Reduce Energy Losses from Uninterruptable Power Supply (UPS) systems

UPS systems maintain power to data centers in the event of a utility power disruption.  They typically use batteries as an emergency power source that may last for a few seconds to tens of minutes – just enough time for either emergency generators to come online, or for computing equipment to be shut down properly.  They also “clean up” dirty power, which can damage sensitive electronic equipment.  (This means correcting for power sags, surges and frequency distortions.)

UPS System

Figure 1: UPS systems can range in size from small rack mounted units of just one kVA capacity (left) to large freestanding models of 1000 kVA or more (right) that interface with racks of batteries (not shown here).  Image credits: Dell and Emerson Electric.

UPSs are part of a data center’s electrical distribution system, which includes utility or generator-supplied power, building switchgear and transformers, and Power Distribution Units (PDUs).  Although they are often overlooked, electrical distribution system losses can account for 10% to 12% of the total energy consumed by the data center, on average.1

There are a number of ways to save energy with UPSs.

Examine your power backup requirements

Do you really need UPSs for all of your server room equipment – and if so, how much is enough?  Many IT applications are not so critical that they cannot be shut down if there is a power disturbance and restarted without adverse effects.2  Analyzing your power backup requirements can help you eliminate capital costs for unnecessary or oversized UPS systems. It will help you save energy that is lost in power conversion and energy used to cool UPS equipment as well. Any IT load that requires high reliability should be a candidate for relocation to a true data center, a colocation facility, or to a cloud solution. Learn about selecting a sustainable colocation facility.

Buy ENERGY STAR-certified UPS systems

Ask your UPS vendor for ENERGY STAR-certified UPS models, which can cut energy losses by 30-55%. Models that qualify for the ENERGY STAR are made by most of the leading UPS manufacturers.  They usually cost and perform the same (or better) than standard products, but they are designed and/or constructed to save energy.

Most of the energy consumed by a UPS is the result of switching losses in the inverter and transformers. To mitigate these losses, energy-efficient UPS systems employ a power management system that precisely controls every pulse of the switching cycle, optimizing the inverter's switching for specific load types and load levels. The resulting switching patterns suffer the least possible losses and surpass the efficiency of older-style systems with fixed switching patterns. In addition, UPS systems can offer a 2% to 3% overall efficiency advantage over a generic lower efficiency UPS due to their high-efficiency transformers.

Use “Eco mode” features where feasible

Some UPS systems feature an “eco mode.”  This is a broad term used to describe any UPS mode of operation that improves the efficiency of the system.  However, the efficiency increase comes with a particular trade-off in performance or reliability, which varies by vendor.  In many UPS systems, eco mode is synonymous with “bypass mode,” in which the UPS system allows utility power to bypass the rectifier and inverter and directly feed the IT load.  In bypass mode, losses from the inverter/rectifier circuits are eliminated.  This can reduce data center energy costs by as much as two to eight percent.3  However, it’s always a good idea to ask your UPS provider if eco mode or bypass mode is suitable for the operating characteristics of your facility.  In some cases, these high efficiency modes can be used periodically -- whenever conditions are right.  (For example, when power quality permits, during non-critical times of data center operation, or when known risk factors such as storms are not present.)

Operate UPS systems at a high load factor

New energy-efficient UPSs generally range from 92% to 95% efficient.  However, a UPS does not always operate as efficiently as its rating suggests. Instead, its efficiency follows a curve based on its “load factor” —how much of its power capacity is being used. If the UPS is operating at 100 percent capacity, it will run as efficiently as its rating implies. But if the UPS is operating at a lower capacity, as almost all do, the equation changes. This is because a UPS loses energy in two ways – proportionally and fixed.4  Proportional losses occur in the form of heat-dissipation and are directly tied to the size of the load.  Fixed losses, however, remain constant regardless of how much current is running through your UPS. When a UPS is running a partial load, fixed losses have a more significant impact on its efficiency. This is especially important when you consider that most facilities operate their UPS’s at less than half of their load capacity.

Efficiency generally drops off when the load is less than 50 percent of UPS capacity, and drops substantially when below 30 percent of capacity.  To illustrate, consider the efficiency curves of two different UPS systems in Figure 2, below.  UPS A has a 96 percent efficiency rating, while UPS B has a 94 percent efficiency rating.  When operating at a capacity below 50 percent, however, UPS B is actually the more efficient system – despite its lower efficiency rating.

Chart Showing UPS Efficiency

Figure 2: UPS Efficiency Curves

Lightly-loaded UPS systems are generally not operating very efficiently, as shown by the curves in Figure 2.  Boosting a UPS system’s load factor, therefore, increases efficiency.  As the case study below illustrates, operating two 750 kVA UPS systems at higher load is more efficient than running two 1000 kVA systems at lower load. 

Modular UPS systems improve efficiency because they allow you to add capacity (e.g., in 10-50 kVA increments) in proportion to the growth of your IT load.  See Figure 3.  This unique feature allows data center operators to add just enough UPS capacity to accommodate load growth, thereby keeping their UPS load factor high.

Modular UPS

Figure 3: Modular UPS systems can grow to match your IT load.  This allows them to be run at a high load factor, which is more energy efficient.

