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How to Improve the Energy Efficiency of Your Datacentre UPS

The energy efficiency of your data centre UPS is important for two critical reasons. The first is running costs and the second is related to the battery runtime available when the mains power supply fails. An undersized UPS is often immediately obvious as load is applied on start-up but may not always be so. Intermittent overloading can occur if there is marginal capacity for extra load. Oversizing, can also be an issue whilst providing what can prove to be a false sense of security.

UPS Efficiency Levels

Today’s online datacentre UPS are highly energy efficient compared to those of 10-20 years ago. In terms of static UPS, datacentre operators typically choose to install one of three types when looking at applications under 1MVA:

  1. Modular UPS: possibly the fastest growing trend in uninterruptible power supplies. Modular UPS systems have use fixed sized UPS frame into which several equally rated UPS power modules are placed. A typical modular system may use 10kW, 25kW or 50kW modules and these can be arranged into a capacity or N+x parallel/redundant configuration into a UPS frame same. If the frame size is 200kW, then the frame can support four x 50kW modules giving a 200kW capacity as a maximum or 150kW in an N+1 configuration.
  2. Monobloc Transformer-less UPS: this type of uninterruptible power supply is, as its name suggest, without a transformer. The UPS has an IGBT-based rectifier which supplies a DC power supply to the load and to the IGBT-based inverter output stage. IGBTs are a high efficiency power switching component used in power supplies. In a transformer-less (or transformer-less) UPS system, the use of an IGBT-based rectifier and inverter makes for a very compact and high efficiency system. It is this electronics design and arrangement used in modular UPS designs. The smallest static type monobloc UPS will be around 700VA and the largest up to 800kVA or more.
  3. Monobloc Transformer-based UPS:possibly the oldest type of UPS design using a transformer-based to provide isolation to the load and from which a rectifier section is connected to charge the battery and provide power to the inverter. This type of UPS system is audibly noisier than a transformer-less UPS, heavy and generally not as energy efficient. It is however more suited to harsher electrical environments including industrial applications and may be available from 5kVA up to 800kVA or more in size.

For larger installations from 1MVA upwards, rotary UPS systems are an alternative option. This is a dynamic type of power supply using kinetic and stored energy to power the load when the mains power supply fails.

Legacy UPS Systems

A datacentre-type uninterruptible power supply can have a working life of 10 years or more provided that the device undergoes regular annual maintenance and cyclical refurbishment.

Uninterruptible power supplies include ‘consumable’ items which require regular placement and within their design life. UPS batteries are the most common example and are generally of the lead acid 5-year or 10-year design life, requiring replacement around years 3-4 or 7-8 respectively and when used within a 20-25˚C ambient environment. Cooling fans are another example of a UPS consumable which should also be inspected annually and replaced according to fan manufacturer recommendations.

Around year 8 of operation a UPS system should be considered for refurbishment. The principle works here include replacement of the AC and DC capacitors and cable harnesses. The capacitors require replacement as they can dry-out and their self-healing characteristics be weakened to the point of a potential critical failure occurring. Capacitors use flammable polypropylene and at high internal temperatures can experience thermal runway and case a fire inside the UPS.
If your datacentre power system is at the 8th year of operation, it may be worth considering a complete UPS upgrade rather a refurbishment and especially if your legacy system is a transformer-based type.

An upgrade to one of the latest triple-conversion designs found in transformer-less monobloc and modular UPS systems can see energy efficiency improvements from 15-20% or more dependent on the load percentage and profile supported.

Correct UPS and Battery Sizing

Most users associate energy efficiency with running costs, and this is certainly true with a datacentre-type uninterruptible power supply. The higher its operational energy efficiency the lower the amount of energy wasted. Energy efficiency savings also mean lower heat output and therefore less load onto the local cooling system.

Right sizing the uninterruptible power supply and load profile also helps to ensure the UPS is supporting a load profile within its optimum range. On an old transformer-based UPS system, the energy efficiency may only be 80-85% and within a highly loaded system. This is because in this older type of UPS system the efficiency profile tails off quite dramatically as soon as the load is less than around 70-80% of the UPS rating. In a more modern UPS design, a load as low as 25% can still achieve high operating efficiency. The use of a modular UPS design also helps to right-size as the ‘correct’ number of UPS models can be used in this high efficiency design to meet the load and any N+X requirements.

