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Considering the Pros and Cons of UPS Eco-mode

In the never-ending effort to squeeze every bit of efficiency out of data center power and cooling, some data center operators are taking a hard look at UPS eco-mode and whether it can save them money. It’s an interesting question, because while eco-mode can indeed save on energy costs, it comes with some risks that data center operators would do well to carefully consider.

Despite what you might read in UPS marketing literature, eco-mode is neither new nor revolutionary technology; it’s been around for decades and is, in fact, the basic mode of operation used in offline UPS where it is called “standby” or “line interactive” mode.

APC by Schneider Electric has calculated the energy savings associated with the use of UPS eco-mode to be about 3.3%, corresponding to an energy savings of about $10,000 per year for a 1MW rated data center at 50% load. Of course your actual energy savings will depend on your specific equipment and architecture, data center load and your cost of electricity.

While 3% is not a major efficiency pickup, it is valuable. Unfortunately, there are consequences associated with operation in eco-mode, which must be taken into consideration.

Loss of electrical protection

A key function of a UPS system has historically been to regenerate “clean” electrical power without the variations of voltage and frequency or transients present on raw utility mains power – the central concept being that such variations interfere with the operation of sensitive IT systems.

While that was true at one time, virtually all equipment sold today is designed to accommodate variations of mains voltage within a specified operating range. It’s a fact that IT equipment today requires less power conditioning than in prior decades.

That said, the UPS load in a data center is a complex system of circuits including many different types of IT equipment and transformers, possibly including other complex devices such as static switch PDUs, fans, and pumps. It’s much more difficult to assure the performance of a complete data center than it is any single piece of IT equipment, such as a personal computer or server.

Added time to respond to power problems

Eco-mode cannot predict the future. It must respond to a problem that already exists and switch to the inverter. This means that the problem will get through the UPS to the data critical load until the following four things happen:

  1. The power problem is detected
  2. The UPS determines whether and how to respond
  3. The UPS inverter is energized
  4. The transfer switch is actuated

In practice these events may take from 1 to 16 milliseconds, during which the data center load is subjected to the power problem. While that may not affect the typical 2U server, it could result in various other problems, including tripped breakers, overloads and dropped loads. Additional issues related to the use of eco-mode include risk of thermal shock and the potential for reduced UPS battery life.

Technical means exist to overcome all of these potential problems but it’s difficult to ensure that they won’t be a factor in a specific installation unless all aspects of the data center design are very well characterized and analyzed.

To learn more about eco-mode pros and cons, and how to determine whether it may be a benefit to your data center, check out the APC by Schneider Electric white paper, “Eco-mode: Benefits and Risks of Energy-saving Modes of UPS Operation.”


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