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Monolithic vs Modular UPS systems for data centres

Luke Eiland

Luke Eiland

Service Manager, SolarEdge Critical Power Division
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Image credit: SolarEdge

Increasing demand for reliable power, coupled with the growing use of essential electrical equipment and digitisation, is driving up adoption of UPS systems.

While data centres, hospitals, and manufacturers were among the first to adopt the technology, demand for UPS systems is expanding to other users such as research facilities, schools, or anywhere with critical power applications. Considering the high stakes associated with energy failure, what are the most important factors to consider when choosing such a system?

UPS system design boils down to two main types – monolithic or modular. The original UPS design, the monolithic system, is a solution made of single sub-systems including one static switch, one rectifier, one inverter etc – but importantly, they have no intrinsic redundancy. The later developed modular UPS is, as the name suggests, made up of several smaller components that can be added or removed to scale the solution up or down. Looking at both UPS solutions, we will analyze the benefits of each in relation to three main decision criteria – cost, reliability and simplicity.

Cost – is it all about price?

When making any investment, cost is always the primary consideration. When it comes to UPS systems, and if comparing systems of a similar power requirement, the traditional monolithic solution has the lower initial price tag. This is why, traditionally, it has been the solution of choice for organisations with smaller power requirements – especially if their needs are unlikely to change in the near future. However, for those businesses with larger power requirements, such as power-hungry data centres, or where increased flexibility is needed, there is more to consider.

While there are different model sizes of monolithic systems, there is less flexibility in terms of power capacity than with modular UPS. Subsequently, monolithic UPS users often end-up paying more for a system that’s larger than they need to reach their power requirements. Additionally, should their power requirements change, due to a lack of design flexibility, a whole new monolithic system is often required.

This is one of the strengths of modular UPS. To increase the capacity of your modular system, it’s as simple as buying additional power modules and slotting them into your existing rack. What’s more, should you have multiple modular UPS solutions at your facility, you can also swap modules from system to system to address changing load profiles – increasing efficiency savings and mitigating risk. As demand for data centres is increasing, so must load size, and therefore the UPS systems to back them up. An easily expandible system is a key benefit in such situations.

Reliability – when you can’t afford to fail

Designed to ensure that your power supply is reliable and uninterrupted, a UPS system must be extremely robust and reliable – obviously, this is critical when powering critical data centres or other essential electrical devices, as power failure can lead to critical data loss or corruption.

Traditional monolithic systems feature a simple design. Consisting of fewer individual components, this means that there are less failure points in the system. Alternatively, modular UPS solutions spread the load over many more components. Which it could be argued, provides many more potential failure points. However, in a monolithic system, failure of one component could mean failure of the system as a whole. But in true modular units, if one module fails, it’s separated from the rest of the system and the other modules pick-up the slack to ensure continued operation/power. This is a key distinction between modular systems. Not all modular solutions are the same, and only ‘true modular’ UPS provide this functionality.

Like many things in life, being handmade is generally better, however UPS systems are not one of them. While large monolithic solutions are manufactured manually with the resulting risk of human error, smaller power modules for modular UPS systems are manufactured and tested utilising automated solutions. This approach adds to reliability, with each of these smaller modules also being machine tested before shipping – something that’s not undertaken within the monolithic manufacturing process.

Simplicity is key

Thankfully, failure of a UPS system is uncommon, but replacing individual components as part of regular system maintenance is both expected and planned. Ensuring that your system remains operational, or that you have back-up power during those service intervals, can be a big issue with considerable consequences for your operation.

For this, modular UPS systems are relatively easy to maintain. In a true modular solution, each essential part is hot swappable and can be removed for maintenance or replaced while the system continues to run. Conversely, a monolithic system will likely need to be shut down for maintenance of core components, which means that you either lose your back-up facility during this period, risking critical data loss, or you have to switch to another system – essentially meaning that you need two units to provide the same service, with all the associated costs of purchasing, installing and maintaining two UPS devices. Now, some users also install two modular UPS systems for ultimate peace of mind, but this trend is beginning to change as depending on the load’s criticality, it’s often not necessary.

Crucially, and unlike the modular approach, if certain individual parts fail in a monolithic environment, the process of replacing them is often complex and time consuming, requiring a specialist UPS engineer representing additional downtime and cost.

Monolithic vs Modular

Both UPS systems are reliable, but they employ totally different designs to provide different levels of flexibility and autonomy. For data centres with lower critical power application requirements, a monolithic solution will be both cheap to install and generally fit for purpose. But as we’ve explored, you could also be installing a system that’s larger and more expensive than you need, as well as being more costly to maintain in the long run. Plus, should your power requirements change, and you need to add more capacity, monolithic solutions don’t have the system flexibility to support you.

For data centres that require ultimate reliability and no downtime, a modular UPS system is very appealing – but on the face of it, they do look more expensive. However, when the total cost of ownership is calculated, considering ease of maintenance, a smaller floorspace footprint and overall system flexibility, the modular argument becomes very compelling.

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