Battery Sizing Formula

The energy efficiency of a UPS and its load size also affects the size of the battery required to deliver a specific runtime or backup autonomy. The formula for battery sizing is:

Battery Load (kW) = (UPS kVA x Power Factor) / UPS Efficiency

The energy efficiency of a UPS system plays a large role in sizing a UPS battery. The greater the efficiency the smaller the battery load.

Oversizing the UPS battery leads to a bigger installation space and investment cost than may be required. Under-sizing has its own downsides, not least of which is less runtime provision than may be required.

Care should be taken when assessing the stated efficiency of a UPS system. UPS manufacturer ratings should be verified to confirm whether the UPS has been tested in full on-line or eco-mode. A benchmark on-line energy efficiency rating is around 95-96% with eco-mode running at 98-99%. The issue with eco-mode UPS operation for most datacentres and server room is the lower grade of power protection provided and with potentially a momentary break in supply. On-line UPS mode provides a no-break supply and digitally generated sinewave output from a constantly running inverter to which the load is connected.

Some UPS manufacturers can also power down parts of their UPS system to improve energy efficiency. A battery charging circuit can be turned off and the battery allowed to self-discharge to a pre-set level before the charging circuit reengages.

UPS Sizing

Sizing an uninterruptible power supply should be relatively straight forward. If there is an existing load, it can be measured. However for new installations, the load needs to be estimated and this can be difficult, especially if ratings are pulled from datasheets and manuals. This is because the power drawn is nearly always shown as being the full-power rating of the power supply. In the case of a server with a 1500W power supply this could be misleading as the 1500W is the maximum power that can be supplied and not that which will be ultimately drawn. Taking this as a typical example, the surge current on power up may equate momentarily to only 80% of the 1500W rating and the true running current drop down to as low as 25-30%.

If the load is ‘steady’ with little anticipated change over time, then a monobloc UPS may be the most appropriate for an installation. The alternative is to choose a modular UPS system which by design can be more easily right-sized for any given load. A modular UPS system can also automatically adjust the number of operational modules required to support a given load at any time, thereby increasing energy savings.

Remote Monitoring

UPS systems should be monitored over an Ethernet/IP network for alarm conditions. Continuously monitoring the UPS can establish load patterns over time and provide enough time to respond to issues that could affect energy efficiency and operational resilience. Two specific alarms that can indicate energy efficiency and operational issues include:

  • Overload and bypass alarms: if the load connected to the UPS is greater than its rated output, an alarm will be issued. If the overload is large enough or there is an internal fault condition, the UPS may go into bypass mode.
  • Overtemperature alarms: can indicate an internal fan failure or that ventilation into and out of the UPS cabinet is restricted. Long term higher internal ambient temperatures can lead to component damage and ‘cooked’ batteries as well as requiring fans to run at higher speeds.

If the battery set is within a separate cabinet or room, the installation of a separate battery temperature sensor is recommended.

Carbon Trust Energy Technology List

One other aspect to remember when selecting whether to upgrade a legacy installation or to deciding on the right UPS for a new datacentre. Capital incentives (first year tax breaks) are available for products listed on the Carbon Trust’s Energy Technology List (ETL). To be included on the ETL each product within its category must be capable of reaching threshold levels of energy efficiency in order to qualify. For uninterruptible power supplies, the tax-related incentive is available for static systems from 10kVA up to 1MVA.

More Information at:


Over the last two decades there has been a significant improvement in the datacentre UPS energy efficiency levels. The use of IGBT-based rectifiers and inverters has helped to dramatically lower fixed energy efficiency losses and flatted the efficiency curve out over a wide load range, from as low as 25% to 100% loaded. There have also be improvements in transformer design and the widespread introduction of modular datacentre UPS by many of the leading UPS manufacturers. Energy efficiency levels have risen from around 70-80% to around 96% for most online systems and 97-98% could eventually become the norm. However, there will always be the need to correctly size both the UPS and its battery set, monitor installations and ensure that they are correctly maintained if the design calculations are to be proven in use.

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