI machine in a hospital. In this case, the UPS is going to be supporting the load on normal operation, and the inrush current needs to be included in the sizing of the UPS.


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:


  1. What kind of redundancy do you want or need in the system?
  2. How would a modular UPS design affect sizing?
  3. What amount of battery run time or types of batteries do you need?


These topics, and many more, are for future discussion. Stay tuned!




Edited by Nicole KristofDigital Marketing Specialist

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