Case Study: New UPS Systems Save Energy in Exelon Data Center

Exelon has made a number of improvements to its Joliet, IL Data Center over the years, but arguably no single improvement was more effective at boosting overall data center efficiency than upgrading the UPS and power distribution systems. About five years ago Joliet, a Tier II data center, operated two 1000 kVA Piller UPSs in parallel. These large UPS systems were inefficient because they ran at such a low load factor. The power distribution board at that time was also sub-optimal as it represented a single point of failure.

Today the data center is served by four smaller (750 kVA) UPS modules configured as two x 2N systems. This new UPS configuration offers full redundancy (meaning that if one UPS system fails, a redundant UPS system can supply all of the power for critical loads) -- and the ability to add capacity as needs grow. The efficiencies of the individual UPS units are above 90% because they are operating at a high load factor. Power is distributed by independent paths to pairs of PDUs. Each PDU pair receives power from at least two UPS modules. Additionally, monitoring equipment is installed in all power distribution panels to measure system efficiency and performance.

"Replacing the old UPS systems was probably our biggest challenge on the path towards a more efficient facility," recalls Dom Lovino, Exelon Corp's Manager of Data Center Operations. "But it also provided one of the biggest payoffs in terms of efficiency." These changes and several other efficiency upgrades (new CRACs with variable speed drives and dual coils, airflow management improvements, and an increase in the allowable temperature and humidity ranges) brought the data center's PUE down from 1.96 to 1.146. Although computing capacity has increased at least 5-fold during the past five years, overall energy consumption at the Joliet Data Center has remained virtually flat. Exelon tapped The Green Grid, DataCenter Dynamics, the Uptime Institute, and nlyte Software for assistance in identifying and implementing energy efficiency measures at the Joliet Data Center.

Savings and Costs

New energy-efficient UPSs generally range from 92% to 95% efficient, while older “legacy” models can be less than 90% efficient.5  (The Eaton Energy Saver System claims to reach 99% efficiency across a wide range of loads.6)  These values indicate how much of the original incoming utility power is used to power your IT load versus the amount lost to operate the UPS. For instance, a 90 percent efficiency rating indicates 90 percent of the original incoming power is powering your load, while 10 percent is being lost to operate the UPS.  Keep in mind that these efficiency figures generally assume the UPS system is operating at 100 percent of its load capacity, and that efficiency declines at lower loads.

An ENERGY STAR certified UPS can cut energy losses by 30-55% when compared to a standard UPS system.  For instance, a 1000 kVA UPS used in a large data center could save $18,000 annually.

DOE estimates that a 15,000-square-foot data center operating at 100 watts per square foot requires 13,140 megawatt hours of energy annually for the IT equipment. If the UPS system supplying that power improved from 90% to 95% efficient, the annual energy bill will be reduced by 768,421 kWh, saving about $90,000 at a rate of $0.12/kWh.7  Plus there would be significant energy savings from the reduced cooling load.

When operating in bypass mode, UPS systems can reduce data center energy costs by as much as two to eight percent.8

In addition to the direct cost of a new UPS system, installed costs may include data center downtime in order to reconfigure electrical loads, labor costs associated with removing the old UPS and deploying new one, and end-of-life disposal costs for systems that are not repurposed.

Tips and Considerations

Efficiency ratings don’t always paint an accurate picture of actual UPS efficiency, so ask your UPS provider to show you efficiency curves, like Figure 2, above.  These will help you determine which system will be the most efficient in your particular facility and application.

Ask your UPS provider for advice prior to activating eco mode or bypass mode, since the efficiency increase comes with a trade-off in performance or reliability.

1 Evaluating UPS System Efficiency, by K. Kutsmeda, Consulting-Specifying Engineer, June 11, 2015.

2 Improving Energy Efficiency for Server Rooms and Closets, Lawrence Berkeley National Laboratory, US DOE, October 2012. (PDF, 673 KB)

3 High Efficiency UPS Operating Modes, a presentation by Chuck Heller, Liebert Corporation, May 2011.$FILE/9-Heller-Liebert-High%20Efficiency%20UPS%20Operating%20Modes%20CRH%2005042011%20Final.pdf (PDF, 454 KB)

4 When an Energy Efficient UPS Isn’t As Efficient as You Think, Quality Power Solutions, September 24, 2014.

5 High Efficiency UPS Operating Modes, a presentation by Chuck Heller, Liebert Corporation, May 2011.$FILE/9-Heller-Liebert-High%20Efficiency%20UPS%20Operating%20Modes%20CRH%2005042011%20Final.pdf (PDF, 454 KB)

6 Information on Eaton’s Energy Saver System is available at

7 Best Practices Guide for Energy-Efficient Data Center Design, National Renewable Energy Laboratory (NREL), March 2011. (PDF,  899 KB)

8 High Efficiency UPS Operating Modes, a presentation by Chuck Heller, Liebert Corporation, May 2011.$FILE/9-Heller-Liebert-High%20Efficiency%20UPS%20Operating%20Modes%20CRH%2005042011%20Final.pdf (PDF, 454 KB)