Monolithic and modular UPS topology comparison

Early uninterruptible power supplies were built as monolithic units; today, these have been mostly replaced by modern, modular UPS designs. Nevertheless, monolithic UPS installations are still very much available, as exemplified by products like KOHLER Uninterruptible Power’s KOHLER PW 6000 and 5000/TP models.

Accordingly, we compare the two technologies. First, we review the key aspects of the modular approach, and the benefits arising. Then we look at arguments for choosing monolithic and see how these stand up to scrutiny. Finally, we describe situations where monolithic can be beneficial.

Modular UPS systems benefits

A modular UPS system starts with an empty frame which can be populated with from one up to typically five or six modules. Each module should consist of a complete UPS system including a static bypass switch, so the system has no single point of failure. Providing that redundancy is allowed for in the design if one UPS module fails, the others will continue to support the load. This means that N+1 redundancy, and therefore high availability, can be built into modular UPS systems efficiently and easily.

An example of this is a modular UPS system like the KOHLER PW 9250DPA being used to support a 250 kW load. It could be populated with six 50 kW modules which share the load across their 300 kW total capacity. If any single module fails, the remaining 250 kW capacity will continue to fully support the load, without interruption, until the faulty UPS module is replaced.
This redundancy has been achieved with just a 20 percent increase in capacity; by comparison, a monolithic UPS solution must provide a complete second uninterruptible power supply – 100 percent capacity increase – to achieve the same N+1 redundancy.

Modular UPS system flexibility also creates other significant advantages. As data centre operations grow over time, more UPS modules can be plugged in, for vertical scaling, simply and without interrupting power to the load. There’s no need to find extra floor space or install more cabling. Once a UPS frame is fully populated, further frames – up to five more for KOHLER PW 9250DPA – can be added, for horizontal scaling. Any capacity from 50 kW to 1.5 MW with redundancy can be accommodated as required. Different modular power ranges allow for smaller or larger UPS systems.

This allows data centre users to future-proof their UPS systems without need for unnecessary capacity, cost, power consumption, cooling, floor space, cabling, or installation downtime.

Monolithic topology advantages examined

One advantage cited for monolithic UPS systems is a lower initial purchase price. While this can be true, modular UPS systems can run more efficiently, with lower cooling requirements, so their total cost of ownership is lower.

Another advantage claimed for monolithic uninterruptible power supplies is that a modular UPS system’s multiple modules contain more components, which reduces reliability and mean time between failures (MTBF). While true, this misses the point that it’s availability, rather than MTBF, that ultimately matters to users. And availability can be optimised by minimising Mean Time to Repair (MTTR) as well as by maximising MTBF.

Accordingly, modular UPS systems can offer the best availability because their reduced MTBF is more than offset by the reduction in MTTR realised by hot-swap modularity; repair times come down from an average six hours to just half an hour.

So where should monolithic UPS systems be used?

Nevertheless, monolithic uninterruptible power supplies like the KOHLER PW 5000/TP or 6000 are sometimes the best UPS solution. For example, if initial cost is an issue, while the risk to the load caused by any power problems is considered as low. If no redundancy is required, then the problem of excessive extra redundant capacity doesn’t arise. Additionally, if the load can safely be left unprotected during UPS maintenance or an extended repair session, a hot-swappable power supply is no longer particularly beneficial.


To find out which approach is best for your particular application, it pays to consult a well-resourced UPS supplier like KOHLER Uninterruptible Power. They can offer impartial advice, backed by an uninterruptible power supply product range encompassing both monolithic and modular UPS topology.

If you have any questions about our uninterruptible power supplies or UPS services please get in touch with KOHLER Uninterruptible Power via our contact page or call us on 0800 731 3269.

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