Understanding the UPS System kVA and kW Debate

UPS kVA or KW debate

In a datacentre, comms room and any general IT environment it is vitally important to ensure that all elements within the critical power path are sized correctly. The critical power path runs from the building incomer to transformers, switchgear, distribution panels, uninterruptible power supplies (UPS) and power distribution units (PDUs). There are two generally accepted ways of calculating the power drawn by IT systems (loads) supplied from the critical power path. These are referred to as VA and Watts.

VA stands for Volt-Ampere and is calculated by measuring the Voltage supplied to the equipment multiplied by the Amperage (Amps) drawn to power it. VA is often referred to as the ‘Apparent Power’ drawn from the power supply in order for the IT component to function. To work out a kVA figure, simply multiply the VA by 1000 (kilo).

Watts is the true measure of the power drawn by any device or system. This is known as ‘Real Power’ and is the value used by utility companies to charge for the electricity used on an hourly basis; normally expressed as KWh or GWh (where K=Kilo and G=Giga). In a colocation datacentre kWs are used to charge clients and in any IT environment kWh are used to calculate Power Usage Effectiveness (PUE). When assessing the critical power path electrical contractors and power protection specialists will look at the VA and Watt ratings in different ways.

An electrical contractor will assess the existing mains supply in terms of a number of factors. These include: the mains supply phase configuration (three or single phase), the load phase requirements (three or single phase), the current draws and upstream circuit breaker types and sizes to ensure safe discrimination in case of short-circuits and system overloads. They will also look at the existing cables types and run lengths to ensure that they are sized to support the load requirements. Electrical contracting companies are therefore more likely to look at the voltage and current (VA) arrangements.

Eco Power Protection specialists will consider the ‘Real Power’ being drawn and focus on the Watts or kW power demands of the load. This is because whatever the UPS system recommended, the uninterruptible power supply must be sized to deliver the maximum kW demand of the load and the batteries have to be sized based on the ‘Real Power’ demands if the autonomy is to be reached. All UPS systems are quoted with both a kVA and kW rating. These two values measure their ability to provide both ‘Apparent Power’ and ‘Real Power’ and are linked through a term known as the ‘Power Factor’.

Power Factor can be related to the Input and Output of a UPS. Input Power Factor is the load placed by the UPS on the mains power supply or standby power generator. Most modern UPS have a near Unity (1.0) or greater than 0.9 input power factor. The higher the value the more closely matched the UPS input demands to the current and voltage waveforms of the supply, leading to more efficient power conversions, less heat output and therefore less wasted energy. When sizing an uninterruptible power supply load the power factor in question is the output power factor. Power factor is expressed as a ratio of the VA/W or kVA/kW. For example, a 60kVA UPS may have an output power factor of 0.8 or 0.9pF meaning that it can only supply a total load of 48kW or 54kW. If the UPS system has an output power factor of Unity (1.0) it can deliver 60kW.

At the heart of many IT systems is a switch mode power supply (SMPS). This converts the AC mains input into the various levels of DC required to power the internal PCBs and assemblies of an IT system. Power supply manufacturers have been driven like all electronics companies, to achieve ever higher levels of energy efficiency and to reduce their impact on the mains supply. To achieve this and meet the need to support more power hungry servers like Blades and those used in virtualised environments, many switch mode power supplies are rated close to or at Unity power factor.

So how does this relate to UPS? Well UPS manufacturers have always used power factor to their advantage when promoting their systems. In many sub 2kVA uninterruptible power supplies it is common to find a power factor lower than 1.0 and in many cases as low as 0.6 for the smallest systems. This allows UPS manufacturers to offer a 300W UPS that can deliver 500VA of ‘Apparent Power’ if VA is used to size the system. This practice is becoming less common but it is just as important to check the kW as the KVA rating of a UPS. This becomes even more important as loads rise in size. Larger UPS tend to be rated with a higher output power factor of at least 0.9. Legacy systems can be found around 0.8 and the latest uninterruptible power supplies tend to be launched to market with Unit output ratings where the same UPS will deliver the same kW and kVA values.

For datacentre and comms room managers looking to swap out legacy systems or simply invest for planned expansion a Unity Power Factor rated UPS system is therefore always recommended by eco power protection specialists. When you do the ‘maths’ the answer is easy to see. For a typical datacentre application using a centralised UPS to supply the critical power path distribution switchgear and PDUs to the server cabinets, the Real Power calculation could come out at 40kW. This could mean offering a modern 40kW/40kVA system with no rom for future expansion. For a 0.9 output power factor rated system would have to look at a rating or 40kW/0.9=45kVA with the nearest UPS sizing available being a 60kVA/54kW system. Whilst this would offer room for expansion it would also be larger, less compact, less efficient and represent a higher capital investment. In the example given, EcoPowerSupplies would recommend an Eaton 93PM uninterruptible power supply which offers the additional benefit N+1 redundancy in a single, compact cabinet.

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