Evaluating the true cost of owning an uninterruptible power supply (Sample Article)

Caught between a growing dependence on IT resources and a heightened concern over power grid reliability, organisations increasingly regard UPS's as critical components of their IT infrastructure. UPS's, 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.

UPS cost factors

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.

Easy upgrades and optimised availability

With increasing pressure and dependence on IT capacity, UPS planning should provide for future upgrades. For traditional UPS's, 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.

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