Skip to content Skip to footer

Evaluating the true cost of owning an uninterruptible power supply

Electrical Review Logo

Caught between a growing dependence on IT resources and a heightened concern over power grid reliability, organisations increasingly regard UPSs as critical components of their IT infrastructure. UPSs, however, call for a significant level of investment, so it is important to understand the true costs of UPS ownership and the factors that impact the total cost of ownership. In this article, Kenny Green, technical support manager at Uninterruptible Power Supplies (UPSL), a Kohler company, discusses these issues and shows how UPS costs can be mitigated by using modular UPS technology


An early decision in the selection process concerns the UPS topology; should it be a traditional transformer-based type or an modular implementation? A traditional system may have a lower initial purchase price, but costs more to operate than a modular solution for several reasons. These higher costs start even before installation, with a transformer-based solution being considerably larger and heavier than its modular equivalent, meaning transportation costs will be significantly higher.

This trend continues after delivery depending on the configuration installation costs for the traditional system can be higher.  It can also occupy a greater area of valuable floor space, which could be used for revenue generating server racks. During operation, its original cost advantage is further eroded, and then reversed, due to its lower efficiency and higher cooling requirement. Other, indirect factors also contribute; these include maintenance, training for maintenance, and spare part holding.

The modular system’s superior commercial appeal is extended if capacity expansion is ever required. If this is the case, modularity makes a system far easier and less costly to upgrade. While offering all these cost benefits, the modular approach also better justifies the reason for installing a UPS at all, as it significantly improves the UPS’s availability.

Scoping the costs

A modular system’s extra initial purchase cost can be recovered within the first year of operation because of its improved efficiency. The example in Fig. 1 below shows how this is possible. Other factors are then discussed, showing how they further reduce the relative cost of owning a modular system.

The example compares the traditional and modular systems’ cost of ownership arising from efficiency and cooling losses over a five year period. Both systems support a load of 96 kW, which equates to 120 kVA at a power factor of 0.8. The traditional or legacy system comprises two 120 kVA modules operating in a 120 kVA N+N redundant configuration. The modular system also provides redundancy in a N+1 configuration, using four 40 kVA modules to cover the load.

Fig. 1: Heat loss and cooling costs for traditional and modular systems

These figures show how modular system efficiency is better than that of a traditional, transformer-based system. This is partly because a modular system’s transformerless technology is simply more efficient, but it is mainly due to the difficulty of ‘right sizing’ a monolithic traditional system. In the example, the system is only 50% loaded, which reduces its efficiency significantly. Fig.2 shows the relative efficiencies and effects of loading for the two UPS technologies.

Fig. 2: UPS efficiency curve

As mentioned above, as well as having lower, more load-dependent efficiencies, transformer-based UPSs are considerably larger and heavier than their transformerless equivalents. For example an 80 kVA transformer-based unit weighs approximately 1150 kg with a volume of 1.3 m3. Two units used for an 80 kVA N+N system would therefore weigh 2300 kg in total, and occupy 2.6 m3. By comparison, an 80 kVA N+1 modular system, built with three 40 kVA modules in a single rack, weighs around 379 kg and occupies 1.15 m3. These differences have a sizeable impact on transportation costs, which are approximately 2.26 times more for the transformer-based system.

Installation costs are similarly affected. A traditional UPS, based on two units, needs two or three times more floor space than a modular type. The transformer-based version of the 80 kVA N+N system specified above would need 1.44m2 floor space, while just 0.58 m2 is needed for the modular equivalent. Installation cost would be 86% higher for the transformer-based UPS. An added advantage for the modular solution is that it delivers nearly twice the power density – 206 kVA/m2 compared with 111 kVA/m2.

To be complete and accurate, calculations of UPS ownership costs should include provision for repair and maintenance. Monolithic, transformer-based systems present logistics challenges because a failure must be identified down to component level by working on the UPS while it is still in situ. This is problematic primarily because it denies UPS availability to the critical load for an extended period of time, but it causes other problems too. Technicians must be highly-trained to perform the depth of diagnostics needed, and specifying a spare part set which is adequate yet cost-effective is challenging. Managing these stock kits also consumes resources.

Modular systems allow for faster, easier and lower-cost repairs. Spares logistic complexity reduces to holding a single module type that can be used even to support UPSs of varying power ranges. Engineers can ‘hot-swap’ modules within 15 minutes, without needing training in a wide range of different systems. The UPS remains on line, while the module can be repaired without pressure off site. This approach allows up to 50% savings on stock management and logistics costs.

Batteries must be included in budgeting for both UPS types, as they have finite working lives and will eventually need replacing. Additionally, budget must be allowed for removal and safe, legal and certified disposal of the exhausted units as batteries are classified as hazardous waste.

Easy upgrades and optimised availability

With increasing pressure and dependence on IT capacity, UPS planning should provide for future upgrades. For traditional UPSs, these mean extra space and costly cabling, with the system taken off-line during the work. A modular UPS eliminates these delays and costs because it comprises a rack containing a set of plug-in modules that can easily be replaced to meet growing critical load requirements. Extra capacity can be achieved simply by swapping, say, three 20 kVA modules for three 30 kVA versions. Provided the UPS’s frame and associated cabling distribution is adequate, this upgrade requires little time or cost, no extra floor space requirement and no interruption to the load.

This article has to show the factors to consider when calculating a UPS system’s true cost of ownership. It also reveals why an advanced modular system ultimately costs less to own than a transformer-based equivalent, even if its initial purchase price is higher. Additionally, it’s worth remembering that as well as these costs savings, the modular topology wins on the most fundamental expectation of a UPS – its availability. Hot-swapping of modules as described cuts Mean Time To Repair (MTTR) from 6 hours to 30 minutes, so modular systems make an availability of 0.999999 (Six Nines) a reality.

For more information about UPSL and the UPS systems they offer please visit

Top Stories

Stay In The Know

Get the Electrical Review Newsletter direct to your inbox, and don't miss a thing.