Written By Aaron Oddy, Sales Engineer, CENTIEL UK 

For UPS installations, correct sizing ensures efficiency in system design and reduces the overall total cost of ownership (TCO) for the client. To right size the UPS, understanding the size of the load is necessary.  UPS systems are commonly measured in power ratings of kVA or kW, depending on the load requirement. Historically, sizing in kVA was standard practice as it is understood that the apparent power is different to the actual power.  For example, a 100kVA UPS designed with a power factor (pf) of 0.8 can only deliver 80kW of true power. Factoring apparent and actual is critical in making the right calculation for your system.

The gap between apparent and actual power has become less over time with the latest UPS systems offering a unity power factor of 1.0, meaning kVA = kW. However, it is still necessary to check to see if the actual load has a power factor to size the system correctly using the equation below.

Actual Power (KW) = (Apparent Power (KVA) x UPS Power Factor)

Here’s a working example comparing 0.9pf and 1.0pf:  If the Unity Power Factor of a UPS is 1.0, a load with apparent power of 60KVA will be 60KW.  Simple. However, if the Power Factor of the UPS is 0.9 then the true or actual power of a 60KVA UPS will be 54KW of actual power.   Therefore, it’s important to understand the actual rather than apparent load to determine the size of the UPS needed.

We also don’t want to oversize a UPS as this is inefficient.  A UPS which is too big can cost more to buy, more to run and more to maintain, significantly adding to the TCO over time.

With standalone or monolithic UPS, there is less flexibility on right sizing.   If a UPS is required to support a 100kW load on day one with projected load growth of up to 200kW, installing a 200kW initially would be inefficient and unnecessary. Although the system is future proofed, it is double the size required from day one.

A modular system is far more flexible and a 100kW (Unity Power Factor) UPS could be installed with the capacity to add modules using a pay as you grow approach up to full capacity.  Only the infrastructure around the UPS needs to be ready for future growth, allowing for system upgrades without the need to transfer the load to bypass or a site shutdown. Rightsizing the UPS from day one reduces overall capital expenditure and minimises running and maintenance costs.

It’s the same with the DC components of an installation.  To right-size batteries we need to understand a client’s shut down procedure.  If there is a power cut, how long does it take to shutdown the critical load safely? Is a three-hour run-time really needed?  For example, Hospitals, adhere to HTM compliance of a one-hour autonomy.  However, other clients may only need, half an hour or 40 minutes. It’s all dependant on site requirements.

Lead is priced on weight, so it’s possible to reduce the investment in the lead-acid batteries proportionally.  This also influences the amount of space needed to house batteries and cooling to keep them at their optimum working temperature of <20 degrees C.

The term efficiency also requires attention.  Most marketing material will state the efficiency level of the UPS, but it’s important to look more deeply at what this means.  Most UPS are incredibly efficient if run at 100% load.  However, when the load demand reduces significantly so can the overall system efficiency.

The pandemic is a prime example of where equipment was not being used to full capacity and a reduced load meant that some UPS systems were not operating at the best point in their efficiency curve. Particularly systems with aging technology.  To mitigate this, look for a UPS that has a flat efficiency curve meaning it sill performs optimally even at lower loads.

So, when it comes to UPS systems, size is definitely important!   Calculate the rating of the UPS system based on the actual power of the load measured in KW; don’t oversize systems; query the necessary autonomy and ensure the UPS can remain efficient if the load tails off.  Our sales engineers at CENTIEL are experienced in making these kinds of calculations to ensure UPS systems are right sized to perform optimally, reducing the TCO for power protection for clients all over the world.

This article originally featured in UK Power News Magazine December 2021


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