For UPS installations, right-sizing ensures efficiency in system design and reduces the overall total cost of ownership (TCO) for the client.
Firstly, understand the size of the load. 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 UPS system correctly, note the equation below.
Actual Power (KW) = (Apparent Power (KVA) x 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.
Oversized UPS are inefficient. They cost more to buy, run and maintain, significantly adding to the TCO over time.
True modular UPS are flexible and can 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 shut down 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.
Therefore, it is 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.
Efficiency also requires attention. Underutilised UPS systems may not be operating at the best point in their efficiency curve. To mitigate this, look for a UPS that has a flat efficiency curve meaning it sill performs optimally even at lower loads.
To summarise: 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 perfectly sized to perform optimally, reducing the TCO for power protection for clients all over the world.
Originally featured in Electrical Trade Magazine July 2022.