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How Do I Correctly Size A UPS?

There are several factors that influence sizing a UPS system, including the combined load of all the equipment the UPS will protect, scope for further system expansion, battery runtime and redundancy.  

As well as choosing the right UPS topology, correctly sizing an uninterruptible power supply is crucial – undersizing inevitably causes immediate problems, while initial oversizing will waste energy, money and valuable floor space.  

The easiest way to ensure a correctly sized UPS system is to get prospective suppliers to undertake a full site survey where they can accurately assess your requirements. However, it is possible to broadly size a UPS yourself by following a step-by-step process. 


Critical or Non-Critical Loads 

This starts with listing and reviewing all the equipment that will need to be protected by the UPS. Establish whether an item of equipment is critical – and therefore will need the emergency backup provided by the UPS – or non-critical, which can be allowed to fail when the mains power supply does so. (Learn more about the difference between critical and non-critical loads).


Power Range 

The next step is to calculate the total power range for the combined critical load that needs protecting. It’s important to base this on use during peak working hours, rather than on quieter times such as an office network during the night. 

Equipment labels and supporting technical data will provide information such as the supply voltage, frequency, number of phases, load current, power factor and power consumption.  

The power consumption of electrical equipment is stated in either Watts (W) or Volt-Amperes (VA). Because UPS systems are rated by VA or kVA ratings, this may require a conversion from W to VA, which can be calculated by dividing the power consumption (W) by the power factor.  

Add up all the VA, then multiply this by a figure such as 1.2 or 1.25, which factors in potential future growth and need for system expansion. That figure is the maximum size in VA or kVA that your UPS should be.  

Note that a UPS should never be sized to run at 100% load capacity, as this isn’t recommended for safe, stable and reliable performance. 


Potentially Problematic Loads 

Certain equipment (i.e. laser printers, blade servers, air conditioners, certain lighting systems, motors and compressors) have an inrush of current during start-up or draw higher currents in normal operation, which can cause the UPS to overload. This can lead to intermittent alarms or potentially send the UPS into bypass mode.  

For these types of load, good practice suggests two options: either remove them from the power protection system (if the equipment can safely power down on mains failure) or oversize the UPS by a factor of at least three.  


Battery Runtime 

This is the amount of time you want the UPS to keep equipment operating in the event of a power failure. How to size a UPS battery depends on the nature of the equipment. In many circumstances, runtime only needs to be for a few minutes as a bridge to let the standby generators kick-in and take over. 


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