UPS - You Want One How Big?!

Sizing Up Your UPSDeciding on the right UPS and, in particular, the right size of UPS for a specific application can be challenging. There are no hard and fast rules, says Paul Norgate of Eaton Power Quality, but there are some very useful guidelines.

In this article, I am not going to dwell on UPS topologies as this subject has already been widely covered and most leading manufacturers offer downloadable guides to topology selection. Suffice to say that, in IT applications, an on-line topology will in almost every case be the best choice, although other topologies, such as line interactive, are sometimes suitable.

But what size should the UPS be? To answer this question, the key points that have to be considered are the configuration of the UPS installation, the size of the load, the required runtime, the possibility of future expansion, and resilience. Let’s take these factors in order, while noting that they are, to some extent, interdependent.

There are two main options for the way that UPS systems are configured: either each server rack is fitted with its own individual UPS – a decentralised UPS system – or one large UPS is used to supply multiple racks – a centralised UPS system. The first approach has the benefit of easy expandability, but the UPS’s do occupy potentially valuable space in the racks.

The second approach typically allows the UPS to be located in less valuable grey space, such as an electrical switchroom, but careful consideration must be given to how easy it will be to expand the UPS system, should the need arise.

Once the basic configuration of the system has been determined, assessing the size of the load is, in principle, a relatively straightforward matter of taking the loading information from the rating plate on each server fed from a particular UPS, adding the loads together and then allowing a safety factor.

In practice, things aren’t quite so simple, as the server rating plates will show the loading in watts (W) whereas the UPS will be rated in VA, which means that the power factor of the UPS must be taken into account. For example, some line interactive UPS’s have a power factor as low as 0.8, which means that a 1,000 VA UPS of this type can only support a load of 800 W (1,000 VA x 0.8). With modern double conversion UPS’s, the power factor is more usually around 0.9, so the difference between the VA rating of the UPS and the power rating in watts of the load is less of an issue, but it should still be taken into account.

If there is any uncertainty about the size of the load or the appropriate safety factor to use it as a good idea to obtain expert advice from the UPS manufacturer, and such advice should always be sought in the case of large data centers with centralised UPS systems, as the load calculations are likely to be more complex and making a mistake can be costly.

The UPS should ideally be selected so that it will be operating at about 75% of its rated capacity under normal circumstances, in order to achieve the highest efficiency while allowing scope for expansion. It is nevertheless worth noting that modern UPS’s lose very little efficiency with loads down to around 40%, so if future increases in load are likely, the installation of an over-sized UPS should not necessarily be ruled out. Over-sizing shouldn’t, however, be taken too far – even the best UPS operating at only 20% or 30% of its rated load will be inefficient.

Let’s move on to runtime. In non-IT applications, runtime requirements vary greatly, and should always be considered on a case-by-case basis. As a rule of thumb for IT systems, however, a runtime of 30 minutes is typically considered appropriate but, in sizing the UPS to provide this, the details of the application must still be taken into account. For example, if there is an on-site standby generator, a shorter run time may be adequate.

Eaton UPSAlso, in some applications, it may be possible to shut down the power to some servers almost immediately, while for others it will be essential to maintain power long enough for complex data backup and shutdown procedures to be executed. In reality, most data centers have servers that fall into both of these categories. In such cases, an effective and flexible power management system, such as Eaton’s Intelligent Power Management (IPM), will allow selective control over which servers remain powered and which do not.

The next sizing consideration is the one that’s most frequently neglected – provision for future expansion, which is often referred to as scalability. As has already been suggested, this depends very much on the configuration of the UPS installation. Where each rack has its own UPS, expansion is very straightforward – simply add more racks with more UPS’s. Scalability is therefore guaranteed in these cases without the need for special measures.

Where a centralised UPS system is used, the situation is rather different. The decision then has to be made whether to buy an over-sized UPS at the outset, bearing in mind the points made about minimum loads earlier in this article, or to invest in a modular UPS system where extra modules can be added at a future date. With this last approach, however, it’s important to be sure that sufficient space will be available to accommodate the new modules and that the electrical infrastructure is in place to deal with the expansion.

The final point to consider is resilience. What happens if there’s a UPS failure or the UPS has to be taken out of service for maintenance? For the most critical systems, it may be necessary to have a redundant standby UPS to cover such eventualities.

As this article has hopefully demonstrated, sizing a UPS system isn’t an intrinsically difficult process, but there are many factors that have to be taken into account. Careful attention to these factors, ensuring particularly that none are overlooked, will allow the best and most economical choice to be made. Much more detailed guidance can be found in the latest UPS Handbook published by Eaton Power Quality, which can be downloaded from Finally, it is also worth bearing in mind the old adage, “If in doubt ask!” The Eaton Power Quality team is always ready to provide expert advice.

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